Build a Pulse Charger for reviving tired Lead Acid batteries

If you own a motorcycle, a motor home, a caravan, a lawn mover, a day cruiser or maybe a vintage car you must at some point had to write off a lead acid battery. When a battery is improperly charged or allowed to self-discharge as occurs during non-use, sulphate crystals build up on the batterys plates. The sulphate preventing the battery from being fully charged and therefore it is unable to deliver its full capacity. When trying to charge a battery in this state it only gets hot and looses water, the gravity of the electrolyte is not increasing to its normal “full charge” state. 

 Pulse Charger for reviving tired Lead Acid batteries Circuit Diagram

The only thing you do is killing the battery completely. If a battery has a resting voltage of at least 1.8 Volts/cell and no cells are shorted, desalination of its plates can be done. This circuit is an add-on and part for a modification of a normal charger and it takes care of the sulphate problem.

The project: get hold of an old charger, big or small it’s your choice depending on the size of batteries you normally handle (bigger is better). There are some tricks to boost the performance if you need it. Start by ripping out everything except the transformer and the rectifier. Some older chargers are equipped with fin rectifiers, which have high voltage drop and must be replaced. Replace with a rugged bridge rectifier that can cope with the amperes. All wiring on secondary should be short and heavy wire. The rectifier should be bolted to the chassis to keep cool. If the charger have a high/low switch it’s a bonus, if not you can in some cases add a few turns of wire on the secondary winding. 

The circuit; a 14-stage ripple counter and oscillator IC 4060 produce a pulse, which is the heartbeat of the circuit. The pulse is feed to the 555 timer that deicide the length of the active output. With the switch you can select long or short pulse output. The output of the 555 timer triggers the zero-cross opt isolator triac driver MOC 3041 via a transistor. This gives the charger transformer a soft start via the triac and the snubber circuit. A small power supply is necessary for the circuit and consists of T1 a transformer 15V 0.1A secondary, a bridge rectifier, a regulator and two caps. Because this project include a charger that is (X) the outcome can differ in performance from one case to another. However this do not mean that your project doesn’t work, but the efficiency can vary. Some notes the snubbercap is a high voltage AC type (X) and the resistors on the mains side is at least 0.5W type. Use a triac that can take 400V+ and 10A+, I use BTA 25.600 but this is overkill in most cases. No PCB sorry!

How it works:
Well the short version. The object is to get the cell voltage high enough for the sulphate to dissolve without boiling or melting the battery. This is achieved by applying higher voltage for shorter periods and let the battery rest for a while. The pulses on short range is about 0.5s on / 3s off and the long pulse range is 1.4s on / 2s off. These times can vary depending on component tolerances. Start on long pulse and if you discover “boiling” (more than with normal charging) in the electrolyte switch to short puls. Don’t leave the process unattended, at least until you know how your specific version of this project turns out. I built ver.1 of this circuit some 10 years ago and have experimented with it but I’m sure someone can improve it further.

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Touch Switch II

This circuit uses a 555 timer as the bases of the touch switch. You can learn more about 555 timers in the Learning section on my site. When the plate is touched the 555 timer is triggered and the output on pin 3 goes high turning on the LED and the buzzer for a certain period of time. The time that the LED and the buzzer is on is based on the values of the capacitor and resistor connected to pin 6 & 7. The 10M resistor on pin 2 causes the the circuit to be very sensitive to the touch.

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Build a Single cell Charger Circuit Diagram

This Single cell Charger Circuit Diagram detects a full-charge state and automatically switches to a float condition —from 240 mA to 12 mA. The circuit uses the 555 timer.

 Build a Single cell Charger Circuit Diagram

 

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Simple Low Power Car Stereo Amplifier

A simple low power car stereo amplifier circuit based on TDA 2003 is shown here. The circuit uses cheap, readily available components and it is very easy to construct. TDA2003 is an integrated car radio amplifier from ST Micro electronics that has a lot of good features like short circuit protection for all pins, thermal over range low harmonic distortion, low cross over distortion etc. 

Circuit diagram :

 A simple low power car stereo amplifier Circuit Diagram

In the circuit given here each TDA2003 is wired as a mono amplifier operating from a 12V supply. Resistors R2 and R3 forms a feedback network that sets the amplifiers gain. C7 is the input DC de-coupling capacitor and C5 couples the speaker to the amplifiers output. C4 is used for improving the ripple rejection while C1 and C2 are employed for power supply filtering. C3 and R1 are used for setting the upper frequency cut-off. Network comprising of C6 and R4 is used for frequency stabilization and to prevent oscillation.

Notes.

• Assemble the circuit on a good quality PCB.
• Heat sinks are necessary for both ICs.
• The circuit can be operated from 12V DC.
• S1 is the ON/OFF switch

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Cheap 12V to 220V Inverter

Even though today’s electrical appliances are increasingly often self-powered, especially the portable ones you carry around when camping or holidaying in summer, you do still sometimes need a source of 230 V AC - and while we’re about it, why not at a frequency close to that of the mains? As long as the power required from such a source remains relatively low - here we’ve chosen 30 VA - it’s very easy to build an inverter with simple, cheap components that many electronics hobbyists may even already have.

Though it is possible to build a more powerful circuit, the complexity caused by the very heavy currents to be handled on the low-voltage side leads to circuits that would be out of place in this summer issue. Let’s not forget, for example, that just to get a meager 1 amp at 230 VAC, the battery primary side would have to handle more than 20 ADC!. The circuit diagram of our project is easy to follow. A classic 555 timer chip, identified as IC1, is configured as an astable multivibrator at a frequency close to 100 Hz, which can be adjusted accurately by means of potentiometer P1.

Cheap 12V to 220V Inverter Circuit diagram:

As the mark/space ratio (duty factor) of the 555 output is a long way from being 1:1 (50%), it is used to drive a D-type flip-flop produced using a CMOS type 4013 IC. This produces perfect complementary square-wave signals (i.e. in antiphase) on its Q and Q outputs suitable for driving the output power transistors. As the output current available from the CMOS 4013 is very small, Darlington power transistors are used to arrive at the necessary output current. We have chosen MJ3001s from the now defunct Motorola (only as a semi-conductor manufacturer, of course!) which are cheap and readily available, but any equivalent power Darlington could be used.

These drive a 230 V to 2 × 9 V center-tapped transformer used ‘backwards’ to produce the 230 V output. The presence of the 230 VAC voltage is indicated by a neon light, while a VDR (voltage dependent resistor) type S10K250 or S07K250 clips off the spikes and surges that may appear at the transistor switching points. The output signal this circuit produces is approximately a square wave; only approximately, since it is somewhat distorted by passing through the transformer. Fortunately, it is suitable for the majority of electrical devices it is capable of supplying, whether they be light bulbs, small motors, or power supplies for electronic devices.

PCB layout:

PCB Layout For Cheap 12V to 220V Inverter Circuit Diagram

Parts List :
Resistors
R1 = 18k?
R2 = 3k3
R3 = 1k
R4,R5 = 1k?5
R6 = VDR S10K250 (or S07K250)
P1 = 100 k potentiometer
Capacitors
C1 = 330nF
C2 = 1000 µF 25V
Semiconductor
T1,T2 = MJ3001
IC1 = 555
IC2 = 4013
Miscellaneous
LA1 = neon light 230 V
F1 = fuse, 5A
TR1 = mains transformer, 2x9V 40VA (see text)
4 solder pins
Note that, even though the circuit is intended and designed for powering by a car battery, i.e. from 12 V, the transformer is specified with a 9 V primary. But at full power you need to allow for a voltage drop of around 3 V between the collector and emitter of the power transistors. This relatively high saturation voltage is in fact a ‘shortcoming’ common to all devices in Darlington configuration, which actually consists of two transistors in one case. We’re suggesting a PCB design to make it easy to construct this project; as the component overlay shows, the PCB only carries the low-power, low-voltage components.

The Darlington transistors should be fitted onto a finned anodized aluminum heat-sink using the standard insulating accessories of mica washers and shouldered washers, as their collectors are connected to the metal cans and would otherwise be short-circuited. An output power of 30 VA implies a current consumption of the order of 3 A from the 12 V battery at the ‘primary side’. So the wires connecting the collectors of the MJ3001s [1] T1 and T2 to the transformer primary, the emitters of T1 and T2 to the battery negative terminal, and the battery positive terminal to the transformer primary will need to have a minimum cross-sectional area of 2 mm2 so as to minimize voltage drop.

The transformer can be any 230 V to 2 × 9 V type, with an E/I iron core or toroidal, rated at around 40 VA. Properly constructed on the board shown here, the circuit should work at once, the only adjustment being to set the output to a frequency of 50 Hz with P1. You should keep in minds that the frequency stability of the 555 is fairly poor by today’s standards, so you shouldn’t rely on it to drive your radio-alarm correctly – but is such a device very useful or indeed desirable to have on holiday anyway? Watch out too for the fact that the output voltage of this inverter is just as dangerous as the mains from your domestic power sockets.

So you need to apply just the same safety rules! Also, the project should be enclosed in a sturdy ABS or diecast so no parts can be touched while in operation. The circuit should not be too difficult to adapt to other mains voltages or frequencies, for example 110 V, 115 V or 127 V, 60 Hz. The AC voltage requires a transformer with a different primary voltage (which here becomes the secondary), and the frequency, some adjusting of P1 and possibly minor changes to the values of timing components R1 and C1 on the 555.

Source: http://www.ecircuitslab.com/2011/08/cheap-12v-to-220v-inverter.html

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Animal Friendiy Mousetrap

This mousetrap is built around a PlC12F683 and uses an infrared transmissive optical sensor that is modulated at a frequency of 38 kHz, so that it isnt affected by the ambient light. The modulation is carried out by the PlC, which generates a 38 kHz signal at port GP2, which is connected to the lR LED. The lR receiver is a type that is usually found for use with remote controls. lt reacts only to 38 kHz signals. lt reports the presence of an lR signal to the PIC via port GP1.

When the lR lightbeam is broken the PIC turns of the relay via port GP4 and FET T1 , which: causes the door of the mousetrap to close. The transmissive optical sensor is housed inside a small wooden box. A small amount of food is placed inside this box.

Animal Friendiy Mousetrap Circuit Diagram

When a mouse walks through the light beam on its way to the food it causes the door to shut behind it and an LED starts flashing. The door is normally kept open by the coil of a relay that has been taken apart. When the coil is no longer powered the tin door is pushed shut by means of a spring. A piece of glass or transparent plastic should be put on top of the box, so that the mouse doesnt have to enter a dark space. When a mouse has been caught it can be let free again somewhere outside, some distance away from the house.

The reset button has to be pressed to ready the trap for its next victim. The author has managed to catch a few dozen mice with this device. The program is written in PICBASIC Pro and can be freely downloaded from the Elektor website, it is found in archive file # 100308-11.zip.

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10 W Audio Amplifier Rise

This design is based on the 18 Watt Audio Amplifier, and was developed mainly to satisfy the requests of correspondents unable to locate the TLE2141C chip. It uses the widespread NE5532 Dual IC but, obviously, its power output will be comprised in the 9.5 - 11.5W range, as the supply rails cannot exceed ±18V.

10 W Audio Amplifier Circuit Diagram

As amplifiers of this kind are frequently used to drive small loudspeaker cabinets, the bass frequency range is rather sacrificed. Therefore a bass-boost control was inserted in the feedback loop of the amplifier, in order to overcome this problem without quality losses. The bass lift curve can reach a maximum of +16.4dB @ 50Hz. In any case, even when the bass control is rotated fully counterclockwise, the amplifier frequency response shows a gentle raising curve: +0.8dB @ 400Hz, +4.7dB @ 100Hz and +6dB @ 50Hz (referred to 1KHz).

Notes:

• Can be directly connected to CD players, tuners and tape recorders.
• Schematic shows left channel only, but C3, C4, IC1 and the power supply are common to both channels.
• Numbers in parentheses show IC1 right channel pin connections.
• A log type for P2 will ensure a more linear regulation of bass-boost.
• Do not exceed 18 + 18V supply.
• Q3 and Q4 must be mounted on heatsink.
• D1 must be in thermal contact with Q1.
• Quiescent current (best measured with an Avo-meter in series with Q3 Emitter) is not critical.
• Set the volume control to the minimum and R3 to its minimum resistance.
• Power-on the circuit and adjust R3 to read a current drawing of about 20 to 25mA.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.
• A correct grounding is very important to eliminate hum and ground loops. Connect to the same point the ground sides of J1, P1, C2, C3 &C4. Connect C9 to the output ground.
• Then connect separately the input and output grounds to the power supply ground.

Parts:

P1_________________22K   Log.Potentiometer (Dual-gang for stereo)
P2________________100K   Log.Potentiometer (Dual-gang for stereo)
R1________________820R   1/4W Resistor
R2,R4,R8____________4K7  1/4W Resistors
R3________________500R   1/2W Trimmer Cermet
R5_________________82K   1/4W Resistor
R6,R7______________47K   1/4W Resistors
R9_________________10R   1/2W Resistor
R10__________________R22   4W Resistor (wirewound)

C1,C8_____________470nF   63V Polyester Capacitor
C2,C5_____________100µF   25V Electrolytic Capacitors
C3,C4_____________470µF   25V Electrolytic Capacitors
C6_________________47pF   63V Ceramic or Polystyrene Capacitor
C7_________________10nF   63V Polyester Capacitor
C9________________100nF   63V Polyester Capacitor

D1______________1N4148    75V 150mA Diode

IC1_____________NE5532    Low noise Dual Op-amp

Q1_______________BC547B   45V 100mA NPN Transistor
Q2_______________BC557B   45V 100mA PNP Transistor
Q3_______________TIP42A   60V 6A    PNP Transistor
Q4_______________TIP41A   60V 6A    NPN Transistor

J1__________________RCA audio input socket

Power supply parts:

R11_________________1K5  1/4W Resistor

C10,C11__________4700µF   25V Electrolytic Capacitors

D2________________100V 4A Diode bridge
D3________________5mm. Red LED

T1________________220V Primary, 12 + 12V Secondary 24-30VA Mains transformer

PL1_______________Male Mains plug

SW1_______________SPST Mains switch

Technical data:

Output power:

10 Watt RMS into 8 Ohm (1KHz sinewave)

Sensitivity:

115 to 180mV input for 10W output (depending on P2 control position)

Frequency response:

See Comments above

Total harmonic distortion @ 1KHz:

0.1W 0.009% 1W 0.004% 10W 0.005%

Total harmonic distortion @ 100Hz:

0.1W 0.009% 1W 0.007% 10W 0.012%

Total harmonic distortion @ 10KHz:

0.1W 0.056% 1W 0.01% 10W 0.018%

Total harmonic distortion @ 100Hz and full boost:

1W 0.015% 10W 0.03%

Max. bass-boost referred to 1KHz:

400Hz = +5dB; 200Hz = +7.3dB; 100Hz = +12dB; 50Hz = +16.4dB; 30Hz = +13.3dB

Unconditionally stable on capacitive loads

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Build a High And Low Voltage Cut Off With Time Delay Circuit Diagrams

The power line fluctuations and cut-offs cause damages to electrical appliances connected to the line. It is more serious in the case of domestic appliances like fridge and air conditioners. If a fridge is operated on low voltage, excessive current flows through the motor, which heats up, and get damaged.

The under/over voltage protection circuit with time delay presented here is a low cost and reliable circuit for protecting such equipments from damages. Whenever the power line is switched on it gets connected to the appliance only after a delay of a fixed time. If there is hi/low fluctuations beyond sets limits the appliance get disconnected. The system tries to connect the power back after the specific time delay, the delay being counted from the time of disconnection. If the power down time (time for which the voltage is beyond limits) is less than the delay time, the power resumes after the delay: If it is equal or more, then the power resumes directly.

This circuit has been designed, built and evaluated by me to use as a protector for my home refrigerator. This is designed around readily available semi-conductor devices such as standard bipolar medium power NPN transistor (D313/SL100/C1061), an 8-pin type 741 op-amp and NE555 timer IC. Its salient feature is that no relay hunting is employed. This draw back is commonly found in the proctors available in the market.

The complete circuit is consisting of various stages. They are: - Dual rail power supply, Reference voltage source, Voltage comparators for hi/low cut offs, Time delay stage and Relay driver stage. Lets now look at the step-by-step design details.

Dual rail power supply.
This is a conventional type of power supply as shown in Figure 1. The power is applied through the step-down transformer (230/12-0-12V/500mA). The DC proportional to the charging input voltage is obtained from bridge rectifier. Two electrolytics are there to bypass any spikes present. Bridge is capable of handling currents up to 1 Amp.
Output is given by: -
V(out) = 0.71 X V (secondary)
= 0.71 X 24V
= 17.04 V
(This equation is similar for the negative rail as well)

Circuit diagram

Low voltage cut off op-amp
Figure 2 shows the use of very common and easily available op-amp 741 as a comparator. The op-amp is available in TO-5 and DIP type packing.

Circuit diagram

In this ckt the zener diode D1 and it’s associated resistor R1 are connected to the non-inverting terminal (+ve) of 741 to give the suitable reference voltage. The DC voltage from the sensor is given to the inverting (-ve) terminal through pre-set R2.This is used to set the input level.
When the sensor input is less than Zener voltage the output from the Op-amp remains high and when it is greater than Zener voltage the output goes low. When the sensing voltage is equal to Zener voltage the output of the op-amp is approximately zero.
This phenomenon is used as a decision for switching the relay and to give cutoff in a low voltage situation.

High voltage cut off op-amp
Here the op-amp is used as a inverted amplifier. See Figure 3.Zener and resistor network gives reference voltage to the inverting terminal (-ve) of op-amp. Sensing voltage derived through the 10 K pre-set is given to the non- inverting (+ve) terminal and this sets the high level cut.

When the input DC from the sensor is less than Zener voltage the output of the op-amp is low and vice-versa. When the input DC voltage is equal to the zener voltage, the op-amps output is approximately zero.

Circuit diagram

Time delay
I’ve selected the 555 timer due to following reasons.
1. Timing from microseconds through hours.
2. Ability to operate from wide range of supply voltages.
3. High temperature stability.
4. Easily Available.
5. Its triggering circuit is quite sensitive.

This is basically a monostable. The external timing capacitor C2 is held initially discharged by the timer. The circuit triggers upon receiving a pulse to its pin 2 when the level reaches 1/3 Vcc. Once triggered., the circuit will remain in that state until the set time is elapsed or power to the circuit cuts off. The delayed period in seconds is 1.1 C2.R1 where R1 is in megohms and C2 is in microfarads. In practice, R1 should not exceed 20 M. If you use an electrolytic capacitor for C2, select a unit for low leakage. The time delay may have to be adjusted by varying R1 to compensate for the wide tolerance of electrolytics.

Circuit diagram

Relay Driver
The output from the voltage level detectors cannot directly drive the relay and hence the relay driver is used.

Circuit diagram

In this a relay (12V <500 ohms) is connected to the collector of NPN transistor. The out put voltage from the comparator is applied to the base of NPN transistor through a resistance R1. When the output from the comparator is low the transistor is in OFF state and the relay is in de-energized state. Similarly when the output from the comparator goes high the transistor switches ON and the flow of current from the collector to emitter of transistor energizes the relay.

Generally in a relay driver circuit, parallel to the relay coil, a diode or a capacitor is used. This is to eliminate the back e.m.f generated by the relay coil when currents are suddenly broken. Capacitor C1 is connected in parallel to the coil, which filters out the back emf but it, slows down the working of relay.

A better method is to connect two diodes (as shown in the figure 5) that stop the relay – transistor junction swinging more than 600mV above the positive rail or below the zero-volt rail. During normal operation the diodes are reverse biased and have no effect on the performance of circuit. But when back emf is induced, the diodes conduct heavily and absorb all transient voltages. However, I have employed the both methods.

The Complete Circuit

Circuit diagram

Under normal operating conditions i.e. when the input voltage is between maximum and minimum limit the output from the both the comparators are low. The transistor Q1 is OFF and the relay is in de-energized (pole connected to N/C pin) state and the output is obtained.

When the input voltage is below or above the limits set by the pre-sets R8 or R9, the output of the Op-Amps goes either low or high and diodes D1 or D2 would be forward biased depending on the situation. Transistor Q1 switches ON and the flow of current from collector to emitter energizes the relay and the output is cutoff.

A small amount of hystersis has been added via feed back resistors R10 & R11 so that the relay turns on when the level falls to a particular value but does not turn again until it raises a substantial amount above this value. Other wise the relay contacts will frequently turn on/off and produce chattering.

Construction Hints
1) I used a piece of varoboard, which has copper strips on one side to mount the components, and housed the entire circuit and the transformer in a discarded ATX PC power supply box.

2) An autotransformer has been used to set the limits. Set the output of the autotransformer to 250V AC and connect it to the primary of transformer T1 (see Figure 1). Then adjust the pre-set R9 such that relay just energizes. This is the high limit. Next set the output of the autotransformer to 200V AC and adjust the pre-set R8 such that the relay energizes. Please note that these are my preferred limits but you may select any range from say 170 to 270V AC.

3) A neon with a suitable resistor could be connected between the AC supply lines as an ON indicator. Alternatively, LED with a current limiting resistor could be connected between the relay coil so when the relay is energized LED will indicate the situation.

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Proximity Switch

This circuit is for an unusually sensitive and stable proximity alarm which may be built at very low cost. If the negative terminal is grounded, it will detect the presence of a hand at more than 200mm. If it is not grounded, this range is reduced to about one-third. The Proximity Switch emits a loud, falling siren when a body is detected within its range. A wide range of metal objects may be used for the sensor, including a metal plate, a doorknob, tin foil, a set of burglar bars — even a complete bicycle. Not only this, but any metal object which comes within range of the sensor, itself becomes a sensor.



For example, if a tin foil sensor is mounted underneath a table, metal items on top of the table, such as cutlery, or a dinner service, become sensors themselves. The touch plate connected to the free end of R1 detects the electric field surrounding the human body, and this is of a relatively constant value and can therefore be reliably picked up. R1 is not strictly necessary, but serves as some measure of protection against static charge on the body if the sensor should be touched directly. As a body approaches the sensor, the value of C1 effectively increases, causing the frequency of oscillator IC1.A to drop.

Consequently capacitor C2 has more time to discharge through P2, with the result that the inputs at IC1.B go Low, and the output goes High. As the output goes High, so C3 is charged through LED D2. D2 serves a dual purpose —namely as a visual indication of detection, and to lower the maximum charge on C3, thus facilitating a sharper distinction between High and Low states of capacitor C3. The value of R4 is chosen to enable C3 to discharge relatively quickly as pulses through D2 are no longer sufficient to maintain its charge. The value of C3 may be increased for a longer sounding of the siren, with a slight reduction in responsiveness at the sensor.



When C3 goes High, this triggers siren IC1.C and IC1.D. The two NAND gates drive piezo sounder X1 in push-pull fashion, thereby greatly increasing its volume. If a piezo tweeter is used here, the volume will be sufficient to make one’s ears sing. The current consumption of the circuit is so low a small 9-V alkaline PP3 battery would last for about one month. As battery voltage falls, so sensitivity drops off slightly, with the result that P1 may require occasional readjustment to maintain maximum sensitivity. On the down side of low cost, the hysteresis properties of the 4093 used in the circuit are critical to operation, adjustment and stability of the detector.

In some cases, particularly with extremely high sensitivity settings, it will be found that the circuit is best powered from a regulated voltage source. The PCB has an extra ground terminal to enable it to be easily connected to a large earthing system. Current consumption was measured at 3.5 mA stand-by or 7 mA with the buzzer activated. Usually, only P1 will require adjustment. P2 is used in place of a standard resistor in order to match temperature coefficients, and thus to enhance stability. P2 should be adjusted to around 50 k, and left that that setting.


The circuit is ideally adjusted so that D2 ceases to light when no body is near the sensor. Multiturn presets must be used for P1 and P2. Since the piezo sounder is the part of the circuit which is least affected by body presence, a switch may be inserted in one of its leads to switch the alarm on and off after D2 has been used to check adjustment. Make sure that there is a secure connection between the circuit and any metal sensor which is used.


Resistors:

• R1 = 10kΩ
• R2 = 4kΩ7
• R3 = 1kΩ
• R4 = 47kΩ
• R5 = 47kΩ
• P1,P2 = 100kΩ multiturn cermet, horizontal

Capacitors:

• C1,C2 = 22pF
• C3 = 22µF 40V radial
• C4 = 10nF
• C5 = 100µF 25V radial

Semiconductors:

• D1 = 1N4148
• D2 = LED, red
• IC1 = 4093

Miscellaneous:

• BZ1 = AC buzzer

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Operational Amplifier Op Amp Basics

The op-amp is basically a differential amplifier having a large voltage gain, very high input impedance and low output impedance. The op-amp has a "inverting" or (-) input and "non-inverting" or (+) input and a single output. The op-amp is usually powered by a dual polarity power supply in the range of +/- 5 volts to +/- 15 volts. A simple dual polarity power supply is shown in the figure below which can be assembled with two 9 volt batteries.

Operational Amplifier (Op-Amp) Basics

Inverting Amplifier:

The op-amp is connected using two resistors RA and RB such that the input signal is applied in series with RA and the output is connected back to the inverting input through RB. The non-inverting input is connected to the ground reference or the center tap of the dual polarity power supply. In operation, as the input signal moves positive, the output will move negative and visa versa. The amount of voltage change at the output relative to the input depends on the ratio of the two resistors RA and RB.

As the input moves in one direction, the output will move in the opposite direction, so that the voltage at the inverting input remains constant or zero volts in this case. If RA is 1K and RB is 10K and the input is +1 volt then there will be 1 mA of current flowing through RA and the output will have to move to -10 volts to supply the same current through RB and keep the voltage at the inverting input at zero. The voltage gain in this case would be RB/RA or 10K/1K = 10. Note that since the voltage at the inverting input is always zero, the input signal will see a input impedance equal to RA, or 1K in this case. For higher input impedance, both resistor values can be increased.

Non-inverting Amplifier:

The non-inverting amplifier is connected so that the input signal goes directly to the non-inverting input (+) and the input resistor RA is grounded. In this configuration, the input impedance as seen by the signal is much greater since the input will be following the applied signal and not held constant by the feedback current. As the signal moves in either direction, the output will follow in phase to maintain the inverting input at the same voltage as the input (+). The voltage gain is always more than 1 and can be worked out from Vgain = (1+ RB/RA).

Voltage Follower:

The voltage follower, also called a buffer, provides a high input impedance, a low output impedance, and unity gain. As the input voltage changes, the output and inverting input will change by an equal amount.

• Source
• Bowdens Hobby Circuits

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Belgian Earth Fault Detector

Having been affected by earth fault accidents,  the author put together this little circuit. It  consist of just three elements: the neon with its  original resistor for example, salvaged from  the switch on an AC power bar and a small  capacitor (class Y) salvaged from the electronics of a low-consumption lamp.

A larger capacitance makes the neon glow brighter. All this  for no money at all. The neon lights only when there is an efficient Earth present. This works  well at the author’s home, with Live or Neutral either way round. In the Elektor laboratory based in The Netherlands, some concerns  were expressed as described in the June 2011  issue [1], as the circuit was sensitive to the relative positions of the Live and Neutral. So the  Earth fault detector can also be used as a Phase  detector, but probably in Belgium only.

The whole thing can easily be incorporated into a power socket; the author used a small transparent cover to protect the neon.

Note. As opposed to the UK and the US, some AC  power outlets in Belgium  and all in The  Netherlands are not polarized, i.e. AC power plugs (both earthed  and non-earthed) can be inserted either way around.

Source : http://www.ecircuitslab.com/2012/05/belgian-earth-fault-detector.html

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Home Made Metal Detector

This homemade metal detector circuit will help you find objects composed of materials with relatively high magnetic permeability. It is not suitable for buried coins discovery that is not sensitive enough but you can detect pirates treasures! The metal detector is powered by 2 x 9V batteries, each of it charges with 15mA. L1 detector coil is part of the sinusoidal oscillator built around transistor T1.

Normally, the center frequency of the voltage controlled oscillator (VCO) from the PLL loop that is contained in IC1 is equal to the oscillation frequency of T1. This changes when entering a metallic object (ferrous or nonferrous) in the field induced by L1. S1 is a miniature 2-pole switch. Meter needle deviation is a measure of frequency change, since the direction of deviation depends on the type of material detected by the coil. The meter tool used for this homemade metal detector is zero as central, +-50µA.

Metal detector circuit schematic

Coil L1 consists of 40 turns of enamelled copper wire, wound on a plastic template with a diameter of about 10 cm. Inductance thus obtained ensure the functioning of the oscillator at a frequency approximately equal to the VCO included in the PLL loop. Use an oscilloscope to check that pin 2 of IC1 delivers sinusoidal signal with frequency about 75 kHz.

Adjust P1 so that fronts rectangular signal from pin 4 to coincide with the peaks of the sinusoidal signal from pin 2. Then, adjust P2 in order to obtain 0 on the meter. Since the neutral zero setting “runs” with the battery’s decreasing voltage it will be necessary to restore it (zero balancing) from time to time during use of the metal detector.

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Bar Mode Lights Sequencer

Can drive up to 15 LEDs or LED-clusters, Selectable Bar-length

This circuit, designed on request, allows up to 15 LED clusters to illuminate in bar-mode sequence. LED sequencing will start at power-on and, after reaching the desired output, all the LEDs turn off and sequence restarts. The number of LEDs or clusters forming the bar can be selected by connecting R7 to the appropriate output pin of IC2 or IC3.

Parts:

R1,R5,R9_________1K 1/4W Resistors
R2______________33K 1/4W Resistor
R3_____________100K 1/2W Trimmer Cermet
R4_______________1M 1/4W Resistor
R6,R10__________10K 1/4W Resistors
R7,R8___________22K 1/4W Resistors
R11______________4K7 1/4W Resistor
R12_____________33R 1/4W Resistor (See Notes)
C1______________10µF 25V Electrolytic Capacitor
C2_____________100nF 63V Polyester Capacitor
C3_____________470µF 25V Electrolytic Capacitor
D1--D14________LEDs (See Notes)
Q1___________2N3819 General-purpose N-Channel FET
Q2,Q3,Q5______BC547 45V 100mA NPN Transistors
Q4____________BC337 45V 800mA NPN Transistor
IC1____________7555 or TS555CN CMos Timer IC
IC2,IC3________4094 8-stage shift-and-store bus register IC

Notes:

• R5 and D1 are optional: they could be of some utility in monitoring the sequence frequency set by means of R3.
• The terminal of R7 bearing an arrow must be connected to the desired output pin of IC2 or IC3 in order to select the number of LEDs or clusters forming the bar.
• For example: if you want to drive seven LEDs or clusters connect R7 to pin#11 of IC2 (Output 8) and the LED or cluster drivers to Outputs 1 to 7 respectively.
• Clusters can be formed by up to 12 LEDs as shown in the circuit diagram, right side. Common cluster types usually range from 5 to 10 LEDs.
• Up to 15 of these cluster driver circuits, each formed by the LEDs, two transistors and three resistors can be built and connected to the progressively numbered outputs of IC2 (the first eight clusters) and IC3 (the remaining clusters).
• If a number of clusters up to 7 is required, IC3 can be omitted.
• Constant output current value for the LEDs can be changed by varying R10.
• The formula is: R = 0.6/I (I expressed in Amperes).
• Wanting to drive only one LED per output instead of a cluster, the above mentioned cluster driver can be substituted by a single transistor, as shown in the circuit formed by D2, Q3, R8 and R9.

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Dimmer With A MOSFET

This circuit shows that dimmers intended for use at mains voltage do not always have to contain a triac. Here, a MOSFET (BUZ41A, 500 V/4.5A) in a diode bridge is used to control the voltage across an incandescent bulb with pulse-width modulation (PWM). A useful PWM controller can be found elsewhere in this issue. The power supply voltage for driving the gate is supplied by the voltage across the MOSFET. D6, R5 and C2 form a rectifier. R5 limits the current pulses through D6 to about 1.5 A (as a consequence it is no longer a pure peak rectifier). The voltage across C2 is regulated to a maximum value of 10 V by R3, R4, C1 and D1. An optocoupler and resistor (R2) are used for driving the gate.

R1 is intended as protection for the LED in the optocoupler. R1 also functions as a normal current limiting device so that a ‘hard’ voltage can be applied safely. The optocoupler is anold acquaintance, the CNY65, which provides class-II isolation. This ensures the safety of the regulator. The transistor in the optocoupler is connected to the positive power supply so that T1 can be brought into conduction as quickly as possible. In order to reduce switching spikes as a consequence of parasitic inductance, the value of R2 has been selected to be not too low: 22 kΩ is a compromise between inductive voltages and switching loss when going into and out of conduction.

An additional effect is that T1 will conduct a little longer than what may be expected from the PWM signal only. When the voltage across T1 reduces, the voltage across D1 remains equal to 10 V up to a duty cycle of 88 %. A higher duty cycle results in a lower voltage. At 94 % the voltage of 4.8 V proved to be just enough to cause T1 to conduct sufficiently. This value may be considered the maximum duty cycle. At this value the transistor is just about 100 % in conduction. At 230 V mains voltage, the voltage across the lamp is only 2.5 V lower, measured with a 100-W lamp. Just to be clear, note that this circuit cannot be used to control inductive loads. T1 is switched asynchronously with the mains frequency and this can cause DC current to flow.

Electronic lamps, such as the PL types, cannot be dimmed with this circuit either. These lamps use a rectifier and internally they actually operate off DC.A few remarks about the size of R3 and R4. This is a compromise between the lowest possible current consumption (when the lamp is off) and the highest possible duty cycle that is allowed. When the duty cycle is zero, the voltage across the resistors is at maximum, around 128 V with a mains voltage of 230 V. Because (depending on the actual resistor) the voltage rating of the resistor may be less than 300 V, two resistors are connected in series. The power that each resistor dissipates amounts to a maximum of 0.5W. With an eye on the life expectancy, it would be wise to use two 1-W rated resistors here.

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10 to 1000 MHz Oscillator

Nowadays, it is no longer necessary to use discrete components to build oscillators. Instead, many manufacturers provide ready-made voltage-controlled oscillator (VCO) ICs that need only a few frequency-determining external components. One example is the RF Micro Devices RF2506. This IC operates with a supply voltage between 2.7 and 3.6 V (3.3V nominal) and provides a low-noise oscillator transistor with integrated DC bias setting. In addition, it has an isolating buffer amplifier that strongly reduces the effects of load variations (load pulling) on the oscillator. If a voltage less than 0.7V is applied to the power-down input (pin 8), the oscillator is shut down and the current consumption drops from 9mA to less than 1µA. The VCO is enabled when the voltage on pin 8 is at least +3V.

Connecting the feedback capacitors C1 and C2 to pins 3 (FDBK) and 4 (VTUNE) transforms the internal transistor into a Colpitts oscillator. A resonator is also needed; here this consists of C4 and L1, and it is coupled via C3. Keep the Q factor of the coil as high as possible (by using an air-core coil, for example), to ensure a low level of phase noise. Since most applications require a tuneable oscillator, the varicap diode D1 (BBY40, BBY51, BB804 etc) can be used to adjust the resonant frequency. The tuning voltage UTune is applied via a high resistance. The value of the tuning voltage naturally depends on the desired frequency range and the variable-capacitance diode (D1) that is used. The table shows a number of suggestions for selecting the frequency-determining components. If the frequency range is narrow, a parallel-resonant circuit should be connected between the output pin and +Vcc, to form the collector load for the output transistor.

This can be built using the same components as the oscillator resonator. With a broadband VCO, use a HF choke instead, with a value of a few microhenries to a few nanohenries, depending on the frequency band. In this case C6 is not needed. The output level of this circuit is –3dBm with an LC load and –7 dBm with a choke load. The table that accompanies the schematic diagram provides rough indications of component values for various frequencies. It is intended to provide a starting point for experimentation. The coupling between the variable-capacitance diode and C5 determines the tuning range of the VCO. The manufacturer maintains an Internet site at www.rfmd.com, where you can find more information about this interesting oscillator IC.

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Video Tracer For Trouble Shooting

This circuit was designed as an aid to installers and maintainers of video systems. It is basically a video sync separator (IC1) followed by a LED and buzzer driver (IC2, Q1 & Q2). In use, the device is connected to a video cable and if there is video present, the LED will flash at about 10Hz. If there is no video, the LED flashes briefly every couple of seconds. A buzzer can also be switched in to provide an audible indication. The buzzer is particularly useful when tracing cabling faults or trying to find a correct cable amongst many, where it is difficult to keep an eye on the LED.

Another use for the buzzer option is to provide a video fault indication. For example, it could be inserted in bridging mode, with switch S1 in high impedance mode (position 2) across a video line and set to alarm when there is no video present. If someone pulls out a cable or the video source is powered off, the alarm would sound. IC1 is a standard LM1881 video sync separator circuit and 75Ω termination can be switched in or out with switch S1a. The other pole of the switch, S1b, turns on the power. The composite sync output at pin 1 is low with no video input and it pulses high when composite sync is detected.

These pulses charge a 100nF capacitor via diode D1. When there is no video at the input, oscillator IC2b is enabled and provides a short pulse every couple of seconds to flash the LED. The duty cycle is altered by including D2, so that the discharge time for the 10μF capacitor is much shorter than the charge time. The short LED pulse is used as a power-on indicator drawing minimal average current. When video is present at the input, IC2b is disabled and IC2d is enabled. The output of IC2d provides a 10Hz square wave signal to flash the LED. The buzzer is controlled by switch S2. In position 2 the buzzer will sound when there is video at the input and in position 1 the buzzer will sound when there is no video at the input.

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CAPACITANCE METER ELECTRONIC DIAGRAM

The Capacitance Meter Circuit, However, when finished, you will have an instrument capable of measuring all but the largest capacitors used in radio circuits. Unlike variable resistors, most variable capacitors are not marked with their values. As well, the markings of capacitors from salvaged equipment often rub off.

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Step Up Booster Powers Eight White LEDs

Tiny white LEDs are capable of delivering ample white light without the fragility problems and costs associated with fluorescent backlights. They do pose a problem however in that their forward voltage can be as high as 4 V, precluding them being from powered directly from a single Li-Ion cell. Applications requiring more white LEDs or higher efficiency can use an LT1615 boost converter to drive a series connected array of LEDs. The high efficiency circuit (about 80%) shown here can provide a constant-current drive for up to eight LEDs. Driving eight white LEDs in series requires at least 29 V at the output and this is possible thanks to the internal 36-V, 350-mA switch in the LT1615.

The constant-current design of the circuit guarantees a steady current through all LEDs, regardless of the forward voltage differences between them. Although this circuit was designed to operate from a single Li-Ion battery (2.5V to 4.5V), the LT1615 is also capable of operating from inputs as low as 1 V with relevant output power reductions. The Motorola MBR0520 surface mount Schottky diode (0.5 A 20 V) is a good choice for D1 if the output voltage does not exceed 20 V. In this application however, it is better to use a diode that can withstand higher voltages like the MBR0540 (0.5 A, 40 V). Schottky diodes, with their low forward voltage drop and fast switching speed, are the best match.

Many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35 A. Inductor L1, a 4.7-µH choke, is available from Murata, Sumida, Coilcraft, etc. In order to maintain the constant off-time (0.4 ms) control scheme of the LT1615, the on-chip power switch is turned off only after the 350-mA (or 100-mA for the LT1615-1) current limit is reached. There is a 100-ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. This current overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values.

This will be the peak current passed by the inductor (and the diode) during normal operation. Although it is internally current-limited to 350 mA, the power switch of the LT1615 can handle larger currents without problems, but the overall efficiency will suffer. Best results will be o btained when IPEAK is kept well below 700 mA for the LT1615.The LT1615 uses a constant off-time control scheme to provide high efficiencies over a wide range of output current. The LT1615 also contains circuitry to provide protection during start-up and under short-circuit conditions.

When the FB pin voltage is at less than approximately 600 mV, the switch off-time is increased to 1.5 ms and the current limit is reduced to around 250 mA (i.e., 70% of its normal value). This reduces the average inductor current and helps minimize the power dissipation in the LT1615 power switch and in the external inductor L1 and diode D1. The output current is determined by Vref/R1, in this case, 1.23V/68 = 18 mA). Further information on the LT1615 may be found in the device datasheets which may be downloaded from www.linear-tech.com/pdf/16151fa.pdf

Author: D. Prabakaran Copyright: Elektor Electronics

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1W BTL Audio Amplifier Circuit Diagram

The TDA8581(T) from Philips Semiconductors is a 1-watt Bridge Tied Load (BTL) audio power amplifier capable of delivering 1 watt output power into an 8-Wload at THD (total harmonic distortion) of 10% and using a 5V power supply.

The schematic shown here combines the functional diagram of the TDA8551 with its typical application circuit. The gain of the amplifier can be set by the digital volume control input. At the highest volume setting, the gain is 20 dB. Using the MODE pin the device can be switched to one of three modes: standby (MODE level between Vp and Vp–0.5 V), muted (MODE level between 1 V and Vp–1.4 V) or normal (MODE level less than 0.5 V). The TDA8551 is protected by an internal thermal shutdown protection mechanism. The total voltage loss for both MOS transistors in the complementary output stage is less than 1 V.

Circuit diagram:

1 Watt BTL Audio Amplifier Circuit Diagram

Using a 5-V supply and an 8-W loudspeaker, an output power of 1 watt can be delivered. The volume control has an attenuation range of between 0 dB and 80 dB in 64 steps set by the 3-state level at the UP/DOWN pin: floating: volume remains unchanged; negative pulses: decrease volume; positive pulses: increase volume Each pulse at he Up/DOWN pin causes a change in gain of 80/64 = 1.25 dB (typical value).

When the supply voltage is first connected, the attenuator is set to 40 dB (low volume), so the gain of the total amplifier is then –20 dB. Some positive pulses have to be applied to the UP/DOWN pin to achieve listening volume. The graph shows the THD as a function of output power. The maximum quiescent current consumption of the amplifier is specified at 10 mA, to which should be added the current resulting from the output offset voltage divided by the load impedance.

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Soldering Iron Tip Preserver

Although 60/40 solder melts at about 200°C, the tip temperature of a soldering iron should be at about 370°C. This is necessary to make a good quick joint, without the risk of overheating delicate components because the iron has to be kept on the joint for too long. Unfortunately, at this temperature, the tip oxidises rapidly and needs constant cleaning. Thats where this circuit can help - it keeps the soldering tip to just below 200°C while the iron is at rest.

Oxidisation is then negligible and the iron can be brought back up to soldering temperature in just a few seconds when needed. In addition, normal soldering operation, where the iron is returned to rest only momentarily, is unaffected because of the thermal inertia of the iron. Two 555 timers (IC1 & IC2) form the heart of the circuit. IC1 is wired as a monostable and provides an initial warm-up time of about 45 seconds to bring the iron up to temperature. At the end of this period, its pin 3 output switches high and IC2 (which is wired in astable configuration) switches the iron on - via relay RLY1 - for about one second in six to maintain the standby temperature.

The presence of the iron in its stand is sensed by electrical contact between the two and some slight modification of the stand may be necessary to achieve this. When the iron is at rest, Q1s base is pulled low and so Q1 is off. Conversely, when the iron is out of its stand, Q1 turns on and pulls pins 2 & 6 of IC2 high, to inhibit its operation. During this time, pin 3 of IC2 is low and so the iron is continuously powered via RLY1s normally closed (NC) contacts. Note that the particular soldering iron that the circuit was designed for has its own 24V supply transformer. Other irons may need different power supply arrangements. The warm-up time and standby temperature can be varied by altering R2 and R5, as necessary.

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Power Buzzer

How often on average do you have to call members of your family each day to tell them that dinner is ready, it’s time to leave, and the like? The person you want is usually in a different room, such as the hobby room or bedroom. A powerful buzzer in the room, combined with a pushbutton at the bottom of the stairs or in the kitchen, could be very handy in such situations. The heart of this circuit is formed by IC1, a TDA2030. This IC has built-in thermal protection, so it’s not likely to quickly give up the ghost. R1 and R2 apply a voltage equal to half the supply voltage to the plus input of the opamp. R3 provides positive feedback. Finally, the combination of C2, R4 and trimmer P12 determines the oscillation frequency of the circuit.

The frequency of the tone can also be adjusted using P1. There is no volume control, since you always want to get attention when you press push-button S1. Fit the entire circuit where you want to have the push-button. The loudspeaker can then be placed in a strategic location, such as in the bedroom or wherever is appropriate. Use speaker cable to connect the loudspeaker. Normal bell wire can cause a significant power loss if the loudspeaker is relatively far away. The loudspeaker must be able to handle a continuous power of at least 6 W (with a 20-V supply voltage).

The power quickly drops as the supply voltage decreases (P = Urms 2 / RL). The power supply for this circuit is not particularly critical. However, it must be able to provide sufficient current. A good nominal value is around 400 mA at 20 V. At 4 V, it will be approximately 25 mA. Most likely, you can find a suitable power supply somewhere in your hobby room. Otherwise, you can certainly find a low-cost power supply design in our circuits archive that will fill the bill!

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SAFELY DISCHARGE X CAPACITORS ELECTRONIC DIAGRAM

SAFELY DISCHARGE X CAPACITORS ELECTRONIC DIAGRAM

When the AC voltage is disconnected, CAPZero automatically and safely discharges the X capacitor by closing the circuit through the bleed resistors and directing the energy away from the exposed AC plug. This approach provides engineers with total flexibility in their choice of the X capacitor used to optimize differential- mode EMI filtering without worrying about the effect of the required bleed resistors on system no-load and standby power budget. The innovative design inherently meets international safety standards for all open and short-circuit fault tests, allowing CAPZero to be used before or after the system input fuse. CAPZero is suitable for all AC-DC converters with X capacitors that require very low standby power. It’s offered with 825- or 1,000-V MOSFETs to support a variety of power supply design needs. It is ideal for a wide range of applications, including PCs, servers/workstations, monitors and TVs, printers and notebooks, and appliances requiring EuP Lot 6 compliance and adapters requiring ultra-low no-load consumption. CAPZero devices are available now in an SO-8 package at $0.40 each for 10,000- piece quantities. [www.powerint.com]

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5V DC REGULATED PHONE CHARGER ELECTRONIC DIAGRAM

5V DC REGULATED PHONE CHARGER ELECTRONIC DIAGRAM

Regulated phone charger which is used as an emergency charger for mobile phones with source from ordinary batteries, and works with 1.5V input DC voltage. At 5V, it can provide output to 70mA. If the current is drawn, the voltage will be drop. A006 microcontroller is used to create square wave which used to drive the Field Effect Transistor BBV93.

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Flickering Light II

Regardless of whether you want to effectively imitate a house fire, a campfire, or light from welding, the circuit described here fills the bill without using a microcontroller, although it does use a larger number of components (including some truly uncommon ones). The circuit is based on three oscillators, which are built using unijunction transistors (UJTs). Each oscillator has a different frequency. The output voltages are mixed, which produces the flickering effect. A unijunction transistor consists of an n-type bar of silicon between two ohmic (non-barrier) base contacts (B1 and B2). The effective resistance is controlled by the p-type emitter region. The designation ‘transistor’ is a somewhat unfortunate choice, since it cannot be used for linear amplification.

UJTs are suitable for use as pulse generators, monostable multivibrators, trigger elements and pulse-width modulators. If a positive voltage is applied to the emitter (E), the capacitor charges via the resistor. As soon as the voltage on the emitter reaches approximately half the supply voltage (for a 2N2645, the value lies in the range of 56–75 %), the UJT ‘fires’ and the capacitor discharges via base B1 and the resistor, generating a positive pulse. The UJT then returns to the non-conduct state, and the process just described repeats periodically. The frequency can be approximately given by the formula f ˜ 1/(RC) The frequency is independent of the value of the supply voltage (which must not exceed 35 V).

The maximum emitter blocking voltage is 30 V, and the maximum permissible emitter current is 50 mA. The values of resistors R1, R2 and R3 can lie between 3 k? and 500 k?. If necessary, the frequency can be varied over a range of 100:1 by using a trimpot instead of a fixed resistor. The frequencies from the three pulse generators are mixed by connecting them to the IR diode of a triac optocoupler via R4. The optocoupler, a type MOC3020, K3030P or MCP3020, can handle a maximum load current of 100mA. The triac triggers at irregular intervals and generates the desired flickering light in the two small lamps, L1 and L2, which are connected in series to the transformer secondary.
The light effect can be noticeably improved by using a MOC3040, which contains a zero-voltage switch, since its generates irregular pauses of various lengths when suitable frequencies occur in the individual oscillators. The zero-voltage switch does not switch while the current is flowing, but only when the applied ac voltage passes through zero. An integrated drive circuit (zero crossing unit) allows full half-waves or full cycles to pass (pulse-burst control) Due to the flickering effect arising from its switching behaviour, it is not suitable for normal lighting, but here this just what we want. This version of the optocoupler is also designed for a maximum current of 100 mA.

For a small roof fire or the light of a welding torch in a workshop, two small incandescent lamps connected in series and rated at 6 V / 0.6 A (bicycle taillight bulbs) or a single 12-V lamp (rated at 100 mA) is adequate. If it is desired to simulate a large fire, a triac (TIC206D, rated at 400 V / 4 A, with a trigger current of 5 mA) can be connected to the output of the circuit and used to control a more powerful incandescent lamp. As continuous flickering looses its attraction for an interested observer after a while (since no house burns for ever, and welders also take breaks), it’s a good idea to vary the on and off times of the circuit. This is handled by a bipolar Hall switch (TLE4935L), which has such a small package that it can fitted between the sleepers of all model railway gauges, including Miniclub (Z Gauge), or even placed alongside the track if a strong permanent magnet is used.

If a magnet is fixed somewhere on the base of a locomotive such that the south pole points toward the package of the Hall switch (the flattened front face with the type marking), the integrated npn transistor will switch on and pull the base of the external pnp transistor negative, causing the collector–emitter junction to conduct and provide the necessary ‘juice’ for the unijunction transistors. If another traction unit whose magnet has it s north pole pointing toward the Hall switch passes a while later, the switch will be cut off and the flickering light will go out. Of course, you can also do without this form of triggering and operate the device manually.

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A 12V Car Charger For ASUS Eee Notebook

The ASUS Eee is a fantastic ultra-portable notebook with almost everything required for geeks (and nothing that isn’t). Plus it features fantastic build quality and is very well priced. If you live in New Zealand you can get them from DSE; at the time of writing they are the exclusive supplier. I worked out it’s the same cost as importing one once you include all the duties and tax, plus you get the advantage of a proper NZ-style mains charger. Anyway, being so small I thought it would be nice to be able to carry this around in the car. Unfortunately I couldn’t find a car charger available anywhere at the time so I decided to tackle the problem myself. As a bonus this provides an opportunity for an external high-capacity battery.

Commercial Equivalent:

I thought at this stage it would be worth noting that a commercial car charger is now available for less than it cost me to build this from Expansys and is available in most countries (select your location on their site). It outputs 9.5v from 10-18v in at up to 2.5A. I’d actually recommend it over the design here is it seems to perform better at lower voltages (that one works down to 10V). However I have kept this page up as a reference for those who enjoy tinkering.

Design:

The charger included with the Eee is rated at 9.5v, 2.315A. There isn’t a fixed voltage regulator available for this exact voltage, so the circuit needed to be designed around an adjustable regulator. I decided to design the charger around the LM2576 “Simple Switcher” IC from National Semiconductor. There are tons of ICs like this available, many of which are a bit more efficient, however I selected this one because it is readily available and relatively cheap. It also has a lower drop-out voltage (~2V) than many other chips I looked at which is important when powering the device from a car or 12v SLA battery.

This circuit could have used a standard three pin regulator IC such as the LM317, however most types require an external transistor when handling so much current and not to mention the fact that they are very inefficient; they draw the same amount of current from the input as the load and the difference in power is dissipated as heat. The main problem with using the LM2576 is the fact it needs quite a large inductor due to its somewhat low switching frequency. The inductor I used is made by Pulse Engineering, part number PE92108KNL. I’d prefer a smaller one, however I couldn’t find one capable of supplying the required current that I could purchase in single units. Besides the PE92108KNL is apparently designed specifically to work with the LM257x series.

The circuit also includes a low voltage cut-out based on a 9.1v Zener diode and BC337 transistor that will shut down the regulator if the input voltage is below 11.5V. This prevents unstable operation of the regulator at lower input voltages, and also helps prevent accidental flattening of the supply battery. Substituting this transistor for similar type may affect the cut-out voltage; the Vbe of the transistor should be 1.2v.All of the components used should be pretty readily available in most areas. I got everything from Farnell. Jaycar also sells everything except the inductor. Make sure you specify high temperature, low ESR capacitors as these help result in more stable operation and better efficiency of the charger.

Unfortunately the end result is a charger that is slightly bulkier than I would really like. I attempted to fit this inside an old mobile phone charger case so the whole thing could hang out of the cigarette lighter, however I ran into trouble making the circuit stable enough and dissipating all the heat. Due to the high current involved compared to a mobile phone charger the components are much bulkier so it’s pretty tricky to get all to fit! If I do get it finished I’ll add an update.

Parts List:

• 2x 10k resistor (R1 & R4)
• 2x 22k resistor (R2 & R3)
• 1x 1.5k resistor (R5)
• 1x 120μF 25v electrolytic capacitor (C1)
• 1x 2200μF 16v electrolytic capacitor (C2)
• 1x 1N5822 Schottky diode (or equivalent)
• 1x 9.1v 0.5W Zener diode
• 1x BC337 NPN transistor
• 1x LM2576T-ADJ IC
• 1x 100uH, 3A inductor (e.g. Pulse PE92108KNL)
• 25°C/W or better minature heatsink (e.g. Thermalloy 6073)
• Cigarette lighter plug with 3A fuse and 2.1mm DC plug (e.g. DSE P1692)
• 2.1mm DC chassis mount socket
• 1.7mm x 4.75mm (ID x OD) DC plug and cable
• Small plastic enclosure

Building It:

Make yourself a PCB using the template below (600dpi). I simply laser print (or photocopy) the design onto OHP transparency sheet and then transfer the toner onto a blank PCB using a standard clothes iron. Any missing spots can be touched up with a permanent marker before etching. This is quick, usually results in pretty tidy boards and hardly costs a thing. There is a tutorial on a variation of this method at http://max8888.orcon.net.nz/pcbs.htm.

Install the components on the PCB and triple check the layout before soldering. It is much easier to start with the low profile components such as resistors and diodes, then install the larger components after-wards. Don’t forget the wire link; this is shows as a red line on the layout guide above. Remember to smear a small amount of heatsink compound on the regulator tab before mounting the heatsink.

For a case I used a small plastic enclosure from DSE, part H2840, as it was all the local store had in stock that was remotely suitable. The PCB is designed to fit into this particular case, however any small box should be suitable. If you have a dead laptop charger lying about it might be worth ripping the guts out of that and salvaging the case. If your enclosure is different you may need to modify the design to suit, so I have provided the schematic and PCB design files for download. They were created using Eagle. The Eee uses a standard 1.7mm DC power connector with a positive tip.

Testing:

Connect the circuit to a 12v supply. If you use a car or lead acid battery ensure you have a 3A fuse fitted in line with the circuit before connecting it, just in case. Use your multimeter to check that the circuit outputs about 9.45v with no load. Connect a 12V, 21W lamp (e.g. old brake lamp from a car) or similar load across the output and check that the voltage doesn’t vary much. You should now be able to connect your Eee. The circuit design should be good for up to 2.5A, so there is plenty of margin for the Eee to fully function and charge its own battery off this supply.

    

SLA Battery Carry-bag:

Jaycar have a really cool carry bag with a shoulder strap designed to perfectly fit a 12v 7AH sealed lead acid battery. The bag features a fused cigarette lighter socket and is the perfect compliment to this charger. It works well with the Eee and provides hours of extra use. The shoulder strap means it’s not too bothersome to carry about and the charger circuit itself zips up neatly inside the bag. The under-voltage cut-off means the battery will never run completely flat, and the Eee will simply cut over to its internal battery once the SLA runs out. I got my SLA battery from Rexel as they are much cheaper (approx NZ$18 including GST last time I bought one) and they don’t sit as long on the shelf as many other suppliers.

Disclaimer:

This circuit is intended for people who have had experience in constructing electronic projects before. The circuit design and build process are provided simply as a reference for other people to use and I take no responsibility for how they are used. If you proceed with building and/or using this design you do so entirely at your own risk. You are free to use the content on this page as you wish, however I do ask that you include a link or reference back to this page if you distribute or publish any of the content to others.

 Source: Marlborough Wi-Fi

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Automatic Air Humidifier

The humidifier circuit is based on a special humidity sensor Type NH-3 from Figaro. Depending on the sensor output, the circuit drives a ventilator that is part of an air humidifying installation. The ventilator is switched on an off by a triac. So as to keep the circuit as simple as possible, the supply voltage and the test voltage are drawn directly from the mains supply. The 240 V mains voltage is converted into an 8.9 V pulsating direct potential by capacitor C1, resistor R1 and zener diode D1. The pulsating voltage is used to drive the sensor.

It is also transformed to a 7.5 V supply voltage by D2 and C2. The sensor needs an alternating drive voltage at a level not higher than 1.5 V. This potential is obtained from the pulsating direct voltage by network R2-R3-C3-C4, which removes the direct voltage component and lowers the level to 1.4 V. At the same time, the network functions as a 50 Hz bandpass filter. To ensure that the drive voltage for the sensor does not fall outside the common-mode range of op amp IC2, an offset potential of 3.9 V is applied to the sensor as well as to the voltage reference source of the op amp.
This potential is provided by zener diode D3. The reference level is set with P1.The op amp is given some hysteresis by R5. When the humidity of the ambient air rises above that corresponding to the level with P1, the output voltage of IC2 is about 6 V. This results in T1 being cut off by D4, whereupon the triac is also disabled. When the humidity drops below that corresponding to the level set with P1, a pulsating potential appears at the output of IC2. This voltage is used to charge capacitor C6.

The charged capacitor thereupon provides a steady current to the triac. When T1 is cut off for some time, capacitor C6 is discharged via resistor R7. Capacitors C1 and C7 are discharged via R9, so that after the mains has been switched off, no dangerous potential remains at the pins of the mains connector (K1). The humidifier is best built on the PCB shown in Figure 2, which is available ready made (see Readers services pages towards the end of this issue). Bear in mind that parts of the board will carry mains voltage, which makes careful working and the enclosing of the board in a plastic case imperative. The humidifier may be converted into a dehumidifier by interchanging connections 1 and 3 to sensor IC1.

Parts list

Resistors:
R1 = 470 Ω, 1 W
R2, R3 = 10 kΩ
R4 = 1 kΩ
R5 = 56 kΩ
R6 = 6.8 kΩ
R7 = 4.7 kΩ
R8 = 470 Ω
R9 = 2.2 MΩ
R10 = 39 Ω, 1 W
P1 = 1 kΩ preset
Capacitors:
C1 = 0.47 µF, 250 V a.c.
C2 = 470 µF, 16 V, radial
C3, C4 = 0.33 µF, metallized polyester, 5%
C5 = 0.1 µF, high stability
C6 = 47 µF, 16 V, radial
C7 = 0.047 µF, 250 V a.c.
Semiconductors:
D1 = zener diode, 8.2 V, 1.3 W
D2 = 1N4001
D3 = zener diode, 3.9 V, 500 mW
D4 = zener diode, 2.4 V, 500 mW
T1 = BC557B
Integrated circuits:
IC1 = NH-3 (Figaro)
IC2 = TLC271CP Tri
1 = TLC336T (SGS)
Miscellaneous:
K1, K2 = 2-way terminal block for board mounting, pitch 7.5 mm
F1 = fuse-holder with 630 mA slow fuse

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