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see TALKING ELECTRONICS WEBSITE email Colin Mitchell: talking@tpg. com. au 70 Interesting Circuits as: zip (1. 9MB) or . doc (1. 7MB) or . pdf (1. 1 5MB) (14-9-2010) For our other free eBooks: 50 – 555 Circuits 1 – 100 Transistor Circuits 101 – 200 Transistor Circuits 100 IC Circuits For a list of every electronic symbol, see: Circuit Symbols. For more articles an ELECTRONICS WEBSI NTRODUCTION This e-book covers a circuits. bYiSt: see TALKING OF17 p They have been presented for a reason. The original circuits come from an Indian Electronics Magazine and most of them had faults.

Either they were ver-designed, poorly designed or contained a fault. In the process of bringing these faults and corrections to you, we have created this egook of70 Interesting Circuits. You can lean a Iot from other peoples mistakes. Not only Will you remember not to make the same mistake but the corrections and improvements generally require less components. The art of designing a circuit is to make it as simple as possible and use the least number of components. But before you put a design into production, get someone else to look at it. f faulty circuits and how to check and test things before releasing them. It is surprising that a magazine with a readership of over 1 could publish items with glaring faults. The faulty projects in this collection have a rewsed circuit included in the article and you can compare the two designs. It only takes a minute to see a circuit Will not work and a few more minutes to create a improved design. This is What we are teachlng in this section … how to look at a circuit with a “critical eye” then design an improvement.

This is called REVERSE ENGINEERING and involves the highest level of technical skill. But it is also the fastest way to test a persons skill and the fastest way to learn. That’s why I consider it to be the most important approach to learning. Colin Mitchell TALKING ELECTRONICS. talking@tpg. com. au CONTENTS AC Detector Auto Cutout Battery Monitor Capacitance Beeper Constant Current Source Door Watcher Electronic Siren 555 Schmitt 10 Output LED Sequencer 12v Lamp Dimmer 16 LED Night Rider Ultrasonic Remate Control Emergencv Light 20F -a 555 and decade counter CD401 7.

The timer IC NE555 is wired as an astable multivibrator that produces 6Hz clock at its output pin 3. The 4017 is a CMOS decade counter with 10 outputs. Inputs include a CLOCK (Pi RESET (Pin 15), and a CLOCK INHIBIT (Pin 13). The ciock input connects to a Schmitt trigger for pulse shaping and allo clock rise and fall times (not needed in our case). The counter advances one output at the rising edge of the clock Signal the CLOCK INHIBIT line is low. A high RESET s resets the counter to the zero output. The circuit may be configured for counts less than 10 by connecting RESET to an o (one after the desired count).

Thus, a five stage sequencer can be made by connecting pin 15 to pin 1. A CARRY-OUT si 12) can be used to clock subsequent stages in a multi-device counting chain. The output from IC2 pin 3 is connected to clock pin (pin 14) of the IC3 for sequencing operations. NPN transistors QI – Q used to increase the output current for the LEDS which is set by the common 150 ohm resistor. In the circuit, only one oft outputs is HIGH at any one time and the output advances by one count With every clock pulse. But the circuit above is poorly designed.

It does not need the voltage regulator as both chips can work up to 15v. The 4017 can supply 10mA to a LED on a 12v supply so that none of the transistors are needed. The circuit below shows the necessary components. The secret to designing a circuit is to look at the circuit below shows the necessary components. The secret to designing a circuit is to look at the final design and ask. “is this component necessary? ‘ Try removing a component and see if the circuit still works. Keep doing this with all the components. The circuit above wa published in an Indian magazine with over 1 readers.

The faults were obvious. How these faults passed an edito committee is beyond me. They are showing very poor design- leadership in allowing this oversight to be published. The fa technical but are obvious to anyone who has constructed the circuit and experimented with it. Obviously the circuit has ne assembled with anyone with technical expertise. to Index 2v LAMP DIMMER Here is a 12 volt @ 2 amp lamp dimmer that can be used to dim a standard 25 watt bulb by controlling the duty cycle of a astable 555 timer oscillator.

When the potentiometer is at the up position, the capacitor Will charge quickly through both Ik resistors and the diode, producing a short positive Interval and long negative interval which dims the lamp to near darkness. When the potentiometer is at the lower position, the capacitor Will charge through both Ik resistors and the 50k potentiometer and discharge through the Iower Ik resistor, producing a long positive interval and short negative interval which brightens the amp to near full intensity.

The duty cycle of the 200Hz square wave can be varied from approximately 5% to 95%. The two 40F intensity. The duty cycle of the 200Hz square circuits below show how to connect the lamp to either the positive or negative side of the supply. But the first circuit has a mistake and some components are not needed. The 555 Will sink 300mA and it can be connected directly the output transistor – you don’t need the buffer transistor. When the 555 goes HIGH, the voltage on Pin 3 is 1 . 5v lower than the 12v rail and thus the transistor does not turn off.

The two diodes in the ircuit below are needed to drop an additional lv so the transistor turns off. 16-LED NIGHT RIDER The bi-directional sequencer uses a 4 bit binary up/down counter (CD451 6) and two “1 of 8 line decoders” (74HC138 or 74HCT138) to generate the popular Night Rider display. A Schmitt Trigger oscillator provides the clock signal for the counter and the rate can be adjusted with the 500k pot. Two additional Schmitt Trigger inverters are used as a SET/RESET latch to control the counting direction (up or down).

Be sure to use the 74HC14 and not the 74HCT14, the 74HCT14 may not work due to the low TTL input trigger level. When the ighest count is reached (1 1 1 1) the low output at pin 7 sets the latch so that the UPIDOWN input to the counter goes low and causes the counter to begn decrementing. When the lowest count is reached (0000) the latch is reset (high) so that the counter Will begin incremen reached (0000) the latch is reset (high) so that the counter Will begin incrementing on the next rising clock edge.

The three lowest counter bits (QO, QI, Q2) are connected to both decoders in parallel and the highest bit Q3 is used to select the appropriate decoder. 555 SCHMIIT The popular NE555 is used as a Schmitt trigger by connecting Pin 2 and hreshold pin 6 inputs. As the light falling on the LDR falls below a preset value the relay energises. This happens when the voltage at pin 2,6 is greater than 2/3 of Vcc. When light increases, the voltage at pin 2,6 falls and at 1/3 ofVcc the relay is deenergised. This gives a Hysteresis range.

Diode IN 4001 is necessary for the safety of the IC. CONSTANT CORRENT SOURCE n the following circuit an LED is used to give a fixed reference voltage to a transistor- The output constant current out is given by: The LED lights up only when a load is connected at the output. Thus it indicates when the circuit is operating. The operation of the circuit can be made clearer by re-arranging components as follows: he output Will be limited 6 OF usine a red LED and IOR line currents of about 250mA or more without making any electrical connections to the line.

Current is detected by passing on ofthe AC lines through an inductive pickup (LI) made witha 1 inch diameter U-bolt wound with 800 turns of magnet wire. The pickup can be made from other iron type rings or transformer cores that allows enough space to pass one ofthe AC lines through the center. Only one of the current carrying lines, either the line or he neutral should be put through the center of the pickup to avoid the fields cancelling. This is most important is very d fficult to achieve. The best method is to make a short extension cord with the three conductors separated from each other. you make a 3-turn loop with say the active line, and pass a straight rod such as a metal bolt, containing 400 or more turns through the centre ofthe 3-turns, you Will produce a very sensitive pick-up. The magnetic pickup produces about 4 millivolts for AC line current of 250mA, or AC load of around 30 watts. The signal from the pickup is increased about 200 times at the output of the op-amp pin 7 hich is then peak detected by the capacitor and diode connected to pin 7. The second op-amp is used as a comparator which detects a voltage rise greater than the diode drop.

The minimum signal needed to cause the comparator stage output to switch positive is around 800mV which corresponds to about a 30 watt load on the AC line. The output of the 1458 op-amp wi 800mV which the 1458 op-amp Will only swing within a couple volts of ground so a voltage divider (1 k/470) is used to reduce the no signal voltage to about 0. 7 volts. An additional diode is added in series with the transistor base to ensure it turns ff when the op-amp voltage is 2 volts. You may get a little bit of relay chatter ifthe AC load is close to the switching point so a larger load of 50 watts or more is recommended.

The sensitivity can be increased by adding more turns to the pickup. AUTO CUTOUT A 12v relay is connected across the 12v supply. When the output is shorted, the 12v falls to Ov and the relay drops-out. The contacts open the 12v is reapplied to the relay and it Will “chatter” ifthe short is not removed. This circuit Will simply not work and the relay Will simply become a “Buzzer. ‘ In the following circuit, the transistor Will only turn on ifthe utput voltage is above 0. 6v. DOOR WATCHER Three reed switches are at the heart of the circuit, one fitted to each doar.

They close when a doar is opened. An associated LED li hts when a door is opened. The remainder of the circu by either Dl, 02 or 03. 80F requires either pin 1 or 4 go high and this happens when a door opens. Because the high on each pin is only momentary (i. e. about 1/3 second, while Cl ,C2 or C3 is charging) there is only a short burst of buzzer activity (two brief beeps) at each doar opening, after which it goes mute again. So the beep calls attention to the fact that a door has opened and he LED indicates which door, staying lit until [i door is closed.

If another door opens before the first door is closed, there is another beep and another LED lights. The circuit above is too complex. It is very poorly designed. The 3 signals diodes are doing NOTHING. The are simply across each other!! One diode could be placed in the supply line to the 555 if it is needed for the reset line to work correctly. R7 is not needed as the output is taken to the pins 2&6 and the 74LS32 chip can be replaced by 3 x 470u electrolytics. It can be simplified to this: EM ERGENCY LIGHT Here is a circuit of an emergency light. As long as the power supply is present, transistor QI conducts.

Since the base ofthe transistor Q2 is connected to the collector of QI, transistor Q2 and Q3 do not conduct and hence the lamp remains off. LED glows as long as the supply is present- When the power supply fails the base drive to QI disappears. Thus QI stops to battery voltage and starts conducting, switching on the lamp instantly. The load current is supplied by the battery. Whenever the power supply is restored, QI starts conducting turning Q2 & Q3 off and the lamp is switched off Transistor Q2 conducts and provides sufficient base drive to transistor Q3. The circuit above is too complex.

The first diode is not needed and the rest of the circuit can be re-arranged. The 2R2 Will overcharge the battery and dry it out in a few months. The ICOR gives 40mA charging with a 12v batteo,’ and 12v DC plug pack. NCREASING THE INPUT IMPEDANCE The input impedance of a low cost analogue multimeter can be improved using this circuit- The approximate impedance increase Will be about 250. The LED provides a fixed reference voltage for zero setting of the multimeter via VRI. BATTERY MONITOR This circuit makes it possible to monitor the charging process of a batterv. After 0