Although ICs 741 and timer 555 are rare and expensive these ICs are frequently used ICs even by an average hobbyist and electronics students. They are very versatile and damage resistant too. Therefore we have designed the IC tester for timer IC 555 And Op-Amp IC 741
The tester described here test both the ICs [Timer 555 and Op-Amp 741] instantly. The circuit here uses only a few resistors, switches, sockets and capacitors which cost around $3 including the PCB and the 7-segment display. The tester we design is equally useful for a factory and the student of electronics.
|| 5mm LEDs [Light Emitting Diode]
||FND507/LT542 Common Anode Display
Resistors (all 0.25 watt 5% carbon)
|R5 TO R12
||5μF, 16V electrolytic
||47μF, 10V electrolytic
||On/Off toggle Switch
||DPDT miniature switch for mounting on PCB
||IC socket, PCB, connecting wires, enclosure, hardware etc
Form the circuit diagram it is clear that the heart of this tester is the astable multi vibrator formed around 1C 555 which is approximately 2 Hz frequency. So, the output LED D1 (connected through link a and b snow in the circuit) blinks at a rate of 2 Hz if the IC 555 inserted in the socket works well.This output at pin 3 of IC 555 is also given through a DPDT (Double Pole Double Throw switch) to the pins2 and pin3 of IC 741 op-amp. The switch S2 selects the inverting or non-inverting input of the IC 741 . In this way the LED D2 ( connected through link c and d snow in the circuit) also blinks at of 2Hz rate if the IC 741 inserted is OK. If S2 selects inverting mode, then D1 and D2 blink alternately and if S2 selects non-inverting mode then D1 and D2 blink simultaneously. You may use 3V to 12V DC supply voltage safely. However, 9V DC is recommended.
More About circuit
As an interesting feature which gives a digital display without any confusion, you may use a common anode seven segment display. With op-amp IC 741 in inverting mode, the display shows 7 and 5 alternately. In the non-inverting mode it shows 9 and 0 alternately.
Common Anode display Configuration for connecting Display with circuit ( Please connect the A , C and B or D to the corresponding point in the circuit.:
Both, the two LEDs and digital display options are given circuit diagram. You can also use a common anode display having a damaged decimal dot and / or segment ‘e’ which are not used here
Common Cathode display Configuration for connecting Display with circuit ( Please connect the A and C to the corresponding point in the circuit.:
You may also use a common cathode display here by just interchanging the connections of common cathode from positive to the negative of the supply on the PCB near the switch S2. Similarly, interchange the connections of R7 and R8 from negative to positive of the supply.
It has been noted that when both IC 741 and 555 are faulty or in non-operating condition, the display shows 7 (but there is no change in the display while changing the switch S2), and when IC 555 is working and 741 is in non-operating condition the display shows 5 and 7 alternately and when IC 555 is not working but 741 is alright, then display shows 7 in inverting mode and 7 in non-inverting mode, but does not flash.
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