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Continuity Tester Circuit using IC 555

In this project we are going to make a simple continuity checker circuit using 555 timer.  And what makes this circuit so special is that no transistor is used in it and hence this is indeed the simplest continuity checker.

By Ankit Negi

We all know the importance of 555 TIMER in electronics.

The fact that they are used even today, 45 years after their first appearance in electronics industry makes it a key component of our day to day circuit.

There’s hardly anything this 555 timer cannot do for you. From using it as a clock generator to voltage regulator. And so here we are, making yet another very useful circuit using this invincible IC.


A continuity checker is a simple electronic tool that checks the continuity between two terminals of a circuit. For let’s say you have a wire, which you want to check for continuity.

So you have to  just connect its two terminal to the continuity checker and if there’s no break in the circuit it will indicate it( either by a glowing led or buzzer) and if there’s  break than nothing will happen.


1. A 555 timer

555 timer pinout diagram

2. One buzzer ( **if you do not have buzzer then use LED)

PCB mount piezo buzzer

3. 9v battery

9V PP3 battery

4. One 4.7 k resistor

4.7K resistor MFR 1%

5. One 47 k resistor

47K resistor 1/4 watt CFR 5%

6. One 10uf ceramic capacitor

10uF capacitor tantalum

7. One 0.1 uf  ceramic capacitor

0.1uF disc ceramic capacitor

8. Two connecting  probes( red and black)

meter probes red black

Circuit diagram:

There are total 8 pins in 555 timer as shown in circuit diagram make connections as shown and don’t forget to connect capacitors as they are as important as any other components in this circuit.

Connecting probes are connected between trigger terminal (2) and ground.

**If you do not have a buzzer than connect led in series with 1k resistor in place of buzzer**


Before I explain its working you must know these two points:

A. If voltage at trigger pin is less than 1/3v of the applied voltage (9v in this case), only than the output will be 1(HIGH).

B. If voltage at threshold pin is greater than 2/3v of the applied voltage then the capacitor (10 uf) starts discharging through discharge pin (7th) to ground.

As you can see in the above iC 555 based continuity tester circuit, to check continuity you place the circuit between probes (connected to trigger terminal and ground).

Case1—if there is a break in circuit

If this case arises then that means there is infinite resistance(open circuit) between pin 2 and ground which causes all voltage drop between pin 2 and ground which is obviously greater than 1/3 of 9 volt, hence(from point 1) we get 0 volt as output from pin 3 at which buzzer or led is connected. Hence buzzer will produce no sound indicating a break in circuit.

Case2—if there is no break in circuit

If this case arises then that means there is almost 0 volts (short circuit)  between pin 2 and ground which causes all voltage drop across 4.7k resistor and thus pin 2 get 0 volt  which is obviously less than 1/3 of 9 volt, hence(from point 1) we get 1 volt as output from pin 3 at which buzzer is connected. Hence buzzer will produce sound indicating continuity in circuit.

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    1. GOOD EVENING sir!
      what is the maximum DC voltage that can be connected to the connected probes?
      Thank u very much!

      1. the circuit is designed for testing continuity between two points in a circuit or component, if a voltage is present then the output will show incorrect results, therefore power should be switch OFF while using this tester


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