Simple Continuity Tester Circuit Using 555 Timer

Normally we use continuity testing to check if a circuit, PCB or cable is continuous or not. It also helps us to check if a bulb is burnt out or not. Simply we can do the test by using a battery and a led or buzzer, but they are not precise or accurate.Usually we use a continuity test multimeter which is little bit costly and bulky too. Also these give a positive indication even when there is 100 ohms of resistance between the probes.So how to make a continuity tester which has no such kind of drawbacks. This circuit is built around a common 555 chip wired in astable mode. The continuity test circuit can precisely judge continuity as that low 6 to 8 ohms(which can be set by using the preset). Speaker is used to indicate the continuity.
You can enclose this low cost circuit in a suitable case and attach it with multimeter probes to have a device look.

Continuity tester schematic:

Continuity tester circuit using 555

Components Required:

  1. NE555 IC
  2. Capacitors (100pF, 22uF, 1ouF)
  3. Resistors (2.2K, 1.5K, 100 Ohm)
  4. Speaker
  5. Test probes-2
  6. Potentiometer (10K)

Working:

  • Output of 555 is connected to a 8 ohms speaker.
  • As you know, 555 is enabled only when the reset pin(4th pin) is connected to Vcc. Here the 4th pin is connected to ground through a 100 ohm resistor, so normally it is OFF. Thus speaker doesn’t produce any sound.
  • One probe is taken from Vcc and other probe from the 4th pin, through a potentiometer.
  • When we are connecting the probes between the testing points, then the 4th pin will get a positive Vcc. Thus enabling 555 multivibrator and hence speaker will start to produce sound.
  • Potentiometer helps to set the resistance to determine the point when the buzzer should turn ON.

Components pinout

555 pin out

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