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 a bit costly and bulky too. Also, these give a positive indication even when there are 100 ohms of resistance between the probes.
So how to make a continuity tester which has no such kind of drawbacks. This simple continuity tester 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 continuity.
Simple Continuity Tester Circuit Schematic
Components Required for Continuity Tester
- NE555 IC
- Capacitors (100pF, 22uF, 1ouF)
- Resistors (2.2K, 1.5K, 100 Ohm)
- Test probes-2
- Potentiometer (10K)
Working Principle of A Continuity Tester Circuit
- Here 555 works in the astable mode of operation. Astable multivibrator working using 555 had been elaborately discussed in one of our previous posts.
- 555 is designed to generate a 2 kHz output frequency. You can use an Astable calculator tool for this.
- The output of 555 is connected to an 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 the ground through a 100-ohm resistor, so normally it is OFF. This speaker doesn’t produce any sound.
- One probe is taken from Vcc and another 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 multivibrators and hence speaker will start to produce sound.
- The potentiometer helps to set the resistance to determine the point when the buzzer should turn ON.
You can enclose this low-cost circuit in a suitable case and attach it with multimeter probes to have a device look.