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.
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:
Components Required:
- NE555 IC
- Capacitors (100pF, 22uF, 1ouF)
- Resistors (2.2K, 1.5K, 100 Ohm)
- Speaker
- Test probes-2
- Potentiometer (10K)
Working:
- Here 555 works in astable mode of operation. Astable multivibrator working using 555 had been elaborately discussed in one our previous posts.
- 555 is designed to generate 2 Khz output frequency. (You can use our Astable calculator tool for this).
- 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
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