Using this circuit you can check continuity of low-resistance paths such as PCB tracks, small coils, intermediate-frequency transformers (IFTs) and low-resistance transformers. However, you can’t check semiconductors with this tester. The tester works off a 9V battery.
The tester is built around optocoupler MCT2E (IC1) and timer IC 555 (IC2) that is wired in free running mode. Optocoupler MCT2E is used here as a continuity sensor. Testing probes A and B are connected to pins 1 and 2 of optocoupler IC1. The phototransistor inside the optocoupler is connected to transistor BC547 (T1) to form a Darlington pair, which improves the performance of the circuit.
When the probes are not shorted, the LED inside the optocoupler glows and the Darlington pair conducts to keep reset pin 4 of IC 555 at ground level and thus no sound is produced.
When probes are shorted via a low resistance, the LED stops glowing and the Darlington pair doesn’t conduct. As a result, reset pin 4 of IC2 goes high to activate the loudspeaker, which generates a sharp audio tone.
To minimise the current through IC1 when probes are not shorted, adjust VR1 until the circuit just stops sounding. The output tone and loudness can be varied by adjusting presets VR2 and VR3, respectively.