A simple continuity tester can be designed using electronic diagram below. The continuity tester can be used to check electrical wiring or printed wiring check. The electronic circuit consists of a piezoelectric buzzer and an astabil multivibrator.
At short circuiting of probes piezo buzzer will produce an acoustic signal.
Using this electronic circuit can be check if there is between two points or not galvanic connection. Continuity indicator can give an answer in case high resistance. The circuit indicate continuity when measured resistance is between 0 and 5 MOhm, over 5 Mohm resistances are shown as interruptions.
Since a FET gate current not practically exist, it is controlled by the voltage gate. If on this way there is a power interruption, the gate voltage is around 3 V and T1 conducts.
The source voltage is close to the supply voltage and T2 controlled and D1 LED lights.
Using this circuit diagram you can make an electronic tester for operational amplifiers. This operational amplifier tester allows a quick functional verification of operational amplifiers. For test this amplifier is connected as a simple square wave generator.