LED Voltage Tester

A universal voltage tester should respond to both dc and ac voltages. The usual types with glow-discharge lamps only work with voltages greater than around 100 V. The circuit shown in Figure 1 uses a Darlington circuit formed by two NPN transistors and can detect voltages of less than 1 V. It can also be used to test continuity. Here the positive terminal of the battery serves as the ‘ground’ connection. Consequently, an input current flows even with a high-impedance connection, but this current increases when a voltage source with the proper polarisation is included in the loop. A supplementary piezoelectric buzzer allows the circuit to also be used as an audio-frequency signal tracer.