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LED Voltage Tester Schematic Circuit Diagram

A universal voltage tester should respond to both dc and ac voltages. The usual types of 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. The tester can be used in the following manners: – Continuity between the two terminals or connected via the fingers: the LED lights. – Testing a battery with the positive terminal connected to the input: the LED becomes brighter. – Testing a voltage with the negative terminal connected to the input: the LED becomes darker or is off. – With an ac voltage, the LED current is modulated, so the LED flickers and the buzzer sounds.

LED Voltage Tester Schematic Circuit Diagram

All of this can be built into the enclosure of a key finder since the essential components are already present: a battery holder, a LED, and a piezoelectric transducer. Alternatively, the tester can be fitted into the case of a ballpoint pen or a length of plastic tubing (see Figure 2). An interesting experiment can be performed using this circuit. One person holds the probe tip, while a second person holds the opposite terminal. If they walk over a carpet or synthetic floor covering, the LED lights up with each step. This is a consequence of charge separation between the floor and the shoes.

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