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LED Voltage Tester Schematic Circuit Diagram

Versatile Universal Voltage Tester

A universal voltage tester should be capable of detecting both direct current (dc) and alternating current (ac) voltages. Traditional glow-discharge lamps are limited to voltages exceeding 100 V. However, the circuit illustrated in Figure 1 employs a Darlington circuit composed of two NPN transistors, enabling the detection of voltages as low as 1 V. Additionally, this circuit can serve as a continuity tester. In this setup, the positive terminal of the battery acts as the ‘ground’ connection. Consequently, even with a high-impedance connection, there’s an input current flow. This current intensifies when a voltage source with appropriate polarization is integrated into the loop. An additional piezoelectric buzzer expands the circuit’s functionality, allowing it to function as an audio-frequency signal tracer.

Multiple Testing Modes

This versatile tester can be utilized in various ways:

  • Continuity Testing: When there is continuity between the two terminals or when connected via fingers, the LED illuminates.
  • Battery Testing: When testing a battery with the positive terminal linked to the input, the LED brightness increases.
  • Voltage Testing: When testing a voltage with the negative terminal connected to the input, the LED either dims or remains off.
  • AC Voltage Testing: In the presence of an alternating current (ac), the LED current undergoes modulation, causing the LED to flicker, accompanied by the buzzing sound from the buzzer.

LED Voltage Tester Schematic Circuit Diagram

Incorporating Components into Everyday Items

This circuit can easily fit into a key finder enclosure since the vital elements – a battery holder, an LED, and a piezoelectric transducer – are already present. Alternatively, the tester can be housed within a ballpoint pen case or a piece of plastic tubing (refer to Figure 2). An intriguing experiment can be conducted using this setup. One person grasps the probe tip while another person holds the opposite terminal. As they walk over a carpet or synthetic floor covering, the LED illuminates with each step. This phenomenon occurs due to charge separation between the floor and the shoes.

Understanding the NPN Transistor

The NPN transistor is constructed from semiconductor materials like silicon or germanium. By sandwiching a p-type semiconductor material between two n-type semiconductor materials, an NPN transistor is created. This type of transistor possesses three terminals: an emitter, base, and collector.


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