Oct 242020

Audible Continuity Tester

This audible continuity tester has been designed as an instrument that can works on live circuits. This is a very important piece of information, as continuity testers are commonly used with the circuit’s power source disconnected, like the continuity tester with an Op Amp.

The test leads have a voltage locked to (set to) at 0.3 volts when there is an open circuit, and the current is only 1 mA when there is a short circuit.

How does the continuity tester work?

The bipolar transistor Q1 forms a constant current source of 1 mA on its collector, and the test lead is fixed at a voltage of 0.3 V with a germanium diode (D2). Germanium diodes have a voltage drop of 0.3 V, when forward biased.

Audible continuity tester - Works on live circuits

Audible continuity tester – Works on live circuits

The op amp IC1 is configured as a Schmitt trigger, with an adjustable trigger voltage level via potentiometer R9.

Piezoelectric Buzzer

Piezoelectric Buzzer

The trigger point determines the maximum resistance with which the circuit operates, and can be preset within the range of 0 to 90 ohms.

The use of the R9 potentiometer in conjunction with the R10 potentiometer makes it easy to fine-tune the circuit for low value resistance values.

Continuity Tester Component List

  • 1 741 op amp (IC1)
  • 1 BC213L (NTE159) PNP bipolar transistor or similar (Q1)
  • 1 5.6 V, 1 watt zener diode (D3)
  • 1 1N914 silicon diode (D1)
  • 1 OA47 or similar germanium diode (D2)
  • 2 1 K (Kilohms) resistors (R4, R8)
  • 1 1.1M (Megohms) resistor of 1.1M (R7)
  • 1 2.7K resistor (R5)
  • 1 12K resistor (R6)
  • 1 2.2K resistor (R3)
  • 1 10M resistor (R2)
  • 1 resistor 47 ohm (R1)
  • 1 20k potentiometer (R9)
  • 1 10K potentiometer (R10)
  • 1 100 nF (nanofarad) capacitor  (C1)
  • 1 piezoelectric speaker U5-35R or similar. (SP/buzzer)

This circuit is based on the original by R. Batty.


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