Innovative Resistance Measurement Using a Buzzer

A New Device Evaluates Resistance through Tones

In the field of electronics, the continuity buzzer has been a standard tool since its inception. While most people have a multimeter, sometimes the exact value of resistance is less important than whether a circuit is open or closed. But what if your buzzer could also provide additional information?

Nick Cornford posed this question and created a buzzer that allows for resistance estimation based on tones. The circuit is relatively simple: a short circuit to ground generates a voltage divider that produces a fraction of the battery voltage, and an FET drives this voltage to a Voltage-Controlled Oscillator (VCO) through a high-gain amplifier. The VCO converts the voltage into frequency, and an audio amplifier transmits it to speakers.

The two amplifiers and the VCO require two op-amp chips. The original design directs the output to relatively high-impedance headphones. To use more practical headphones, the circuit can forgo one op-amp and use another FET with a separate battery. There are many design options, particularly for audio amplification. Namerous VCO circuits exist, and a small microcontroller with an A/D converter and PWM output could be a viable alternative. Additionally, a VCO can be created using a 555 timer.

VCOs are common due to their essential role in Phase-Locked Loops (PLLs).