Frequently Asked Questions

DIY Audio Detection Circuit

This article describes a build-it-yourself project. Sine Systems offers model AFS-3 Audio Failsafe that performs the same function.

It is sometimes necessary to monitor the presence or loss of an audio signal using the RFC-1/B. Absence of audio at the transmitter site is an indicator of many types of failure all of which are usually critical.

The schematic for a very simple audio detector is shown below. Any audio level of -6 dBv or greater will maintain at least 0.5 volts DC at the output. This covers most "line level" audio sources. If greater audio sensitivity is needed, use germanium diodes (1N34) or Schottky diodes (1N5819) for D1 and D2.

audio detection schematic

Audio Detection Schematic

Qty Description Location Digi-Key Part #
1 470 Ohm resistor, 1/4W, 5% R1 470QBK
2 470 mF 16V aluminum electrolytic cap C1,C2 P6230
2 1N4001 diode, general purpose D1,D2 1N4001MSCT


The easiest way to set this up to detect loss of audio with the RFC-1/B is to connect the output of this circuit to a telemetry input and turn the calibration pot fully on (clockwise). Then set the upper limit for this channel to 9999 and the lower limit to around 0150.

With audio present the channel will read "status on" most of the time. During long pauses the reading will change (drop) to numerical values. The longer the silence lasts the lower the reading will drop. Eventually it will drop below the lower limit which will trigger an alarm in the RFC-1/B.