RFC-1 Accessories and Miscellaneous Circuits page 4.3
4.2 Auxiliary Circuits
Accessories are available that give the RFC-1 extra capabilities. Some functions are simple to add with just a few
extra parts.
4.2.1 Audio Detection
In some cases it is desirable to monitor the presence or loss of an audio signal with the RFC-1. This signal can be
used to trigger an alarm in the RFC-1. The circuit shown below is a simple audio detector. It does not provide the
features of AFS-3 Audio Failsafe but it can provide a basic audio status indication.
Figure 4.1; Simple audio detection circuit
The circuit simply rectifies the audio voltage and charges the capacitors. Any audio level of -6 dBv or greater will
maintain at least 0.5 volts DC at the output. Most line level audio sources are sufficient.
The easiest way to set this up as a loss of audio alarm is to adjust the calibration pot all the way up until you hear a
soft clicking sound—the calibration pots are 22 turn trim pot. Then set the upper limit for this channel to 2040 and the
lower limit to around 0150. With audio present, the reading will be "status on" almost all the time meaning that the
telemetry is pegged against the upper end of the scale. During long pauses the reading will change to numerical
values. An alarm will trigger when the value drops to 0150 or below.
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