It actually consists of a tunable field strength meter. The circuit is of an old documentation and the circuit must be tuned to the frequency to be monitored. The circuit offers the option to use a headset.
In the figure we show how to use an LM324 as a voltage comparator the reference voltage is...
We found this circuit in a 1964 Electronic Design. It provides 0.5 V signals from 4 kHz...
This circuit is from a 1992 documentation, but the components used can still be easily assembled. The...