This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
This circuit, for its simplicity and the fact of using a super-regenerative detector only works...
For counting to values higher than 16, many 4520 can be cascaded as shown in Figure . The EN...
We find this circuit in American documentation from the 1980s. It consists of a frequency folder...