This circuit is from a 1980s Signetics documentation. The circuit separates the modulation signal from a carrier. The used components are not common today.
Relays from 20 mA to 200 mA can be driven by this sensitive circuit.6 or 12 V relays can be used...
This is a conventional double T oscillator, where the frequency is determined by the double T...
This circuit can be used to recognize a tone by its frequency. When this tone is present in a...