This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset or speaker must be at least 80-ohm. Observe the low supply voltage.
This circuit generates a pulsed tone whose frequency depends on C2 and whose flashing depends on...
This circuit of a Popular Electronics from the 80's selects tones around 60 Hz, acting as a filter...
This circuit samples the current in a load and in case of excessive value it activates a...