This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and can be changed. The circuit already includes an amplification stage.
This re-triggerable circuit was found in the April 1988 Radio Electronics magazine. Timings are...
This circuit, from a 1994 publication, shows how to polarize a phototransistor to receive...
This circuit for some amps was found in a 90s Radio Electronics. The voltage can be between 6 and 25 V...