This old 1 W audio amplifier transistor circuit is from a 1967 Japanese magazine. The transistors are germanium of the time and the type transformers found in similar equipment of the time.
The recommended distance between the emitter and the phototransistor is 10 mm for this...
This circuit was found in a 1994 documentation. Speed measurement pulses are obtained by a reed...
Here is a high-quality intercom for demanding readers. The circuit is from a 1982 documentation. Of course,...