This circuit was found in a 73 magazine for radio amateurs from the 90s. The circuit uses high voltage transistors and the relay is high voltage as well.
This intercom for the 27 MHz band was found in a recent English documentation, working with a...
This interface was found in a Modern Electronics magazine from 1990. Although the circuit is...
The amplifier gain shown in the figure is given by R1. The circuit is valid for other photodiodes and is...