This circuit is from an old American documentation from the 70s. Transistor 2N3904 can be replaced by BC548. The relay must have 100 mA of maximum current.
The circuit shown is suggested by National Semiconductor's Linear Applications Handbook. The base...
This circuit uses a pickup coil to take the signal from a telephone line without contacts. The circuit...
In this circuit, the transistors are NPN for general use and the phototransistors can be replaced by...