This circuit was obtained from a 1969 Electronic Design. Transistors admit modern equivalents. The frequency of the signals is set to R2.
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...