This circuit is powered by a voltage of 4.5 V, being common in radios of the 60's. The circuit is from Mullard's transistors reference manual.
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...