This circuit for old analog telephones is from period documentation. It controls a relay that can be used to turn the extension on and off.
Found in a Philips documentation from 1968, this circuit detects the trimpot set input voltage....
This circuit is from a 1966 American documentation. The circuit uses a photo-FET, an unusual...
This is a typical application circuit of a 565 PLL. The circuit is suitable for selecting a 67 kHz...