This circuit uses an optical coupler to indicate a phone call through a lamp. The circuit is from a Hands-On Electronics from the 90s. The lamp must be incandescent.
The transistor of this circuit found in an old American documentation can be the BC548 for T1 and...
This circuit was found in Motorola documentation from 1969. The transistors are from that time. The...
The crystal in this circuit can have frequencies from 100kHz to 8MHz. Capacitor C2 adjusts the...