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 circuit in the figure trips with a negative transition of the input signal, its output...
The basic frequency of this circuit is determined by the capacitor of 470 nF and the adjustment of the...
The load current can be adjusted up to about 1 A. The transistor can be TIP32 or TIP42. The...