This circuit is a telephone line simulator and can be used for internal communications. It is designed for a main phone and 3 extensions but can have up to 7 extensions. Just add diodes and 1-pole x 2-position switches, wires and LEDs to the TIP122 transistor emitter, via R2. Being a line with several extensions, due to the diodes, we can trigger the call to the extension we want by pressing the NA switch or the key that is adapted to the telephone corresponding to the extension number. The author pasted 2 thin metal plates (15 mm) on the keyboard plates, under the extension number keys (1,2,3 and 4). On the keys glued other plates (3 mm) for contact when it is pressed.
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