INTERCOMMUNICATION EQUIPMENT
Intercommunication units are classified into master and remote types. A master station can call any one of the remote stations. On the other hand, a remote station can call the master station only. An intercommunication wiring system may employ master stations throughout. In this case, any station can call any other station. This type of intercom set has remote units with beep buttons that permit a caller to signal the master station even when the system is turned off.
The intercom circuit shown in Fig. 26.21 is a 3-transistor amplifier with a slightly sophisticated switching circuit. Each speaker serves the dual function of microphone and loudspeaker, with the switches determining what that function is. When S1 is ...
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