The installation of the doorbell button.
Installation of a doorbell in an apartment : The Installation Of The Doorbell Button.
Quote from Article entitled Installation of a doorbell in an apartment:
There is often a situation when you need to connect two buttons to one bell. This is necessary, for example, when there is a common tambour for several apartments. In this case, one is put in a tambour, and another one - in front of the door. The device connection circuit will then have two parallel buttons. Pressed any of them the circuit will close and the mechanism will operate.
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