You can usually find the transformer in an out-of-the-way place where AC power is available, such as the garage or a utility closet. (Electrical codes preclude placing an unprotected transformer in the attic or outdoors.) Most doorbells operate on 12 volts AC so the transformers typically have 120 volt AC inputs and 12 volt AC outputs.

The pushbutton is a normally open switch that keeps the transformer current from flowing to the ringer until pressed. Lighted pushbuttons allow a small amount of current to flow all the time; enough to light the pushbutton, but not enough to activate the ringer.