Do-It-Yourself Repairs and Basic Wiring Projects:
Understanding Doorbell Systems

By Terry Peterman, the Internet Electrician

Although I think that the standard doorbell system will soon be replaced by cordless systems that are now available, the traditional transformer-based system is still widely used within most residential installations.

The key to installing, repairing, or replacing a doorbell system is to understand the components, and how they work.

The power for the system comes from a voltage transformer. It is usually installed on a light box in the attic, basement, near the electrical panel, or somewhere else accessible (or at least it should be accessible!).

A transformer, as the name suggests, transforms voltage from one value to another, and in this case, takes normal residential voltage of about 120, and transforms it down to a safer, lower voltage of usually about 16 volts. this allows you to wire the system using low voltage wire of smaller gauge, and rating. (18awg/2-wire LVTN is a common cable used.)  (see figure 1)

Figure 1

The doorbell unit usually will have two coils in it, one for the back and the front door to distinguish between them) with moveable cores that act as strikers to ring the chimes when a pushbutton is activated.

The system is wired by running a wire from one side of the transformer to the common terminal of the doorbell unit, and the other side through the pushbuttons at each door, and then back to the one ring, and the two ring terminals in the unit. ( In most cases, the front door is two rings, and the back door is one.)

Here is how the whole system is wired (see figure 2).

Figure 2