this should be a simple question, but no product pages seem to describe this situation. I want to hook up a wired doorbell with two buttons and two chime units. Both bells should respond to both buttons (i.e., one chime back door, two chime front door). However, my dad tried this years ago and the result was the transformer burned out. I'd like to try again. Can anyone tell how to build such a circuit properly?
You don't need a relay and you don't need two transformers. You do need a transformer powerful enough to operate both chimes, so you can't buy doorbell kits with transformer included. You'll need to select the transformer separately.
You should purchase two identical chime units. If the chime units are different, their voltages must match. The transformer voltage must match the chime unit voltage, and the transformer amperage must equal the sum of the two chime unit amperages.
A single dual-tone chime is easy to wire up. Usually there are wiring diagrams in the package and sometimes they even make sense.
Select one low-voltage terminal on the transformer to be the supply leg and the other to be the return leg. It doesn't matter which because it's AC and isolated from your house mains. Connect the supply leg to both doorbell buttons. Connect the other terminal on the front door button to the ding-dong terminal on the chime, and the other terminal on the back door button to the ding terminal on the chime. Connect the common terminal on the chime to the return leg on the transformer.
Now, to wire two dual-tone chimes is almost as easy. You just connect the second chime unit to the first in parallel, ding-dong terminal to ding-dong terminal, ding to ding, and common to common.
This diagram has the transformer at the circuit breaker service panel, but you will probably put yours in your utility room. In any case, for safety reasons, please mount your transformer on a junction box.
The aqua ring indicates where all the wires come together in your utility room. Black indicates the low-voltage supply leg, and white the low-voltage return leg. Of course, the colors are in the diagram only; I'm not trying to suggest that you buy red and blue wire.
For the sake of completeness, and in honor of Thevenin and Norton, I offer a circuit to use when the amperage requirements of the two chimes are equal, and a transformer is available with that amperage rating and an output voltage equal to the sum of the two chime units' voltages:
In the special case of this doorbell arrangement, the loads can be connected in series. This circuit is suitable only for low voltage applications. Never wire household power loads with the switch in the middle.