How do I replace a video doorbell with Cat5 wiring in place?

Question

I'm trying to replace a video doorbell in my new house. The installed video doorbell wiring was using two of the wires each of a 2 CAT5 cables.

• At the transformer:
  • two Cat5 cables are coming out of the ceiling next to the transformer.
  • The set of blue wires (solid blue and white with blue stripe (white/bs)) have been used.
  • The previous install had two blue wires (one from each cat5 cable) twisted together, and two white/bs wires (one from each cable) twisted together. These were attached to the posts of the transformer (blue on one, white/bs on the other).
• At the doorbell chime, the cat5 cables come out of the wall.
  • Here, however, they have blue wire from cat5 cable-1 and white/bs wire from cat5 cable-2 twisted together and attached to the trans post and the blue wire from cat5 cable-2 and white/bs wire from cat5c cable-1 twisted together and attached to the "front" post.
• At the doorbell, there is only one cat 5 cable coming out of the wall.
  • The blue wire was attached to one post of the doorbell and the white/bs wire was attached to the other post.

I'm thoroughly confused on how this should be wired now. Do I need to be using wires from both of the cat5 cables or can I just use wires from one of them.

How do I determine which of the cat5 cables is the one that reaches the exterior doorbell?

Answer 1

First, label each cable and take good photos so you can find your way back to square 1 in case all goes wrong.

Second, disconnect the wiring at the transformer, chime, and bell button. Short the blue pair together at the bell button (twist them together, secure with a spring clip, use a wire nut, or whatever).

Using a volt/ohm meter, measure the resistance between the conductors of the blue pair (meaning the blue and the white/blue) of each cable at the transformer and at the chime. If all goes well you'll find a resistance under 100 ohms between the blue pair in only one of those four cables.

Open the connection at the bell button location and measure resistance at the transformer and chime locations again. The pair that had low resistance before should now be open-circuit.

With any luck there'll be one pair that shows resistance and then open, and that'll identify the cable that goes from the bell button to.. wherever it goes. You can use the same process to identify the cable that goes between the transformer and chime.

It seems there's one extra cable-end in the mix. This process won't help you figure out where it goes, but at least it'll help you figure out which one is unrelated to the doorbell.