Nest Hello Doorbell wiring with DETA C3500 / C3501 Chime UK

Just writing up this post because I struggled to find good information on how to wire up the Nest Hello Doorbell with a common UK chime box the DETA C3500. Please note I’m not an electrician, just a normal guy that is pretty decent at some basic DIY. Make sure you seek advice of your own if you don’t know what you are doing.

There were some instructions online that showed how to wire up the Nest with different chimes and some instructions bypassing the chime box completely. I wanted to see if I could try and keep my existing chime box without having to fork out for a new one.

You obviously have your existing door chime box and the Nest Hello but additionally, items I used for my installation:

The transformer you require must be able to supply at a minimum of 12v to be able to power the camera. I got mines from Screwfix, link to it is above.

British General 8-24V 8VA Bell Transformer Module
British General 8-24V 8VA Bell Transformer Module

Here is my chime box opened up.

Chime Box
Chime Box
Inbuilt Transformer
Removed cover of inbuilt transformer

First obviously turn off electricity to your door chime at your consumer unit. I then unplugged the two blocks of wires with the left block blue and brown being the mains voltage supply, and the right block white and blue thinner wires that go to the door bell itself. I removed the chime box from the wall so it was easier to work with wiring it up.

Mains and door bell wire
Mains and door bell wire

What is needed are the two coming out of the solenoid, i.e. the hammer to the chime, mines were covered in a plastic see through sheath. You need to keep these 2 wires connected to the solenoid so you have to cut them off as close to the circuit board as possible so that you retain maximum length on the wiring. I had troubles with the wiring in that one of them snapped off the solenoid, so I had to dismantle to see how I could salvage but luckily it was just coil wiring so there was plenty. The hardest part was actually trying to pry off the cover to the solenoid, it took almost an hour!

Where to cut the wires
Where to cut the wires highlighted in red and me dismantling the solenoid

And here is the wiring diagram for my installation, note the yellow wire represents white.

Wiring Diagram
Wiring Diagram

I did a little test with the wiring before I actually installed the Nest doorbell to the outside. I installed the app and started doing the setup to make sure the wiring was ok.

Testing the camera
Testing the camera

After I was happy with the wiring, I then wired the Nest doorbell to my existing bell wires outside. It doesn’t matter which way around the two wires go here according to Nest’s video instructions.

Nest doorbell
Nest doorbell

Wired up but as you can see, the new transformer is a monster so it will not fit into the chime box. I had to order an enclosure to make it tidy.

Nest Hello Chime box wiring with transformer
Nest Hello Chime box wiring with transformer

A week later with the new enclosure the installation is now looking decent.

Tided up wiring
Tided up wiring
Project Complete
Project Complete

Hope you find this useful. I have also uploaded a YouTube video in which I describe my installation too.

9 thoughts on “Nest Hello Doorbell wiring with DETA C3500 / C3501 Chime UK”

  1. Hi Paul, super helpful post. I am installing a Nest Doorbell and I really appreciate your detailed guide. I was wondering what type of enclosure did you get to house that monster transformer?




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  2. Hi Paul,
    Firstly thanks for the details post and youtube video, very informative. Wondering if you can help me, i don’t have existing chime but i do have my google home mini’s which does that job in my smart home system, so my issue is how to i get power from the consumer unit to the transformer? (Might be a silly question) i have the transformer coming from screwfix, my consumer unit is directly above my front door, any help would be hugely appreciated.




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    1. Hi Anthony, thanks for reading my post. I’m not an electrician so I wouldn’t know how to create a connection from the CU to doorbell. However, if you don’t have an existing mains connection to the doorbell then I’d suggest using something like this – Power Supply for Video Doorbell,LANMU Power Adapter for Nest Hello Video Doorbell (5M) https://www.amazon.co.uk/dp/B07MJ953JD/ref=cm_sw_r_cp_apa_i_8JUwDbP52TTGM
      It’s got an inbuilt transformer with the right voltage to power the Nest and you can just plug it into a uk wall socket. You wouldn’t need an external transformer.




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      1. Thats how im currently running it, however these transformers are not designed for 24/7 usage and how hot it gets is a concern.

        Thanks anyways, iv took from your post all the bits i needed and will find out with breaker i need to add👍🏻




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  3. Hi, I just moved into my first house and this is one of my first projects. Just wondering, did you just place the transformer box over the old chime unit and drill new holes for the chime box? 🙂 Thanks




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    1. Hi Thomas, no the original chime unit is still in the same place. I had to place the transformer beside it, so drilled new holes for the transformer housing. I passed the wires from the chime box to the transformer box. Hope this helps?




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