Retrofiting a 5-wire (4+N) 12 VAC analog intercom/entryphone with a Shelly UNI. Tegui Horizon N. Tasmota. Diagram and help.

Hi, I have a classic analog intercom/entryphone system based on a Tegui Horizon N that I thought would be cool to domotize, so that I can actuate the door-opening switch remotely and get notified via Telegram notifications when the 12 VAC buzzer rings.

I have been able to switch my Shelly UNI to Tasmota using this intermediate OTA firmware, knowing that I can always go back to the original Shelly firmware with this. Tasmota supports the direct Telegram message integration when enabled in custom builds. Super neat.

So everything works from the software-side when hooked to a 12 VDC wall wart adapter, great. Now the fun part begins.

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Here's the official layout for the five terminals:

• 1Z: Buzzer (white wire)
• 2: Common/neutral (yellow wire)
• 3: Door opener (green wire)
• 4: Electret microphone (gets disconnected when the earpiece rests on the base) (bluish-gray wire)
• 5: Speaker (gets disconnected when the earpiece rests on the base) (red wire)

Here is what I aim to do, roughly:

^{(Image mirror: https://i.postimg.cc/ty8knnpn/imagen.png)}

In the official diagram it says that the electric door lock is rated for 12 VAC 880 mA +- 5%.

Reference PDFs: Official Tegui E-14 installation diagram, Shelly pinout.

I measured the voltage of the terminals with a multimeter. Here's what I found:

2, 3: 13 VAC

3, 4: 13.23 VAC

5, 4: 4.91 VDC

5, 3: 13.26 VAC 50 Hz

4, 2: 5.~ VDC

5, 2: 0 VAC

1, 2: 11.20 VAC while buzzing, zero otherwise.

3, 2: If I bridge these for a few seconds with the disconnected multimeter leads the electric door lock opens as normal. That probably means that I don't have to disconnect the speaker/microphone lines beforehand, reducing complexity.

Attaching the Shelly UNI outputs directly to 2, 3 does not work, I can hear it buzzing through the earpiece when the Shelly's TLP172GM is running but the electric door lock doesn't budge, so I'm thinking I'll be needing a small external relay that can handle more than 100 mA and still fits inside.

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Now, the two big questions. Can I feed the Shelly UNI directly without adding noise to the line/interfering with/damaging the speaker and microphone? Do I need an external power supply? It would be neat to have it all in a self-contained way. But I guess I can always go for the AC to 12 DC adapter route if nothing pans out. If I use external power without matching reference voltage for IN_1 will I be risking destroying the Shelly UNI? Will that still work?

What's the best avenue for an external relay that fits inside and can be actuated by the Shelly UNI? Do I need to add flyback diodes to protect it, any premade kits out there I can use?

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While I have a very rudimentary knowledge of electronics I can handle the software side just fine. This is a learning experience for me, hopefully someone can give me some pointers. I don't want to break anything. Please be kind and thanks in advance.

PS: Maybe the documentation helps other people thinking about doing something similar with their own analog intercoms. The order of the terminals may be different, but from what I've seen on the Internet they are all pretty similar.

Updated the post with a handy diagram of what I aim to do. Let me know what you think.