Putting a relay in parallel with the intercom's door release to open the building door from your phone is one of the automations we get asked about most. It's also one of the worst explained out there: there are three details that decide whether it works or whether you end up burning out the door release.
First: is your intercom analog or digital?
This is the filter that decides whether the project is viable.
Classic analog intercoms, with four wires plus neutral, have a button that physically closes two terminals. Putting a contact in parallel just works. It's the majority case in Spanish buildings.
Digital two-wire bus intercoms (Comelit Simplebus, Fermax Duo, 2Easy) don't close a contact: they send a coded message to the street panel. There, a contact in parallel does nothing, because there's nothing to close. The solution goes through a relay module from the manufacturer itself, and the Shelly just drives its input.
How to find out: cut the power, open the intercom and put a multimeter in continuity mode on the terminals of the door-release button. If there's continuity when you press, it's analog and those are exactly the terminals where the Shelly goes.
Where it connects: to the door release, not to the button
Many tutorials tap directly into the button. It works sometimes, but it's the fragile option: on intercoms that require a prior call, the button is disabled until the bell rings, and your relay will close the contact with no effect.
The door-release terminal is further down the chain and always responds. It's also the one identified in the manufacturer's diagram as the output to the strike.
Important: you connect in parallel, you don't replace. The original wiring stays where it is. If you remove it, the intercom stops opening from its own button and you've traded a convenience for a fault.
The pulse is mandatory
This is the mistake that damages the most equipment.
An electric door strike is designed to receive voltage for one or two seconds. If the relay stays closed, the door release is powered continuously, heats up and eventually burns out. And the building door, meanwhile, stays unlocked.
In the app you have to set up two things:
- Auto-off at 1 or 2 seconds, so the relay returns to rest on its own even if the Wi-Fi drops right after it activates.
- Detached mode, which decouples the switch input from the relay. Without this, the intercom button can end up triggering the relay and the behavior becomes unpredictable.
Which model to choose
You need a potential-free output, that is, a dry contact that doesn't inject its own voltage into the intercom circuit. In the catalog there are three Mini units that have it, and the choice depends on the ecosystem you use:
- Shelly 1 Mini Gen4 — the recommended option today. In addition to Wi-Fi and Bluetooth it has Zigbee and Matter, so it fits into HomeKit, SmartThings or Alexa without intermediate gateways.
- Shelly 1 Mini Gen3 — same format and same dry-contact output, but only Wi-Fi and Bluetooth. It's the economical alternative if you control everything from the Shelly app or from Home Assistant.
- Shelly Wave 1 Mini — the Z-Wave version, for those who already have a Z-Wave hub set up and prefer not to add another Wi-Fi device to the installation.
Watch out for the PM versions
The 1PM Mini, both in Gen3 and Gen4, has an identical format and costs barely a few euros more, but it's no good for this. By measuring consumption it needs the load within its own circuit, so its output isn't potential-free: it delivers voltage. Connecting it to the door-release terminal is putting 230 V where the intercom expects a simple contact.
We'd rather say it before the purchase: if you see "PM" in the name, it's not the unit you want for the intercom.
And about the power supply
A nuance that's often misunderstood: the output being potential-free doesn't mean the device needs no power. All three run on 110-240 V and need live and neutral near the intercom. If there's no 230 V in that box (a common situation, because the intercom bus runs at 12-24 V), you'll have to run a spur from the nearest point.
And a warning: a smart plug is no good for this. It delivers 230 V, and what you need is to close a contact.
What it's really used for
- Opening without getting up. A widget on the home screen and no more dash to the hallway.
- Deliveries and parcels. You open from work when the courier calls your phone. Combined with another Shelly on the stairwell light, you leave the entrance lit the same minute.
- Short-term rentals and cleaning. You give one-off access to the team or to a guest arriving late without duplicating keys or paying for a lockbox.
- Accessibility. A BLU Button next to the armchair avoids the trip to the intercom every time someone rings.
- Integration. When it opens, the hallway light comes on and a notification reaches your phone. All with scenes in the app itself, with no intermediate servers.
Frequently asked questions
What's the difference between the 1 Mini Gen3 and the Gen4 for this setup?
Electrically none: both close the same dry contact and connect the same way. The difference is connectivity. The Gen4 adds Zigbee and Matter, which matters if you want the opening to appear in HomeKit or SmartThings. If you're going to use the Shelly app or Home Assistant, the Gen3 does exactly the same for less money.
Do I need the residents' association's permission?
If the work is limited to the intercom inside your home, which is a private element, usually not. If you have to touch the street panel or the shared bus, yes: that's a common element and requires agreement. Check with the manager before, not after.
Can I power the Shelly from the intercom's own bus?
It's not advisable. The bus is sized for the intercoms' consumption, and hanging added loads on it can degrade the audio or cause voltage drops that affect your neighbors. Run an independent power supply.
What if I lose Wi-Fi?
The intercom keeps working the same from its button, because you haven't replaced anything. You only lose remote opening. If you have a BLU Button paired, that keeps working even if the router is down, because the link is direct Bluetooth.
Is it safe for the building door to open from a phone?
It depends on how you set it up. Enable the device password, use two-factor on the account and don't share it with anyone you wouldn't give a key to. The risk level is comparable to that of any physical key.
Can I know who opened it and when?
Yes. The history logs each activation with its timestamp, and you can set up push notifications on each opening. It's one of the advantages over the traditional key.
Is it good for my apartment door?
Technically yes, if you have an electric strike. But weigh the risk: the building door opens onto a shared space, while your home's door is your last barrier. Many people automate the first and deliberately keep the second on a key.
Can I add a keypad or a fingerprint reader?
Yes. Any device with a dry-contact output can drive the Shelly's input or be placed in parallel with it. It's a common extension in short-term rentals.
Summary
- I want Matter, Zigbee or HomeKit → Shelly 1 Mini Gen4
- Shelly app or Home Assistant, at the best price → Shelly 1 Mini Gen3
- I already have a Z-Wave hub → Shelly Wave 1 Mini
- An extra button next to the sofa → Shelly BLU Button
Before closing the box
Cut the power at the panel before handling anything and verify the absence of voltage. If the intercom wiring shares a box with 230 V circuits, take extra care with the insulation. If you're not comfortable with electrical installation, an installer will solve this in under an hour.
Questions about your intercom model? Send us a photo of the intercom's terminal block and we'll confirm whether it allows the setup. Technical support in English and shipping from Spain.
Leave a comment