Smart Intercom Unlock with Shelly

The Problem

Living in an older Singapore condo means dealing with a lobby intercom that was installed long before smartphones existed. Mine is a Fermax REF 3303 — a beige analogue video intercom that rings when someone presses my unit number downstairs. To unlock the door, I have to walk to the handset and press a grey button. Every time. Even at midnight when a delivery rider shows up. Even when I am overseas and a trusted person needs access.

I wanted to fix this without replacing the existing system, without involving the building management, and without spending a fortune.

Why I Built This

The obvious solution was the Fermax WiBox — an official Wi-Fi add-on module. But after researching, I discovered the WiBox only works with Fermax VDS system intercoms. My unit uses the older 4+N analogue system. WiBox is incompatible.

There was no complete English guide for smartifying a Fermax 4+N intercom with an iPhone. So I built the solution from scratch, documented everything, and this is that guide.

What It Does

• Receive a push notification on iPhone when someone presses the intercom downstairs
• Unlock the door remotely from anywhere in the world with a single tap
• Physical button still works — nothing changes for guests or family members
• Video and audio unchanged — the Fermax handset works exactly as before

Total cost: approximately SGD 50.

How It Works

The solution uses three key components:

Shelly 1 Gen4 is a tiny Wi-Fi smart relay the size of a matchbox. It connects to your home Wi-Fi and can be controlled from anywhere via the Shelly app or Apple HomeKit. It does two things in this project — closes a relay contact to simulate pressing the unlock button, and detects the ring signal on its SW input to trigger iPhone notifications.

PC817 optocoupler module electrically isolates the Fermax 18V DC ring signal from the Shelly's 3.3V logic input. Without this, connecting the Fermax signal directly to Shelly could damage it. The PC817 uses light internally to pass the signal between circuits without any electrical connection between them.

LM2596 buck converter steps the Fermax 18V DC power supply down to 12V to power the Shelly. This means no separate wall adapter is needed — the entire system is powered from the existing Fermax supply.

The flow from end to end:

1. Visitor presses intercom downstairs

2. Fermax sends ring signal on terminal 4

3. 1N4007 diode → PC817 → Shelly SW input

4. Shelly detects button press event

5. Shelly cloud sends push notification to iPhone

6. You tap unlock in Shelly app

7. Shelly relay closes for 1 second

8. Fermax terminals 1 and 3 connect briefly

9. Door unlocks

Identifying the Intercom

The first step was confirming the exact model. The Fermax Loft monitor slides upward off its metal back plate — no screwdriver needed. The back plate stays on the wall and exposes the green terminal block where all wiring happens.

The Terminal Block

The Fermax 4+N system uses a labelled green screw terminal block on the back plate. Only five terminals matter for this project:

Terminal Function

+ 18V DC power — powers buck converter

− Ground — shared reference for all circuits

4 Ring/call signal — triggers iPhone notification

1 Door unlock side 1 — connects to Shelly relay

3 Door unlock side 2 — connects to Shelly relay

Key Configuration Settings

In the Shelly Smart Control app, three settings are critical:

Detached mode — separates ring detection from door unlock. When someone calls, Shelly detects the ring but does NOT automatically unlock. You decide when to unlock. This is essential for security.

Auto-OFF 1 second — the relay fires for exactly 1 second then turns off automatically. This mimics pressing the physical button and prevents the door from staying unlocked permanently.

Power ON default = OFF — if Shelly reboots after a power cut, the relay stays open. The door remains locked. This is the safe fail behaviour.

Final Wiring

The Enclosure

All three components [Shelly, PC817 and buck converter] fit inside an 83×83×40mm IP44 junction box mounted directly above the Fermax back plate. Wago 221 connectors keep all wire joints clean and accessible.

What's Next

Doorbell chime — using a HomePod mini or Google Home Mini to play an audible chime when the ring is detected via Shelly webhook or HomeKit automation.

HomeKit/Matter integration — enabling Matter on the Shelly to add voice control via Siri and HomeKit automations.