Wiring two contacts with one switch (DPDT bridge)
How to use a single Shelly relay to close two isolated contact pairs at once — useful for alarm zone inputs and dual-sensor signalling.
A single Shelly Mini Gen3 has one relay — one pair of contacts. If you need to close two separate contact pairs simultaneously (e.g. bridging A1↔A2 and B1↔B2 on two different sensors), you'll need an external DPDT relay.
This is a hardware pattern, not a software feature. We document it because it's the most common physical wiring question for alarm-zone, smoke-detector, or dual-sensor scenarios.
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| Part | Recommended | Cost (UK) |
|---|---|---|
| DPDT relay, 230V AC coil, plug-in 8-pin | Finder 40.52.8.230.0000 | £5–8 |
| DIN-rail socket for the relay | Finder 95.05.20 | £3–5 |
| Mains wire (0.75 mm²) for the coil | Any | £1 |
| Signal wire (0.25 mm²) for the sensor pairs | Any | £1 |
Sources: RS Components, CPC Farnell, Rapid Electronics, TLC Direct. Equivalent parts from Schrack (PT270230) or Omron (MY2N-GS) work identically.
Wiring
The Shelly switches mains to the DPDT relay's coil. When the Shelly is ON, the coil energises and the DPDT closes both contact pairs at the same instant (mechanically linked). When the Shelly is OFF, the coil de-energises and both pairs open.
Mains 230V is on the Shelly and on the DPDT coil. The signal pairs (A1/A2, B1/B2) are completely isolated from mains — that's the point of using a relay. Wire the mains side first, test with a multimeter that the signal side is dead, then wire the sensor pairs.
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| Pin | Function |
|---|---|
| A1, A2 | Coil terminals (connect to Shelly's O output and Neutral) |
| 11 | COM, pole 1 |
| 12 | NO (normally open), pole 1 — closes when energised |
| 14 | NC (normally closed), pole 1 — opens when energised |
| 21 | COM, pole 2 |
| 22 | NO, pole 2 |
| 24 | NC, pole 2 |
Connections
- Mains L → Shelly
L(live in) - Mains N → Shelly
N(neutral) AND pin A2 of the DPDT coil - Shelly
O(output) → pin A1 of the DPDT coil - Sensor A wire 1 → pin 11 (COM)
- Sensor A wire 2 → pin 12 (NO)
- Sensor B wire 1 → pin 21 (COM)
- Sensor B wire 2 → pin 22 (NO)
Testing with a multimeter
Before connecting the actual sensors:
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1
Power everything up. The Shelly should be Offline or OFF initially.
-
2
Set your multimeter to continuity (beep mode).
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3
Probe pins 11↔12 → should be open (no beep). Same for 21↔22.
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4
In dynamik.app, click Toggle to turn the switch ON. You should hear the relay click audibly.
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5
Re-probe 11↔12 → should now be closed (beep). Same for 21↔22.
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6
Toggle the switch OFF. The click returns to open state on both pairs simultaneously.
You can now connect the sensor wires. The Shelly looks like a single switch in dynamik.app, but physically it controls two independent contact pairs.
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If you don't want an external relay:
- Two Shelly 1 Minis grouped in dynamik.app — register both, toggle them together. Drawback: 50–200 ms skew between the two switches, no guaranteed simultaneity. Not safe for alarm-zone signalling.
- Shelly Plus 2PM — two relays in one device. Currently registered as two separate switches in dynamik.app (group control comes in a later release).