You open the wall switch box to fit a smart relay and find two wires: the incoming live and the switched live that runs up to the lamp. No blue neutral. That is normal in many older installations, because a plain switch never needed one. A smart device does, or at least it needs some way to stay powered while the light is off.
There are three practical ways out: leave the wiring alone and use smart bulbs with a switch that never cuts their power, fit a relay designed for no-neutral boxes, or move the smarts to where a neutral exists. Each has a catch, and most of the catches only show up after installation.
A note before any of this: mains wiring is work for a qualified person, and local rules apply. Philips Hue says the same in its own in-wall installation guide.
Why the neutral matters
A relay or dimmer in the switch box has electronics that run all the time. They listen for Wi-Fi or Zigbee commands, even when the lamp is off. With a neutral, the device takes its supply between live and neutral, and the lamp is just a load it switches.
Without a neutral, the only path back is through the lamp itself. The device has to pass a small current through the bulb while it is off, and draw power from the same circuit while it is on. With an incandescent bulb this works fine. With LED bulbs, that small current can be enough to make the lamp glow faintly or flicker, and the driver electronics inside some LED bulbs react badly to it. This is why no-neutral products come with load limits and extra parts.
Option 1: smart bulbs, with the wall switch taken out of the power path
Smart bulbs avoid in-wall work entirely, but they bring their own problem. Flip the old switch off and the bulb loses power. It drops off the network, and no app or automation can turn it back on until someone flips the switch again.
On Zigbee there is a second cost. Many mains-powered Zigbee bulbs act as routers and relay traffic for other devices. Switch them off at the wall and every sensor that routed through them has to find another path, or drops off. Not every bulb routes, though. Hubitat's documentation says that unlike most Zigbee bulbs, Sengled bulbs are only end devices, and it recommends building a mesh of always-powered repeaters, such as smart plugs, before adding end devices. You cannot assume either way, so check how your bulb joins the network.
The fix is to keep the bulb powered and turn the wall switch into a remote. There are a few ways to do that.
Battery switch modules behind the existing switch
Philips Hue makes a Battery Wall Switch Module that sits behind your existing switch. According to Hue's in-wall guide:
- It runs on a CR2450 coin cell, with battery life given as at least five years.
- It needs no neutral, and it requires a Hue Bridge.
- It works with rocker, toggle and push-button switches, but not with built-in dimmers or combined socket and switch plates.
- The original live wires are joined with a supplied push-in connector, so the bulbs stay permanently powered.
- It measures 43 x 38 x 10 mm.
The result looks like a normal switch, but the bulbs never lose power, and pressing the switch sends a Zigbee command instead.
Hue also makes a Wired Wall Switch Module, mains-powered and slightly larger at 41 x 37 x 15 mm. That one needs a neutral, so it does not help in this situation.
A relay in detached mode
Many smart relays can separate their switch input from their output. Shelly's Gen3 devices call this Detached mode: the wall switch reports its state but no longer drives the relay. The relay stays on, powering the smart bulbs, and an automation turns switch presses into bulb commands.
James Ridgway describes exactly this setup with a Shelly Plus 1 Mini controlling Zigbee bulbs through Home Assistant. He set the relay's power-on default to on, so the bulbs come back after a power cut. He also names the drawback: if Wi-Fi or Home Assistant is down, the wall switch does nothing.
SONOFF's ZBM5 smart wall switch has a similar Detach Relay mode. SONOFF says that if the device goes offline, the physical switch takes over normal control again, which softens the main weakness of detached setups. The catch here is wiring. According to SONOFF's documentation, Detach Relay mode and support for smart lights are only available when the ZBM5 is wired with a neutral.
So detached mode is a good answer when you have a neutral but want smart bulbs. In a no-neutral box it usually is not an option.
Direct binding as a safety net
With Zigbee, you can make a battery remote or switch module talk to bulbs directly, without the coordinator in between. Zigbee2MQTT calls this binding. Its documentation says a bound remote keeps controlling the bulb or group even when Zigbee2MQTT, the coordinator or Home Assistant is down. You set it up in the device's Bind tab in the Zigbee2MQTT frontend, or by publishing to zigbee2mqtt/bridge/request/device/bind with a payload such as:
{"from": "my_remote", "to": "my_bulb"}
Not every device supports binding, and the docs say so. Where it works, it turns the most common complaint about smart bulbs ("the switch stopped working because the server rebooted") into a non-issue.
Option 2: a relay built for no-neutral boxes
If you want to keep ordinary bulbs, a no-neutral relay behind the switch is the classic route. Shelly 1L Gen3 is a good example of what to expect from this category. According to Shelly's knowledge base:
- It is designed for installations without a neutral.
- It runs on 220-240 V at 50 Hz, switches up to 200 W and consumes under 1.2 W itself.
- It connects over Wi-Fi 802.11 b/g/n and Bluetooth 4.2, and supports MQTT and scripting.
- A Shelly Bypass is required for all LED lights. It comes in the box and is not needed for incandescent bulbs.
The bypass is a small component wired in parallel with the lamp, at the light fitting, where it connects across the lamp's live and neutral. It gives the relay's standby current a path that does not go through the LED driver.
Australian distributor Synced has a troubleshooting guide for flickering lights on the 1L and 2L Gen3, and it is a useful checklist even if you never buy from them. Most flicker comes from a missing or wrong bypass, or from too little load. Even with the bypass, the LED lamps on the circuit should draw more than 5 W in total. The older white bypass is meant for the first-generation 1L and Dimmer 2, not the Gen3 models. Low-voltage downlights with a step-down transformer are not supported at all.
Zigbee no-neutral switches are end devices
On Zigbee the same constraint shows up in the network layout. Zigbee2MQTT lists the SONOFF ZBMINIL2, a no-neutral switch module, as an end device that cannot act as a router. In a Zigbee2MQTT GitHub issue, maintainer Koenkk notes that end device behaviour is "very common for switches which have no neutral wire". A router has to keep its radio listening all the time, and a device sipping current through the lamp has little power to spare.
The SONOFF ZBM5 makes the trade-off very clear because it supports both wiring modes. Per SONOFF's documentation:
| ZBM5 wiring mode | Zigbee role | Minimum load | Smart bulbs | Detach Relay |
|---|---|---|---|---|
| No neutral | End device | 3 W | Not supported | Not available |
| With neutral | Router | None | Supported | Available |
In no-neutral mode the ZBM5 also does not support inductive loads such as fans. The lesson carries over to other brands: a no-neutral switch will not extend your mesh, and it should not have smart bulbs behind it.
Option 3: replace the switch, or move the smarts
Some wall controls are designed for no-neutral boxes from the start. Philips Hue's wired dimmer switch for non-smart lights works with or without a neutral. Hue's guide also explains the cost: without a neutral, the light can be less bright, some LED lamps may not work and a bypass capacitor may be needed. A neutral is still preferred. The on/off version of the same switch requires a neutral.
The dimmer's stated ranges are worth reading before buying, because they show how narrow LED compatibility can be:
| Lamp type | Trailing edge | Leading edge |
|---|---|---|
| Mains voltage LED | 5-200 W | 5-20 W |
| Incandescent and halogen | 20-200 W | 20-150 W |
Fluorescent lamps and iron-core transformers are not supported. The dimmer appears in Matter as a light, so it can be controlled from other ecosystems too.
There is one more place to look before giving up on a neutral. Hue's guide points out that in some countries, a switch box without a neutral often sits next to a ceiling junction box that has one. If there is room at the fitting or in the junction box, a module that needs a neutral can sometimes go there instead of behind the switch.
Which option fits which room
| Situation | Practical choice | What to watch |
|---|---|---|
| No neutral, want colour or tunable white | Smart bulbs plus a battery switch module or bound remote | Battery replacement, hub required for some modules |
| No neutral, ordinary bulbs are fine | No-neutral relay with bypass | Minimum load, LED compatibility, no mesh routing |
| No neutral, want dimming of ordinary LEDs | No-neutral smart dimmer | Narrow LED range, possible bypass capacitor |
| Neutral available, want smart bulbs | Relay or switch in detached mode | Behaviour when the network or server is down |
| Neutral available, ordinary bulbs | Any standard smart relay | Zigbee models usually also act as routers |
Common mistakes
- Putting smart bulbs behind a no-neutral relay or dimmer. SONOFF says smart lights are not supported in the ZBM5's no-neutral mode, and the same logic applies to other no-neutral devices. The bulb's own electronics are the load the relay depends on.
- Skipping the bypass with LEDs. Shelly requires it for all LED loads on the 1L Gen3. It goes at the light fitting, in parallel with the lamp.
- Counting no-neutral switches as Zigbee routers. They are usually end devices. Plan routers elsewhere, such as plugs or relays wired with a neutral.
- Forgetting the minimum load. A single low-power LED lamp may be below the 3 W or 5 W floor a no-neutral device expects.
- Leaving a detached switch without a fallback. If switch presses go through a server, decide what happens when it is offline: direct binding, a device that falls back to local control, or at least a power-on default that turns the lights on.
- Assuming every bulb is a router or every bulb is not. Check the device page in your Zigbee software before you plan the mesh around it.
The quickest way to choose is to check the box first, then pick the bulbs. If there is no neutral and you want smart bulbs, the wall switch has to become a remote. If you want to keep ordinary lamps, buy a relay made for no-neutral boxes and read its load limits before you read anything else.