Dimmer/Motion - one device or two

I will be adding two Smart Dimmers some time this month. I want motion capability.
Is there any preference between a Dimmer/motion switch (1 device) or a Dimmer switch and a separate motion detector (2 device). I’m primarily concerned about lag time between the two.

Different people have different preferences, and in some cases it will depend on the specific placement. You’re right that the single unit devices are almost always slightly faster. But on the other hand if you use two devices you can place the motion sensor exactly where you want it to be which may counterbalance that.

Normally a single unit will be less expensive than buying a smart switch and a separate motion sensor, but the single unit has a very different look to it and some people may prefer the aesthetics of a “regular“ looking switch and a fairly hidden motion sensor.

Single unit device with built-in motion sensor:


Smart switch without built-in sensor:

We should also note that as of this writing, there are no single unit models on the official “works with SmartThings” list, which may mean they will not work with the new app. So that’s just something to be aware of.

Thanks for the replies. Any idea of a ‘typical’ lag time with two units. Is it measured in tenths of a second or in seconds?

Depends on the specific devices, the local conditions, and how you have it set up. So there’s a lot of variation.

I forgot to mention one of the advantages of the single unit devices, and that’s that the motion sensor is being powered from the mains, so it doesn’t have a “sleep” period. That in itself would probably save you a second in most situations.

There are other mains powered motion sensors that you can use, but most people use battery operated ones. So it just really varies a lot. The shorter the sleep period For a battery powered device, the faster it uses up the battery, so it also depends whether you are willing to trade quicker response for shorter battery life.

The usual target time would be under three seconds, and most people with a strong enough mesh should be able to achieve that. But three seconds can feel like a really long time.

Most people want it to be under half a second, and to achieve that depends on exact placement of the single unit device relative to the doorway.

It also makes a huge difference if there is a door that gets opened before you go into the next room because the door itself may trigger the motion sensor (a good thing). But if you are walking from the hallway into the kitchen where there is no door, then it’s a question of where you’ve placed the device as to how long it feels to the person like the lag is. That is, it might not be true lag, the time between when the motion sensor is triggered and the light comes on. It might be an apparent lag caused by the person not having yet entered the detection field.

This is a classic problem with single unit devices in bedrooms where the door is typically left open until someone goes to bed.

Many rooms have a switch in the perfect position in terms of motion detection: perpendicular to the person walking into the room.


But other rooms have a “reach around” switch position. As you’re entering the room, you can put your hand through first and feel where the switches and turn it on. Those are awful for single unit motion detection. The person would have to walk all the way into the room in the dark before the switch comes on.

image image

( I myself use a wheelchair, so I am very aware of these issues.)

If the switch is in a reach around position, having a separate motion sensor that you can place, typically on the ceiling right before you enter the room, will result in the light coming on much more quickly.

So it’s just hard to say.

It is common in homes with motion sensor lighting that people who live there learn to slow down a little or wait just a beat until the light comes on to allow for that two second pause. But once you get to four seconds, you get a lot of complaints.

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Perfect answer, thanks.