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There are smart light switches in the market that are Wi-Fi controlled like Plumlife (formerly ube) and Belkin WeMo. I wonder how they control the electricity that they operate on.

When I unplug my old fashioned light switch, I see that there are two wires. I believe that they work in this way.

Poorly drawn light switch

But in smart switches, there must be some inner circuit that contains a relay inside of the smart light switch. If current goes through that circuit, that means smart light switch is closed too; therefore bulb is on all the time.

How do these products run on and control the same electric wire. If there is an external power source that feeds smart light switch like a fellow stackexchanger mentions in this question , I understand. But there is not. So what I ask is how is it possible to feed a circuit that controls it's own electricity.

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Ekrem Doğan
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3 Answers3

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Most likely it uses a transformer and a relay. The transformer brings it down to a DC current with usable voltage for the wifi and anything else that is needed. The relay then turns on and sends power to the bulb. The power is always going to the switch and the transformer to keep the wifi working.

I would think since the wifi uses such a low power and there are only 2 wires that there is always a small amount of power going through. It must use a current limiter to not let enough power through to turn on the light bulb as it is using the power it needs to let the wifi work properly.

deathismyfriend
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    Is what you suggest something like this one ?

    If it is so, you mean that there are always some current will go through in the light bulb even it is closed.

    What about when relay is on, will there be some sort of short circuit or some oscilation in current flow that may affect smart light circuit?

    And finally, what is current limiter? Is that some IC that I can buy from my local hardware store?

     – Ekrem Doğan Sep 06 '14 at 20:21
  • As commented above, how does the WiFi receive power when the relay closes and the voltage seen on the two wires is shorted. – Andy aka Sep 06 '14 at 23:18
  • That's pretty much the gist of it, yes. The control circuit and the DC rectifier that's providing power to that circuit is in parallel with the AC signal that powers the light as well, so even if the light relay is off, the control still receives power. There is likely some sort of resistance, either from the light (lights are not shorts, per se, they generate resistance, more when they're hot) which is maintaining potential over the control. – Daniel B. Sep 07 '14 at 03:49
  • According to Wikipedia, tungsten bulbs have around 144 ohms of resistance when lit, which is probably adequate to keep the control circuit powered; if not there may be a power resistor of some sort in series with the bulb. – Daniel B. Sep 07 '14 at 03:54
  • @DanielBall Sir, so you say that it can be accomplished as mentioned above with a bulb. Is it till true with flourescents? – Ekrem Doğan Sep 07 '14 at 07:57
  • @Ekrem Doğan Probably. Florescents have some additional hardware, as do led "bulbs." But they should still have some internal impedance. Again this is just speculation; I've never opened one of these to look inside. – Daniel B. Sep 07 '14 at 15:51
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The WeMo web page specifically says that a neutral wire is required!

enter image description here

Another method, which I think 'smart' thermostats use, is to have a battery to maintain power (the battery charges when the switch is off). The batteries will eventually wear out, and Belkin lists the lack of a battery as a feature.

Spehro Pefhany
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  • Ok, let's put WeMo out of the picture. But in Ube, they succeeded this somehow. Please check this video – Ekrem Doğan Sep 07 '14 at 07:51
  • In the Faq page of Ube they say that their product will work in 2 - wire homes.

    Q: Will this work in 2-wire and 3-wire homes?
    A: Yes

     – Ekrem Doğan Sep 07 '14 at 08:36
  • @EkremDoğan promotional videos aside, it's not clear to me they actually do this with a viable prototype (they apparently do not have a shippable product yet, after a couple of years). Two wire and three wire refers to the presence of a ground not neutral. I can think of ways to do this technically but I'm afraid the disadvantages would outweigh the advantage. They'd have to leach power both in series and parallel (difficult to do reliably give the variety of loads .. And possibly switches in series). – Spehro Pefhany Sep 07 '14 at 12:16
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The simple answer is that the switches' electronics are in parallel with the load, which it controls via a SSR or triac, solid state devices, instead of a mechanical relay.

schematic

simulate this circuit – Schematic created using CircuitLab

It is in parallel because these switches normally require the neutral wire be connected, and typical house wiring has both wires going to the light switch box, then run to the light fixture.

Passerby
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  • How can it be in parallel when neutral conductor is not connected? – venny Sep 07 '14 at 08:07
  • @venny yes, that question popped into my mind, too. – Ekrem Doğan Sep 07 '14 at 08:17
  • @Passerby What you suggest requires an additional third wire in the wall. In the case I ask, there are only two wires. Please check this picture from my home. – Ekrem Doğan Sep 07 '14 at 10:06
  • @EkremDoğan this is how a standard light fixture should be wired, imho http://www.diyhowto.co.uk/images/projects/light_sw2.jpgIt's technically a single cable, cut in the middle. Apparently, wiring to the fixture, then having a single cable run to the switch is somewhat common, but that seems wrong to me. – Passerby Sep 07 '14 at 18:00