How exactly should I build a smart wall light switch for a smart home

Electrical Engineering Asked by mostafanfs on December 10, 2020

I’m planning to build a smart home system and as you know smart wall light switches are a important part of it. I’ve been digging around for almost two weeks but still couldn’t find what I’m looking for.
I have designed a small zigbee module so I can implement it inside of wall switches and make a network of them so one can control entire network with something like a touch panel board or even his smart phone.

Mechanical wall switches:

I believe I can’t use traditional mechanical light switches because they make contacts all the time and they produce big sparks and they would mess everything while I want a reliable wireless link. Besides I need a relay or something else to bypass the switch when it’s going to be controlled wirelessly. Plus one has to consider a small AC/DC to provide DC voltage the wireless module need. I think It doesn’t seem like a very good idea after all.

Touch wall switches:

I searched for touch wall light switches which are not very expensive but apparently they are packed and there is no room for you to implement your wireless modules inside of them. It was nice if I could do that because they have built-in power transistors and you can control the switch functionality with your wireless module. Plus these kind of switches have AC/DC converter circuits inside and one can use it’s output for their specific purposes like powering wireless modules.
But apparently they have their own problems. First of all they are packed and even if they weren’t there isn’t enough space. Second is the matter of stability and life time. Apparently they start showing functional problems after a couple of month and you have to replace them after a while unless you use very high quality products which isn’t worth for my purpose.

Although there are some touch switches with built-in zigbee controller but they have their own remote controller and I guess you can not control them because you wouldn’t know what command to send.

So I think I facing a dead end here. I’m asking you What would you do?
Is there any way around this problem?

2 Answers

EDIT: As suggested

Danger Will Robinson! Mains voltages (or any voltage above 60V) can be very dangerous. If you are toying around with this and get the sudden urge to lick your creation while in operation, I shall not be held responsible for loss of tongue and/or life. And expand upon that as you like for any and all other body parts.

End of Edit

To expand Samuel's comment into an answer, if you use the normal mechanical wall-switch and make sure no wires get stuck anywhere you solve two possible issues:

  1. You have a nice standard-looking switch
  2. You can use dirty tricks to get your low voltage, because the switch already makes it safe.

What do I mean? Well, Samuel said, if I'm not wrong, do something like this:


simulate this circuit – Schematic created using CircuitLab

In the modified situation, when it is as drawn, the light is off. When you flip the relay the light turns on. When you then flip the switch, it again interrupts the path.

If you make that relay a Latching Relay, such as these: EE2-5SNU NEC Relays you can change their connection with just a single pulse, which will not take much power if you keep the pulse short.

BEAR IN MIND! These are just an example I thought of, they are low current and not very high voltage (may do for light lamps or such, but no big halogen or high power LED lamps or Fluo's). But just to give you an idea.

You can then use a small non-isolated power supply, such as a dirty capacitor divider (they aren't allowed any more in production stuff, AFAIK) to get your low voltage.

For more insights into power supply decisions and why some are better or worse, depending on the application, this Stack answer is a pretty decent one:

With the addition that a viable solution could be to take apart one of those tiny block 5V USB Chargers and making them safe-enough for inside your wall: i.e. glueing everything securely in place to make sure no shorts exist. The cheap eBay ones won't be safe to use for normal stuff, but in the wall, behind a mains-rated switch and rebuilt so that no fire can erupt from a short, they are actually nice, cheap solutions to 5V problems.

Because you use a Mains-Rated AC wall switch, you are allowed to power your electronics with any non-isolated solution, because the switch and the wireless module will take care of protecting you. The MCU, Relay and Zigbee module do not care if they are "unsafely connected", because that only relates to you getting zapped or not.

Just during testing/experimenting you would need to be extra careful not to zap yourself. If you are worried that might be one step too far, you might want to use a regulated fully isolated adapter until you're almost done tinkering. Just for your own safety.

Correct answer by Asmyldof on December 10, 2020

You can static use automatic wall switches and cable it up with the relay as a 3way switch that switches are to control lights from two or more places.

Answered by Nirav Patel on December 10, 2020

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