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I'm working on a project where I use a Raspberry Pi to build smart home devices. One of the devices I want to control is my room's light switch. I've done a bit of research and found out that my light is single pole switch with an identifiable positive and negative lead. My plan is to get rid of the switch and replace it with a relay.

My problems:

  1. The relay isn't able to hold two states unless I tell it to close the circuit continuously (which I heard isn't good for the contact pads inside the relay.)
  2. Can my relay handle the voltage coming through a light switch?

My relay is a JQC3F-5VDC-C

Single Pole Light Switch

Relay

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  • \$\begingroup\$ The relay is rated for 10A 250V. The wires you have on the relay aren't. Besides unless you are an electrician and have permit from the building owner to change the wiring, you should not do this. Since you had to ask, I suppose you are not an electrician and I recommend not to rewire mains wiring yourself. Get a proper electrician to do the job. \$\endgroup\$
    – Justme
    Commented Dec 3, 2020 at 22:50
  • \$\begingroup\$ Hey, thanks for the response! I knew about the wires on the relay not being suited for the application, it was just what was lying around from an old project. Since adding a relay at the light switch doesn't seem to be the safest option, are there any other spots I could add to (maybe where the actual light bulb is screwed in)? \$\endgroup\$
    – Siddappa
    Commented Dec 4, 2020 at 1:07
  • \$\begingroup\$ Continuously running the coil "isn't good" except for how defined in the datasheet. It should have on time/lifespan and/or number of times it can switch on. You could also get bistable or latching relays if the coil on time is a concern. These complicated the design. \$\endgroup\$
    – Passerby
    Commented Dec 4, 2020 at 6:46

4 Answers 4

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A relay may be used, instead of a switch, to control a light.

The relay coil is to be kept energised for as long as the light is to remain lit. A standard electromagnetic relay would be designed for continuous duty with respect to its coil and contacts and would not get damaged, provided that its voltage and current ratings are not exceeded and snubbers are used to minimise damage to its contacts while switching inductive loads. Likewise, with capacitive loads, it should be ensured that the contact rating is not exceeded, while high charging currents are being switched, that could result in welding of contacts.

Latching relays may also be used to avoid coil consumption after the switching action has taken place. A latching relay would require only a pulse input to its coil / coils, for it to be set or reset.

Considering its voltage and current ratings, the relay referred to would be good enough to control AC/DC domestic lights.

It would be advisable to take the assistance of competent personnel to carry out the modification.

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Where is no positive or negative wire. It is AC. One wire come from electrical panel, another going to your light. Work with that you must be sure, the circuit is not energized. Even what condition on picture dangerous if it is powered. It is electricians work. In North America electrical code not allow low and high voltage in same box without separation. Also each electrical device must have visual disconnect.

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At Lowes and Home Depot there are a large number of remote-controllable light switches and even individual light bulbs. I've never drilled down into what technologies and protocols they use, but it probably is something that you can drive with an rPi. If nothing else, you can take apart the remote control unit and drive its switch contacts with outputs from the rPi. Way safer than dealing with house power.

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  • \$\begingroup\$ I've thought of this route but never wanted to do it. The point of my project is so that I can make smart home devices without buying an outrageously priced wifi light bulb. I also wanted to expand my electrical engineering knowledge \$\endgroup\$
    – Siddappa
    Commented Dec 4, 2020 at 1:27
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    \$\begingroup\$ Also note that many of those cheapy relay boards have inadequate creepage distance. This means the distance between the mains circuit and your low voltage circuit is not enough to guarantee safety. \$\endgroup\$
    – Kartman
    Commented Dec 4, 2020 at 2:13
  • \$\begingroup\$ @siddappa there are wifi enabled light adapters that screw in and ones that replace the switch and also ones that go over a standard switch so it does not actually modify anything in the wall. \$\endgroup\$
    – Passerby
    Commented Dec 4, 2020 at 6:42
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Siddappa, I would agree, this is a good way to learn. Here's the short story. You do have a single pole switch. AC wiring colors are different than DC. White is neutral, black is hot, red is hot from the light fixture. In the commercial/industrial world we purchase special relays for this. Unfortunately, they will not fit in the back of a double gang box like yours. So, you'd have to mount it above the ceiling at the light. Each has a low voltage trigger relay, or circuit, which pulls in a heavy duty relay designed to remain either on or off indefinitely. The relay you have is not designed to do so. Many cheap devices try and use the wrong relay and either last about 1 year, or cause a fire. So, after you've played around, replace your device with something tried and true. For examples, go to the kele.com site and look under Lighting Controls, and look under Relays at the Functional Devices RIB relays. Also, go to the functionaldevices.com website and read up. Good luck and at least wear your PPE while learning with 120V - it does hurt!

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  • \$\begingroup\$ please disclose your relationship with the kele.com site (whether you are or aren't' associated with them) Thanks \$\endgroup\$
    – Voltage Spike
    Commented Dec 4, 2020 at 22:39
  • \$\begingroup\$ Thanks for the tip! I've looked into the site you mentioned but there doesn't seem to be any section you're talking about. Also, my relay says it's rated for 10A 125VAC which should be plenty of headroom for a 3 light fixture (I'm still not sure if I'm reading those numbers right). Finally, where is the best place to get electrical PPE \$\endgroup\$
    – Siddappa
    Commented Dec 5, 2020 at 5:56

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