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Safety First! Switching 120vac loads with a microcontroller

Inside Rib

Outside Rib


Rob writes in…

I’ve seen quite a few hacks related to controlling appliances, lights, etc over the years and just wanted to share a little info so that everyone has access to a cheap way to do it relatively safely. By trade I work in the building controls/integration industry and as a result I use these relays at work and at home(chicken coop control,light,etc)quite a bit.

The interface between your microcontroller of choice and the relay is a simple 555 relay driver circuit. I have included the pdf that inspired me to do it this way. The relay I use is the RIBTU1C. The reason I prefer this relay is that the coil will run on 9VC @ 20mA and the contacts will switch 10A @ 120VAC. Total cost for the RIB and a 555 is under $15 if you shop around. In addition the RIB has a partition inside the box between the line and control sides. There’s also room for a Radio Shack breadboard in there!

PDF here.

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  1. I love seeing the ways different people solve problems.

    I’m also working on a small project for switching 120VAC right now.

    I’m using a S102S01 Triac Output SSR. It’s rated at 8A, and operates on a forward current of 20mA at 1.2-1.4VDC. DigiKey is out of these (but they have the S202S01 rated for 220VAC), but Mouser, Arrow and all have them in stock. These SSRs are under $5.

    I’m then using a small power transformer, rectifier, regulator, etc to power the AVR I’m using as the controller. I’ll be hooking up a DS18B20 (from Adafruit) and using this device to regulate a freezer (that was going to be trashed) that we’re using as a frige here in Afghanistan.

    I’ll have to document the build when we’re done.

  2. *fridge

  3. Maybe I am a little slow, but I don’t actually see a 555 in the pic, actually I think I see a transistor. There looks to be a small black box like thing the the bottom left of the relay but it doesn’t look like a 555.

  4. Nice article. I am curious why not using a triac as Inventorjack described above? Also, if you prefer using a relay, how about using a MOSFET to drive the relay. It consumes almost no current and can sink a large amount of current.

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