Proofing box project -- help me re: relay

Hi everyone,

I'm new to the forum and to Arduino. I am trying to build something that allows me to check a temperature and trigger (on/off) a relay when the temp is below/above a certain threshold. The relay controls a simple lightbulb and it's using 220v. [the goal is to create a proofing box for my baking needs]

I figured out the temperature control part (using a LM35) but since I am very rusty with eletronics I cannot decide which relay to get. Could someone please suggest me a proper relay (and any other component I may need for the relay to work) for my project?

I'll release the code and project under a permissive opensource license (MIT) and give proper credits -- a decent and cheap proofing box is a nice thing for a home baker to have and since it's an easy project too it may interest other people ;)



Okay, so I googled “proofing box” (which doesn’t mean the same thing to a baker as it does to a photographer :)), and I’m guessing you’ll be using an incandescent bulb of about 60-100W to keep things warm.

You’ll want a relay good for at least 1 Amp at 120V (or .5A at 250V if you live somewhere with 220V mains). Double that is better for longer relay life, if you can get it for not too much more (which you probably can). Much more than that is a waste.

However, if I were doing this, I’d go to, and buy one of their solid-state relays. It’ll cost an extra buck or two, but you don’t need to add an external transistor, resistor, and diode to drive it. And the physical design of the package makes it much easier to isolate the AC wiring and reduce the risk of electrocuting yourself (always a plus in my book ;)).

Make sure you put some hysteresis in your control loop: if you’re turning the heat off at, say, 90 degrees, don’t turn it back on until the temperature drops to at least 89 or so. If you don’t, you can beat your relay to death as temperature readings wobble around the setpoint.


Hi Ran!

Thank you for your reply; I gave a look at the website you mentioned but unless I did something wrong the only 220V+ SSR they keep is the 480VAC one, and it's a bit too much :)

So I searched a bit on digikey and others, and I found this LC242R by Crydom (datasheet, pdf: it looks right, doesn't it?



Oops. Sorry, I hadn’t noticed that the less-expensive SSRs at All Electronics were only rated for US mains voltages.

If you’re in Europe, you should be able to find inexpensive mechanical relays through the usual hobbyist sources like Farnell and Radio Schlock. You may be able to find SSRs cheap on the surplus market there, too, but I know nothing about surplus dealers outside the U.S.

Thar Crydom unit is much more than you need, and not really worth its cost for your purposes.

You could also do the job with an inexpensive triac, but I’m hesitant about recommending that to a novice, because it’s so easy to make a harmful/fatal mistake when dealing with mains voltages. Much better to make those newbie mistakes with voltages that are merely painful, like the rest of us did ;D


Hi again!

Well, I ended up buying that SSR I showed you, it wasn’t too much expensive after all ( < 9 eur). I wrote a quick sketch, plugged stuff in and it automagically worked!

I pushed the “code” to github – in case someone is interested. it’s at

I’ll be adding my prototype an LCD and some button to let me enter the temperature without the need to plug it into a computer, and maybe will move from the LM35 to some other sensor being able to read the air humidity so that it could be easily used for other stuff (a greenhouse, a reptilarium, etc.). When I had to google for the LM35 I noticed there’s a lot of people in need of dealing with temps and humidity, so I’m hoping it’ll come handy for them.

Thanks again for your tips, Ran :slight_smile: