Author: Andrew June 13, 2015

I started on building a Raspberry Pi based fermentation controller.  This is a device that turns my chest freezer on and off based on the temperature of the freezer and what the Raspberry Pi thinks (which is essentially what I programmed it to do).

This is one of those posts that gets a disclaimer: BE CAREFUL IF YOU DO SOMETHING LIKE THIS.  You can get killed by touching the wrong thing here, and it will hurt then entire time you’re dying.  People will not be able to see your bones as you flash from electrocution. Marv wouldn’t have lived. I’m not an expert or licensed or even qualified to do this (and since I’m doing this, does that make me a Redneck)? Your mileage may vary, of course.

There are really only two major parts to this, a solid-state relay (SSR) and an outlet.  I removed the little tab connecting the two hot-sides of the outlet together to allow for one to be on always and the other to be on just when the SSR says it should be.  This is all wired to a plug (I used an old computer cord that was conveniently colored on the inside.  A diagram of the connections is below.

Some notes:

• This should not be hard-wired into the electrical system.  It is probably code to have things like this be easily able to be disconnected, and the plug allows for that.  Also, intelligence dictates that things like this should be able to be unplugged should some failure happen down the line (and since electricity is involved, those failures can be spectacular and bright… and deadly).
• One could make the case for needing a fuse in this.  I don’t think it needs one.  However, this is protected by both a circuit breaker (as is everything in the house) and a GFCI (because the plug it will use is near a sink).  I’m truthfully not concerned, but I don’t know if I’m right to be unconcerned.
• Solid state relays should ALWAYS have heat sinks, and thermal grease between the sink and the SSR.
• SSRs are rated.  It needs to be able to handle at least enough current as the freezer, although I recommend double. My freezer claims 2.5 amps, my SSR is 40 amps. Should be safe.
• Test everything using a safe tester and test method.  For outlets, the $5 3-prong tester like I use is adequate and safe – much much safer than a multimeter.  Truthfully, every homeowner should have one of these (and when my house was built, I used this little tester on every single outlet).
• Everything should be in a box, in case something happens.  Said box should be waterproof as much as reasonably possible.

Pictures are below.

The only problem I ran into was that the $3 Chinese solid-state relay is bad.  The switched outlet is on regardless of the input (although I’m still testing things, so not 100% sure if I’m right, but I’m ordering like 3 more… after all, they’re like $3 and out of the last two I purchased, one is probably bad).  I’ve tested (with the unit unplugged) across the SSR terminals and did not find a short, and across the two hot sides of the receptacle and did not find a short.  But in the case of the SSR, it may not display a short to my tester because it is a much lower voltage than the outlet.

EDIT

The relay was probably fine.  It appears that there is a minimum current required to turn on/off the SSR, and the 1/4 watt tester was not enough of a load.  I replaced the relay (with one of the FIVE I bought), and plugged the freezer into it, despite the new one showing the same signs as the old one.  The freezer did not turn on when I powered-on the Raspberry Pi (I had turned it off to plug it into the box’s unswitched outlet).  I made a quick administrative code edit, and it worked.

Cheers!