Well I have a cold laundry room and the thermostat on my heater has broken down so now I can choose between heat all the time and the electric bill that comes with that or to have it freezing cold in there or of course buying a new heater that I can’t afford because of all the electrical components that I’ve been purchasing lately… I thought why not make my own thermostat…
What’s Connected
I have a Raspberry pi 3 model B, MCP3008 (analogue converter), TMP36 (thermometer), Relay, Breadboard, Jumper wires and a broken heater.
Triggers & Alerts
I used triggers to set at what temperature to turn on and off the heater.
Only waiting to be moved in to the final location and hooked up to the motion sensor light that I’ve made previously for the garage that is next to the laundry room. Just need to get some soldering on to hook it up some more permanent wiring so I can enjoy both light and heat.
This is a pretty cool project, but I’d be careful with that setup. Heaters can draw a lot of current, so you need to make sure you have less than 8A drawn from the heater. Those relays you are using are rated 10A, but that doesn’t mean the circuity on the boards is also rated for 10A. If you start to go over 10A at the best you’ll have a burnt out relay (experience), at worst you can start a fire. I’d suggest studying the actual thermostat and mimic its output instead of just turning the power on/off and use the relays/circuitry that is already in the heater and rated to do that job.
It’s a 2000 watt heater so when I started I used one of those online watt → amp calculators (to sick to do the math myself) that returned 8,6 amp maximum. I using the heater on the 1000 watt setting so it shouldn’t and hopefully won’t draw more than 4,3 amps.
You could take a chance on it. It would be best to see how wide the traces on the bottom are. The problem with most relay modules like that is the traces on the board can’t handle 30A.
You never want to exceed printed limits. I’m assuming he is using the relay below, or at the very least the actual relays are the same. They are rated for 10A no matter what the voltage up to 250VAC. Any more than 10A at any voltage starts to melt things. Going over 250VAC runs the risk of arcing in the relay which leads to the relay being on in any state.
My concern is what happens if the heater is on and your internet goes down?
In any situation like this, it is essential that you have a local failsafe. A hardware temperature sensor that turns off the heater when it hits too high a temperature, for example.
I work on industrial control systems like this and we operate on multiple levels of redundancy.
Best practice is to ask yourself, “When X system fails, am I still safe?”, and if the answer is no, implement at least one backup plan.
On the Arduino systems, we have virtual I/O, so you can easily setup local timers to shut the heater off after so many seconds or when the temperature exceeds, but even then, I would make sure there is also a hardware based over-temp switch.
I think I would be safe, the thermostat broke last winter and only resulted in a more than average electric bill, but I could always add a second plug in the wall thermostat.
I think I read in a post that Cayenne has on the roadmap some offline runtime that would act as a backup against internet going down.
There is also fire alarms if shit hits the heater so too speak. Me and my girlfriend also walk through the laundry room at least twice in the morning and twice at night to and from work and would then notice if the setup isn’t running properly and would turn it off and leave the door open instead and let the rest of the house warm up that space to even if it isn’t really enough.
Just had a quick look now, and it seems like something I will do my best to keep in mind for some of the upcoming projects. Can do some upgrading on this too.
More of a teaching moment than anything. As we continue to build more and more things with Cayenne, we have to be clear that remote communications and even software for that matter will fail from time to time - even the most well tested systems. I crash Windows at least once a day for instance
Anyway, your heater will have a high temperature limit switch if it is an approved heater anyway. Fire alarms make it triple redundant. LOL.
