Temperature Controlled Switch For Fridge

[kuntushi]

I made a pretty basic temperature controlled switch that I use with an old fridge so I can ferment at set temperatures. It seemed cheaper than buying a wine fridge and/or one of those thermostats from the brew store. I thought a few of you might be interested.

Here is the circuit diagram:

Pretty dodgy looking, but it works. You don't have to run off 12V, but the hysteresis resistor, R6, will need to change if you don't. The hysteresis is set to 2C with that resistance.

Anyway, if you use it as is, R7 is a 0-500 ohm var for tuning what temperature you want the fridge at. Measure between the LM35 GND pin (pin 3) and LM741 +ve pin (also pin 3) to tune the temperature. Temp will be 10mV/C... so 200mV measured between those pins will have your fridge tuned to 20C. It will turn off at about 20C and on at about 22C.

I used an extension cord with it, only cutting the neutral wire and splicing the relay in there. Then I put the relay inside a plastic box with the temp sensor board (with var on it) in the fridge.

If you do attempt this, please only do it if you know what you are doing. By law you need a licensed electrician to install things like this, so you can always ask one to build it for you. You can NEVER be too careful when working with mains AC, ever.

 

[Ivan Other One]

Just found this thread. looks interesting. Have made a few of tricky dicky's kits as a young bloke (power supplies etc) this seems fair easy to make.

Kuntushi, would you care to mention how the components are mounted? Base board? Also wher did you get all your bits?

Cheers Ivan.

 

[A3k]

looks pretty good...

do you have any pics of the mounted unit?

i made one using a PIC. was pretty good, and allowed me to trend my temperatures over time, but got annoyed at it as i needed to get the laptop out just to change the temperature (couldn't be bothered adding pushbuttons to it).
I had always intended on going further with it, making PID out of it, but decided i couldn't be bothered.
then i decided to buy a cheap E-Bay unit, and am happy with it. But it's always cool to make your own thing.


just a quick thought though, aren't you meant to switch the Active?

Cheers,
Al

 

[kuntushi]

I just used protoboard to make it all up then insulated the wires with heat shrink and hot glues it into the box. I basically encased the whole thing in hot glue inside the plastic box because I was paranoid about the AC wires shorting to the DC ones. The DC wires leave the box and are fairly exposed. I bought all the parts from RS components.

I don't have any pics sorry, but it just the board is just in a plastic box with 2 holes, one for power in, one for power out (and the wires to the temp sensor).

As for the cutting, I did infact cut the active. You can cut neutral and get the same effect, but it's much safer and advisable to cut the active. I can't seem to edit my first post, but yes, SWITCH THE ACTIVE! Thanks for pointing that out.

I was going to use a PIC or small micro as well, was even going to add wifi so I could get real time readouts and make changes. I even considered having my PC on with a webserver running so I could see the temp from anywhere and adjust haha. Maybe in the future. For now, it's just controlled by turning the resistor and measuring with a multimeter. I just made it for the fun of it really