How to Transform Your Old Electric Oven Into a Modern Digital Thermostat Controlled One (aka: the Meringues Solution!)





Introduction: How to Transform Your Old Electric Oven Into a Modern Digital Thermostat Controlled One (aka: the Meringues Solution!)

About: I'm an Italian freelance structural engineer, graphic designer and photographer, and I'm teaching physics in Waldorf schools. I always investigate electronics, robotics and science in general, I'm a passiona...

If you ever tried to cook meringues you knot that it's almost impossible if your oven can't keep a constant temperature over the time. Modern electric ovens have a digital integrated thermostat so to achieve that goal with no efforts, but if you like meringues and you have an old oven you could probably feel in troubles.. old thermostats only have a graduated scale, and the only way to maintain the exact temperature is to put a thermometer into the oven, then set the knob consequentially.
A better solution is to buy a digital temperature controller with a thermocouple, and connect it so it will regulate the oven in place of you.

NB: this PID controller support 250V at 3A, it means 750 watt. For any oven more powerful than that value an external 5A relay is needed.

Step 1: The Tools

You need few tools for this project.
To avoid to cut the power cable of the oven you need to use a female socket, and connect it to the thermo controller with some wires. So you need a pair of screwdrivers and a wire stripper.

Step 2: The Connections

Here you can see the diagram of the connections to the back side of the controller. This model works with AC 100-240V, but you can find controllers powered with different voltages. You have to screw the power cables to pins 1 and 2. One of these same cables has to be connected with one of the relay pins (I used the number 3). One of the remain relay pins has to be connected with the female socket. The pin 5 is NC (normally closed) and the 4 is NO (normally open), connect to pin 4 so you'll need to change no parameters in regulator settings.
Connect the K type thermocouple to the 9 and 10 pins, paying attention to the positive and ground wires.
We don't use the "alarm" funtion in this project.

Step 3: The Settings

Many PID (proportional integral derivative) thermo controllers models do much more than turning on and off the relay depending from the current temperature. Usually they predict the tendency of the temperature and act on the switch to reach the final value in the best way.
You can read  the instruction manual of the RKC INSTRUMENT Rex C-100 PID controller.
Holding  "set" you can change the parameters of the controller calculations. I list the values I used:
AL1 (not used): 0400
ATU: 0000
P: 0030
I: 0240
d: 0060
Ar: 0025
T: 0020
Sc: 0000
LCK: 0000
Holding "set" again it will save your values, and pressing "set" a single time it will let you change the temperature value  for your oven (green value).

Step 4: The Match

Now you need to find a proper place where to glue the thermo regulator near the oven. I've secured it at the same oven to take the picture, but it's better to avoid to keep it near hot surfaces, also attach it far from the cooker plates.
The thermocoupler has to be inserted near the cooking surface, so to retrieve the right temperature, you see it in the picture embedded in the grate.
Happy cooking!

Step 5: [UPDATE] the Case

I finally completed the transparent plexiglass snap-fit case,
read the dedicated instructable!



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    21 Discussions

    Hello, I just found your site and am attempting to mod your control into a convection style wall oven. The controls are already electronic that have gone bad and are no longer available. If you get this would you be able to direct me in the process for implementing controls for the broil element seperate from the bake element ( it was a 7 sec broil element on for each 40 sec of bake element ). Additionally the convection fan needs to be switched which of course is far simpler given the fact that it runs continously while the oven is powered on.

    Also this REXC-100 you have in this instructable is it a model ending V*AN or M*AN the V stands for Voltage Pulse ( which I am not clear if that is the correct type of switching for my application ) ... and the M stand for relay contact.

    Thank you for your time


    I could not download the instruction manual from the link provided, but I am quite sure that the build in relay in the PID is only rated to switch a couple of amps. That is not enough for an oven. I would use a separate relay and let the build in relay switch that.

    2 replies

    you're right, I should had specified it in the ible, this PID controller support 250V at 3A, it means 750 watt. For any oven more powerful than that value an external 5A relay is needed. I'll update the instructable, thanks!

    Hi, i know im like 2 years late. But i got a question. My oven is 2000w. Is a reply with 7A sufficient?


    Nice! I've been wanting to try this ever since I cobbed together my sous vide controller. Its always nice to how somebody else has already been there and done that! Thanks!

    thanks verence, this could indeed easily adapted to other appliances, as a fan or an air conditioning system (although it probably already has something similar), and of course the little ovens to warm up the bread in the morning, which usually lack of the thermostat

    I didn't know that you can get PID controllers that cheap. The ones I used for work (years ago) did cost hundreds of dollars. My oven has a working thermostat but just with a knob to set the temperature and without any display. For this cheap price, I'm tempted to do it just for the nerd factor.

    I believe you, that's the reason which pushed me to modify my oven (more than the love for meringues!). Now I'm working on the container for the controller and the socket.

    This would be a great solution to using the oven to raise dough!
    I never understood why my oven manufacturer would have the lowest setting of a high-end digital oven be 160F. I've always wished I could set it someplace between 80F and 110F to let bread dough rise at a controlled temp.


    1 reply

    Yes, it works great, and it's also a very cheap solution. I'm writing a new instructable about the new plexiglas case.


    5 years ago

    No work, like my meringues, must try this soon

    Great idea, but I don't know what a "grate" is that you have the thermocouple attached to. Should it be in the oven somewhere?

    1 reply

    I've been wanting a controller for my thermoelectric cooler and it's nice to learn there is a low cost alternative.

    A step further could include the controller to switch on and off only tehe resistors , some ovens have a small fan that can be kept working to uniform temperature inside the oven.
    We ourselves use brands as Fuji PXR3 , a 1/32DIN controller , half the size that can eve perform temperature patterns as heat to 50º in 3 mins, hold the temperature and rise again to another value and even to "cool" down in a precise program.Try it , you'll be amazed the way these gadgets work.

    Very interesting, years ago ,we did something like that , but for our truck's a/c , worked beautiful , mantained the temperature in the cabin plus-minus 1ºC.

    You must take pictures of the meringues YOU made using this thermostat to prove that it works... :P