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, now I'm teaching physics in Waldorf high-schools. I always investigate electronics, robotics and science in general, I'm a passi…

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!