With this instructable, we will take a cheap toaster oven and turn it into an accurate, temperature controlled tempering oven that will be able to achieve a stable and accurate temperature controlled by a microprocessor.

We will combine some common and off-the-shelf components to easily and safely achieve this goal.

Why do we want to do this? After heat treating steel, it's in a very hard and brittle state. So we can use it with less risk of it snapping, we want to temper it. Tempering will slightly soften steel, but more importantly will restore flexibility to it. Most steel manufacturers specify particular temperatures to temper steel at so you can achieve a given hardness rating.

Having the temperature accurate to a fraction of a degree isn't critical for tempering, but I want more accuracy and repeatability than the simple bimetallic strip thermostat that most inexpensive ovens use. ±10°C could easily be a difference of 1-2 Rockwell C points in hardness.

Step 1: Requirements

Here's what you'll need

Toaster Oven
PID Temperature Controller
Solid-state Relay
Temperature Sensor
Insulation

Step 2: Toaster Oven

I was aiming for something pretty low-cost for the toaster oven. I specifically don't want anything with fancy digital controls or anything like that as I'm only going to be bypassing them.

I wanted a toaster oven that has a shield over the heating elements. In an oven with an exposed element, you have radiative heat (essentially infra-red light) where heat is transferred directly from the heating element into the workpiece in the oven. Whilst this heats the part up nice and quickly, and is often what you want in a toaster oven as it will give you a nice crispy exterior, it's harder to measure an accurate temperature of the workpiece as the air temperature in the oven (that the sensor is measuring) will often be lower than the temperature of the workpiece. I couldn't get the exact one I found online that had a shield, so I'll have to fabricate one myself.

Convective heat transfer (or, more simply, convection) instead means that the heating element is heating the air and the air is heating the workpiece and the sensor. This is less efficient and slower, but it allows our sensor to more accurately measure a temperature that will be the same for the workpiece. Speed of heating is less of an issue in this case as tempering cycles are typically 1-2 hours each, so a few more minutes to heat up at the beginning doesn't make a lot of overall difference.

Radiative Heat Transfer

Convective heat transfer

Step 3: PID Temperature Controller

This is the bit that works the magic to make the whole thing sing.

I have gone with an ITC-100 temperature controller from Ink Bird. It's widely available, relatively inexpensive and seems to be pretty well supported.

Without this, the toaster oven either has a wildly inaccurate temperature control or no temperature control at all. The PID is able to take many measurements of the temperature over time in the oven (the particular model I've chosen samples the temperature twice per second), it works out what the current temperature is, it knows what I want the temperature to be and it can see how fast it's heating up or cooling.

Armed with this data, the PID can then ramp up the temperature and when it's getting close to the temperature I've dialled in, it will slow the rate of heating (by turning the heating element on and off) and then keep the temperature at a stable level (again, by turning the heater on and off). As it's able to calculate the effects of inputting heat, it can maintain a constant temperature (generally within 1°C) whereas using a traditional bimetallic strip thermostat, it will swing up and down by 10° or more.

Step 4: Solid State Relay

The Solid State Relay, or SSR, is essentially a switch with no moving parts. By using a small trigger voltage (from the PID) it is able to switch a much larger voltage (240VAC from the wall).

The PID I'm using has a built-in SSR, but it is only able to switch up to 3A of current - as I'm running on 240V that limits me to around 700W. Most toaster ovens are at least twice this, so I'm using an Ink Bird SSR that's capable of switching up to 25A. This is far more than I'm going to need, but keeping the current lower than the maximum will extend the lifespan of the device and keep it running cooler. I'm also using a heatsink on the SSR to further keep it cool.

Furthermore, you can never be too sure about the actual ratings of eBay components, so with a 1600W oven, I'm only drawing 6.7A which gives me plenty of headroom, even if the component is only suitable for half of it's rated load.

Step 5: Temperature Sensor

The PID I'm using came with a Type K Thermocouple which will suffice to get everything up and running.

This thermocouple is inexpensive and rugged, although not as accurate as some other ways of measuring the temperature.

I have ordered (but not yet received) a 3-wire PT100 resistance thermometer. Instead of relying on the thermoelectric effect to measure the temperature, a PT100 sensor instead has a tiny platinum coil (the PT part of it's name) with a known resistance of 100Ω at 0°C (the 100 part of it's name) and the resistance of this coil varies in a stable and repeatable fashion with changes in temperature.

The PID can be programmed to use almost any commonly found temperature sensor, it defaults to a Type K but can easily be changed to PT100. It is very important to let the PID know what kind of temperature sensor you're using as they all behave very differently and you will get wildly inaccurate readings if you're using the wrong type.

Step 6: Insulation

Most cheap toaster ovens have little to no insulation. They're a metal box for the oven cavity and then an air gap between that and the outer shell.

In order to help keep a more stable temperature in the oven, and to protect the electronics in the PID from the heat of the oven, I've taken the shell off the toaster oven, wrapped the oven cavity in high temperature insulation wool (HTIW) and then put it all back together again.

The fine ceramic fibres in HTIW are really not good to breathe in, so ensure you wear appropriate protective equipment when working with it.

Isowool is an Alumino Silicate Wool (ASW) which is classed as hazardous. It may cause cancer by inhalation and is irritating to the skin. It only takes a minute to put on your safety gear, so play it safe. Do not breathe the fibres from it and protect yourself against skin and eye contact.

Step 7: Connect the Components Together

Before you put all of this gear into your toaster oven, you should first check that it's all working correctly, that you have the relevant configuration in the PID and that you have calibrated your temperature sensor.

How it all goes together will vary depending on the PID and sensor that you have. If you have the Inkbird PID and the Type K thermocouple, then follow my instructions.

WARNING - YOU WILL BE WORKING WITH MAINS VOLTAGE - THIS IS REALLY DANGEROUS IF YOU DON'T KNOW WHAT YOU ARE DOING.

IF YOU ARE NOT SURE HOW TO CONNECT EVERYTHING, PLEASE GET AN ELECTRICIAN TO WIRE IT UP FOR YOU.

With that out of the way, let's proceed.

In the instruction manual for your PID and, possibly on a sticker on the case, will be a wiring diagram like the one above. If you know how to translate it, wiring everything up is quite simple.

Let's start with the temperature sensor. If you are using a 3-wire sensor, then it connects to terminals 3, 4 and 5. Most 3-wire sensors (generally resistance type sensors like a PT100) will have two leads that are one colour and a third lead in a different colour. If you're using 2-wire sensor, then it connects to terminals 3 and 4 and you put a short wire link between terminals 4 and 5. Some PT100 sensors are 2-wire and all thermocouples are 2-wire. A 2-wire PT100 sensor is likely to be slightly less accurate than a 3-wire version.

I've got a Type K thermocouple, so it's a 2-wire sensor. Thermocouples are polarised, one terminal will be positive and one will be negative. PT100 sensors are not polarised, so can be installed either way. My Type K sensor had a red and a blue terminal. I guessed that red was positive and blue was negative and it was correct.

The temperature sensor is the trickiest bit, and you're not going to blow anything up if you get it wrong, so hook it up how it looks like it goes and then test it to make sure it works as expected. If you have a thermocouple connected the wrong way, it will read negative temperatures, not positive. If you have a 3-wire sensor hooked up the wrong way, it will probably just give you a constant, low reading that won't change with temperature.

For the Power input (terminals 9 and 10) check what voltage your PID expects. Mine will take anything between 110 and 240V AC (and you can power it off DC as well). I have a mains lead that I have stripped the ends off to connect it temporarily. When it's in place in the toaster oven, I'll be using some of the internal wiring to power it. In this case, it doesn't matter which one you hook up to Live and Neutral, so connect them up, tighten the screws and make sure there's no bare wire exposed.

Lastly, connect the SSR. Connect terminal 8 to the + input and terminal 6 to the - input on the SSR. You won't be switching anything with the SSR at the moment, but you can watch to see if the LED lights up when the PID switches the output on.

Step 8: Calibrate Your Temperature Sensor

Out of the box, the PID and sensor will probably give you wildly inaccurate temperature readings.

Fortunately there are two things that you should have easy access to that will ensure you can get a pretty accurate calibration. Definitely accurate enough for the purpose of running the tempering oven. What are these magical, mystical items with a known temperature?

Ice Bath Boiling Water A properly made ice bath can be within 0.1°C of 0°C and boiling water will be almost spot-on 100°C at sea level, however this will vary with your altitude above sea level. I recommend starting with an ice bath and then checking with boiling water.

To make an ice bath, you need a container FULL of ice and then 3/4 topped up with water. You MUST have ice going all the way to the bottom of the container and extending above the surface of the water.

Then, once you've added the water to the ice, give it a minute or two to cool, and stir it well.

While you're measuring the temperature, you want the tip of the probe to be in the middle of the water and ice, and keep stirring it around. You don't want it to be up against a piece of ice (which will be colder than 0°C) and you don't want it up against the sides of the container (which will be warmer than 0°C) and you don't want to keep it still as it may slightly warm up the water it's in.

When I first hooked up my sensor and chilled it in an ice bath, it was reading something like 74.3°C for PV (the Process Value, or the current temperature of the process). Don't be alarmed that it's not reading 0. Every thermocouple is slightly different in the voltage level that it will produce at a given temperature. Fortunately the PID has a Sensor Calibration setting (Sn on the LED Display) for this offset.

To program in the offset of 74.3°C (or thereabouts, it was fluctuating a bit even in the ice bath), hold down the SET button for more than 2 seconds to enter setting mode. Once you're in setting mode, keep pressing the SET button until the display reads Sn on the upper display and 0.0 on the lower display.

Using the arrow buttons put in a value of -74.3 - or whatever you were reading in the ice bath, but make sure it's negative if the value it was displaying was positive, or if the PV temperature reading was negative, then put in a positive value. Then, press and hold SET to cycle through the rest of the setting blocks and back to the home screen. This will lock in your temperature offset and this setting will be retained even after you power the PID off.

To check that the calibration has worked, boil some water in a saucepan on the stove.

Once the water has come to a boil, dip the sensor in the boiling water (and be careful to keep your fingers out!). Stay well clear from the sides of the saucepan, make sure the sensor is in the middle of the water. After 10-20 seconds or so, the PID should be reading pretty close to 100.0°C on the top PV LED Display. If you're reading within, say, half a degree of 100.0°C then you're good to go. If it's out, go back to the ice bath (making sure there's still sufficient ice in the bath) and start the calibration process again.

Step 9: Check the Programming in the PID

Now that your temperature sensor is calibrated, you need to check the programming in the PID.

Out of the box, my PID has a SV (Set Value) of 50.0°C and the output set to Heating.

So, to recap, PV is the Process Value - in this case it is the currently measured temperature. SV is the Set Value - the temperature you want to achieve. The PID needs to know if it's controlling heating or cooling. If it's set to heating then it will turn on the output when the Process Value is lower than the Set Value - it wants to heat things up to the SV. If, however, it's set to cooling, then it will turn on the output if the PV is higher than the SV - it wants to cool things down to the SV.

To configure the mode for heating or cooling, hold down the SET key for more than 2 seconds, and then press it repeatedly until you see CF on the LED. Use the arrow buttons to change the value to either 2 for heating (the default) or 3 for cooling. We want it to be set to 2 as we're controlling a heater. Then, hold down SET to cycle through the other setting blocks until you're back at the home screen.

To configure the Set Value, when you're on the home screen (PV reading current temperature at the top and SV reading set value at the bottom) use the arrow buttons to change the SV. PV will probably be giving you room temperature at this stage, assuming the probe isn't still in the ice bath or boiling water. If you set the SV to a temperature less than PV, you should see the OUT LED light up on the PID and at the same time the input LED should light up on the SSR. Play around with the SV and make sure that when the OUT light on the PID lights up that the input LED on the SSR lights up too, and when the OUT light goes out on the PID that the LED on the SSR goes out too. This confirms that the SSR is receiving the correct input signal from the PID.

Step 10: Install the PID and SSR Inside the Toaster Oven

Now that you've verified that everything is working, you can take the toaster oven apart and install the PID, SSR and temperature sensor.

Your toaster oven will probably be different to mine, so you'll need to work out the best way to take it apart (and put it back together again!)

With mine, I couldn't get the sides of the case off until I'd taken the front off. I couldn't get the front off until I'd undone the spring that holds the door closed (and this was pretty tricky).

Once I got it all apart, I had to then make a cutting template the same size as the PID. My PID is a 1/16 DIN which is 45 mm x 45 mm. I cut out a 45x45 square from some cardboard and using the cardboard as a template, marked a square where the PID would go.

Luckily for me, the thermostat dial on the front panel was ~45x45 so this was the perfect place to install the PID.

I marked the square to cut and then slowly and carefully cut it out with a Dremel and a cut-off wheel. WEAR EYE PROTECTION when you're using the Dremel. Make sure you're not cutting with a grinding disk or grinding with a cutting disk or you are more likely to break it. Also, with the cutting disks, only cut straight lines. They are incredibly brittle and if you try and cut a curve (not that you need to for this project) you'll probably break it and have little bits of disk flying everywhere at very high speed.

Once I had my square cut out, I broke the sharp edges with a file so I didn't cut myself when fiddling around with it.

Step 11: Wire It All Up

This bit is pretty fiddly and difficult to describe what to do as your toaster oven will probably have different wiring to mine.

Here's how I went about working out what went where.

I located the point where the mains wiring came into the case. From here it went first to the thermostat and then from the thermostat to the selector switch, from there to the timer switch and then on to the heating elements.

Seeing as I'm just replacing the thermostat, and I've already wired up the other connections to my PID to test it, I have to take the two wires that go into the thermostat and instead put them through the OUTPUT connectors on the SSR. This means that the PID is switching the SSR on and off, and this switches the power to the heating elements that was otherwise switched by the thermostat dial.

If your toaster oven doesn't have a thermostat (lots of cheaper ones don't) then you'll need to wire the OUTPUT connectors on the PID in series with the mains power to the heating elements.

Don't worry if you don't have one with a timer, I would have preferred one without a timer as well, but this was the cheapest one I could get.

I have used some regular mains cord to make the extra connections because I plan to put thermal insulation in to protect the PID and the wiring.

If you have access to high-temperature wiring, I'd recommend using that as it's going to be safer in the long run.

So, to recap, I have wired the SSR in place of the old thermostat. The PID then switches the SSR on and off which replaces the functionality of the thermostat.

I also took a lead from the mains input and sent this to the power inputs on the PID as it needs power to run as well.

Step 12: Add Insulation

First up, locate your safety equipment. Mask, glasses and gloves at a bare minimum. If this stuff gets in your lungs you can get cancer. If it gets into your skin, you'll be really itchy. Just take a minute now to get safe.

Next up, locate your supply of High Temperature Insulation Wool (Isowool, Rockwool etc).

I measured out a strip that was the depth of the oven from front to back and it was wide enough to cover the top and right-hand side.

I did want to cover the left-hand side too, but there's the spring that holds the door closed, and I didn't want to have the spring breaking fibres off. I couldn't cover the rear as there wasn't a separate outer case over the back.

I had to take the whole control panel off, and then carefully and gently get the insulation behind all the wiring and against the side of the oven. It was really fiddly to do, especially as the only gloves I could find were welding gauntlets - at least my arms were covered too.

Once the insulation was in place, I put the outer case back on and it all held in quite well.

Step 13: Put It All Back Together

In putting it all back together, at this point you want to try and add some insulation. I'm going to fill the void in between the oven cavity and the outer case with Isowool. You'll notice a distinct lack of Isowool in my photos - that's because I don't have it on hand yet so I'm going to have to take it apart and add it later.

I can't really run the oven like this - the compartment where the PID lives is designed to get warm and it's likely too warm for the PID.

I will be putting a sheet of Isowool on the oven side of the cavity where all the controls live. This will regulate the temperature in there, keeping it within the operational range of the PID and ensure that my wiring doesn't melt.

I will be packing Isowool in between the oven cavity and the outer case on the top and left-hand side, this will help regulate temperatures inside the oven. I'll also see if I can find some way to put Isowool on the rear of the oven as well - the better insulated it is, the better the PID will be able to keep an even temperature.