For those who don't already know, sous vide is a way of cooking that requires a carefully temperature-controlled water bath. The trouble is that a "proper" sous vide water bath costs upwards of around £450, which feels rather steep for such a simple device. There have recently been some new, cheaper sous vide circulators which you just immerse in a tank of water and let it control the temperature regulation and heating, but these still cost around the range of £150-£200.

The idea of this project is to turn an ordinary slow cooker or rice cooker into a sous vide cooker for around £25. The project also aims to leave the slow cooker/rice cooker entirely unmodified, so that it (a) it can still be used for its original purpose, and (b) you can easily replace or upgrade the cooker itself if, say, you wanted a larger capacity for your sous vide cooker. It also means that the actual heating side of things is handled by a commercial appliance, which significantly reduces the complexity and the inherent risk of the build itself - you don't need to worry about waterproofing or earthing, for instance.

The idea is that you insert this device between the slow cooker/rice cooker and the mains, and the device turns the power to the cooker on and off as required to maintain the temperature as needed. It's not a new idea at all, and Googling "DIY sous vide" will turn up many similar projects, but there aren't too many good write ups of the whole build process.

This is a nice, simple project that requires only a little in the way of tooling - but it does involve working with mains electricity, so obviously you need to be careful. If you are not competent at dealing with mains voltage electrics and aware of the risks involved and safety precautions required, you are strongly advised not to attempt this project!

Step 1: What You Need (and Where to Get It in the UK)

(Incidentally, I'm not affiliated to any of these suppliers!)

Parts list:

Temperature controller, STC-1000, including temperature probe*: £9.54, inc P&P (I got mine from eBay. You can also easily source these from Amazon, Deal Extreme, DHgate, LightInTheBox, AliExpress or indeed pretty much any of the major direct-from-China suppliers). The advantage of using the STC-1000 is that it's a really commonly used one, so (a) it's really cheap and (b) there's tons of help available online if you need it. There's even an open-source, reverse-engineered firmware available for it if that's your sort of thing.
ABS enclosure, 120mm x 120mm x 90mm outer dimensions: £6.80, inc P&P (eBay, Maplin, CPC, RS - pretty much anywhere that does electronics components)
M12 Cable gland: £1, inc P&P (eBay, but you can also pick these up from Wickes, CPC or RS)
1-gang plasterboard back box: £0.68 (Toolstation or any DIY place)
1-gang unswitched socket: £1.32 (Toolstation or any DIY place)
Terminal block (choc-block), rated to at least 10A. I had some lying around, but you can get a strip rated to 15A for £0.64 from Toolstation, or check most DIY places or RS)
Ordinary 13A fused plug**: £0.74 (Toolstation or any DIY place)
1m (or longer if you prefer) 1.0mm^2 3-core mains flex**: £0.60 (You can buy this by the metre in store at Wilkinson, or buy it in longer lengths at any DIY store). 1.0mm^2 cable is rated to 10A at 230V (the switching circuit on the STC-1000 is only rated to 10A, so there's no need to go for cable thicker than this. If you do choose to go with 1.25mm^2 or 1.5mm^2 cable instead, be aware that you may need to get a larger cable gland).
Small rubber grommet (optional): I paid £2.25 for an assorted set of them from Toolstation, but have a look on eBay or RS). I don't think it's absolutely necessary, but it just neatens up the way the temperature probe exits the enclosure, and adds a tiny bit of strain relief.
An ordinary, manual rice cooker or slow cooker. What is important is that it heats the bowl when the power is turned on without having to press any buttons, as the control device we are building here will simply turn the mains power on and off. The cooker should be rated at less than 2.3kW power, as the cable and temperature controller are only rated to 10A current, but I've yet to find any rice cooker or slow cooker that gets anywhere close to this power.

The total cost for parts came to just £22.93, including the complete rubber grommet set but excluding the terminal block and rice cooker (which I already owned).

*In case you ever need to replace the temperature probe, what you need is an NTC with 10 kiloohm resistance at 25C and a beta coefficient of 3435. I won't bother explaining what all that means, but if you look for an NTC listed as 10k/3435 or something like that, that'll be the right part. It shouldn't cost more than about £1, take this one for example, so if you're being charged more than about £3, you're really being ripped off!

**Alternatively, you could very easily salvage a mains lead with a moulded plug attached by just cutting it off a mains appliance that is being discarded. If you do this, make sure that the cable is of an appropriate size - one way would be to check the current rating of the original appliance, and make sure that it's 10A or higher (or the power rating is 2.3kW or more). If it is not, you can still use it safely as long as the current/power rating on the original appliance is higher than your rice/slow cooker.

Tools list:

Drill and drill bits
Coping saw or Dremel and cutting wheel to cut out apertures from the plastic enclosure (If you don't already have something suitable, I found a coping saw with blades for £3 inc p&p on eBay)
File
Screwdrivers
Wire cutters or side cutters
Wire strippers (optional)
Stanley knife (optional)

Step 2: Measure and Cut Out Aperture for the Socket

The layout for the enclosure will be to have the temperature controller slot in to the side at the bottom of the enclosure, and the socket will be in the lid. Measure your back box to find out what size aperture it needs (mine was 74mm square) and mark it out on the lid in pencil.

With an electric drill, drill holes in all four corners of the aperture, taking care to offset the holes from the edges so that the entirety of each hole is within the material to be removed. The holes need to be large enough to take the coping saw blade. Although you technically only need one hole to get yourself started, it's definitely worth drilling one in all four corners as it makes changing sawing direction much easier. Unmount the blade of the coping saw and place it through one of the holes to cut out the aperture, taking care to stay on the waste material side of your lines - you can always remove more material later on, but you can't put it back! If you're like me and can't saw straight, you'll want to clean the cuts up with a file.

Once you're done, push the back box into the lid and push the "wings" out to secure it. The wings will get tightened up against the lid by the screws that hold the socket on. You may need to pack them out with some scrap material if your lid is too thin for your back box - you could cut a couple of pieces of the scrap plastic from the lid for this.

Step 3: Cut Out the Aperture for the Temperature Controller

Repeat the process, but this time on the side of the enclosure base. You probably need to be a bit more careful cutting this opening out, as the rim of the temperature controller is a bit narrower than the back box.

The temperature controller is held in place by sliding plastic clips on its sides, but don't attach it just yet - you will need to wire it up before fitting it into position.

Step 4: Fit the Plug

Cut a 20cm length off the end of the mains flex and save it for later; we will use the wires from this to wire up the temperature controller. Undo the sealing nut (the one with the rounded face) on the cable gland and slide it carefully onto the mains flex. As I discovered, a cable gland will thread more easily onto the flex in one direction than the other, so you probably want to thread the cable gland onto the flex before you fit the plug. I learned this the hard way, and it's not impossible to thread the cable gland on "backwards", but it is more fiddly! Don't tighten the sealing nut on the cable gland up yet - you will need to be able to slide it around on the flex for a little bit.

Strip the end of the flex (on the sealing nut side of the cable gland) and fit the mains plug. I'm not putting any detailed instructions for wiring a plug, as if you don't already know how to do this safely, you really shouldn't be attempting this build!

Step 5: Drill Holes for Cables

The mains cable will need a 12.5mm diameter hole for the M12 cable gland. I only had drill bits going up to 10mm, so I drilled the biggest hole that I could and abused my cordless drill a bit to nibble away at the edges until it was big enough for the cable gland - it's not a perfect hole, but it's good enough. The correct size for the sensor cable hole will depend on your grommet; mine was about 8mm diameter. Drill and deburr both holes. You can go ahead and fit both the cable gland (having already slid it onto the mains flex) and the grommet now. To fit the cable gland, you just remove the locknut (the flat one), slide the body of the cable gland through the hole in the enclosure and refit the locknut.

Step 6: Wire and Fit Temperature Controller

Take the trimmed part of the mains flex from earlier and carefully remove the sheath to recover the live and neutral wires from inside. Use these wires and the terminal block to wire up the temperature controller, making sure to thread the sensor cable through the rubber grommet before wiring it up! You can see the appropriate wiring pattern clearly in the second photo.

Once the temperature controller is connected up, refit its rear cover and slide the plastic clips on the side up to secure it firmly in place. It's a good idea to secure the terminal block to the enclosure in some way - I had originally planned to use a blob of hot glue from a glue gun to stick it to the side, but I decided that some double-sided foam tape would be adequate and less messy.

Step 7: Wire Up the Socket

Poke out one of the breakouts in the back box, then thread the remaining wires through it. Fit and tighten up the lid of the enclosure. Now just wire up the socket and screw it on to the back box - again, no detailed photos of this step, as you should know how to do this already if you're going to attempt this yourself!

Step 8: Calibrate

To calibrate the temperature sensor, we need a known fixed temperature. Pure melting ice will give us a very convenient 0C point to use, so fill a small glass with lots of ice and top it up with water. You will need quite a bit of ice, as the calibration process is quite slow and it's important that there's still ice in the water at all times. Plug in the sous vide controller and drop the temperature sensor into the iced water. Now wait and be patient! You need to record the lowest temperature it reads, but it will take 5-10 minutes to reach equilibrium. Input this value as your temperature offset on the temperature controller (this is setting "F4"), and check that the temperature of the iced water is correctly measured as 0C. While you're at it, I would recommend reducing the minimum temperature difference (setting "F2") to its minimum value (0.3C), as this will minimise the temperature variation when cooking.

Step 9: Cook!

Set your desired sous vide temperature on the controller (setting "F1"). Fill your rice/slow cooker with warm water, as close to the desired cooking temperature as possible, and drop the temperature sensor into the bowl, along with the food to be cooked. Plug the rice/slow cooker into the controller, and get cooking!