Introduction: Digital Timer for Any Electrical Appliance

In late 2006 Jaycar were selling a microcontroller based flexi-timer kit for $90 (now discontinued). In the advertising blurb they stated "As easy to use as a microwave timer!".

Well I always figured that if the interface for a microwave timer is touted as being the yardstick for a quality timer, why not simply use a microwave timer. You'll can save some cash and do your bit for recycling at the same time.

If I am lucky, I can make a trip down to my local dump and pick up a microwave oven for nothing, otherwise I can drive to the city dump and buy one for $5. You may even have an old one sitting in your basement, or have a friend who does.

If you decide to build one of these yourself, you will need to have it checked by a qualified electrician before connecting it to any mains electrical supply.

What is this project useful for?
Most applications where you need to run a mains powered device for a specific period of time and then have it automatically turn off. Most microwave timers will run for a maximum of 1 hour 40 minutes (99 minutes 99 seconds). There are probably dozens of potential applications.

Update: I have since discovered that this timer can be programmed to run for anything up to 3 hours and 20 minutes. By utilising the "cook time" feature, 2 separate time periods can be entered and the machine adds them together.

What you will need for this project:
1: A disused microwave oven
2: Mains input and output sockets
3: An enclosure for your timer
4: screws/bolts to hold everything together.
5: Electricians wire nut to connect the electrical wires.
6: Common sense and caution regarding electricity

Tools I've used for the project: (you may get by with less)
Standard Drill
Wire cutters
Dremel style handpiece
Dremel cutting disc attachment

Step 1: Acquire the Timer Board From a Microwave Oven

Dismantle your microwave oven, and retrieve the front panel and circuitry.

When pulling apart the microwave oven, try to keep all the wiring connected to the front panel if possible. This part may be a bit tricky, a bit like a complex puzzle. If you need to cut some wires, then go ahead. But if you need to look at the wiring to work out which cables on the board go where, it is easier to work it out with all of the electrical stuff still connected.

Essentially, once you are finished, the only wires that you need at the end are the two that directly power the board, and the two that lead to one of the door switches.

The photo below shows a white plug with a red, blue and black wire. These are not needed for my project, I only hadn't removed them when I took the photo. The power wires on my board are down at the very bottom. The wires which connect to the door switch are black and yellow and are still connected to the door switch. The switch is a normally closed switch meaning that if the wires are disconnected then the board will think the door is open and the timer won't run.

Step 2: Turn the Front Panel Into a Face Plate

First undo the screws holding the electronics board to the front panel and remove it completely.

Next remove the excess material around the bottom of the front panel (the opening mechanism).
Remove the button itself and cut the rest from the faceplate completely.
We only need the buttons and the display screen.

Lastly cut the sides off the faceplate and all other plastic protrusions with a dremel cutting disc. We want to turn it into a flat faceplate with nothing else sticking out save for the parts that will hold the circuit board.

You will only get so close with a cutting disc, change to a grinding disc and/or file to smoothen up the end result.

Take care not to damage the wire strip wich connects to the front buttons during this process.

Step 3: Prepare the Enclosure

I have used an empty 4 litre drum of olive oil for the enclosure.

I had no special reason for using this, save for the fact that it was the right size and I happened to have one lying around. Eg, it was free and sticks with my whole ethos of this being a recycling endeavour. There are many other options for the enclosure.

If you choose to follow my lead, just make sure that the olive oil container is empty before proceeding any further. All that you need to do is roughly mark out where the recess needs to be cut for the electronic board of the timer. Screw the timer circuitry back onto the microwave oven faceplate and measure and mark the required dimensions for the recess onto the olive oil can.

Then using the dremel and cutting disc, trace around the line and cut the reccess.

Step 4: Tidy Up the Board and Enclosure and Fit Them Together

My enclosure ended up pretty dirty after cutting, the interior also had a bit of oil residue.
Give it a good clean up on the inside and out. Be careful you don't scratch yourself on the sharp edges left from the cutting process.

Then fit the timer board and face plate into the enclosure.
Hopefully if you've cut it correctly it will fit tidily with no problems.

Decide where you will need to place the bolts to secure them together, then mark them out on the enclosure and timer board and drill the holes.

At this point I did a test fit and found that the door switch which I left plugged into the board was going to bounce around inside the enclosure. I then cut the wires shorter and used a single wire nut to connect them together.

In order to tighten the bolts from outside the case, I made them point outwards and cut a groove at the ends with the cutting disc. This made it possible to tighten them from the outside using a small flat screwdriver. There might be a better way to do this, but I couldn't think of one at the time.

Step 5: Connect the Electrical Stuff

I've kept and used the original microwave power cord along with an old power board I had lying around. Cut the ends of both squarely and push them through the enclosure from the outside.
Make sure that they are not plugged into anything before you do this.

Strip the coatings off the wires and tie the two cables into a knot to ensure that the cables cannot be pulled out when everything is finished.

I also salvaged 2 wires from the microwave oven for the connections to the relay on the timer board, hence the reason the colors from those 2 wires don't match my diagram.

Make sure that you match the wires correctly between the input and output cables.

This is the point where you will need to get a qualified electrician to take a look to make sure that everything is okay. Do this BEFORE plugging it into the mains electricity.

Step 6: Seal It Up

Slowly pull the input and output cables from the outside until the knot hits the entrance hole and carefully fit everything inside the enclosure and seal it all up.

After the bolts are done up tight I used copius amounts of hot glue around the cables in the top to secure them in place.

Step 7: Wrapping Up

I couldn't understand why my wife was less than impressed when I showed her that if I wanted to, I could program a desk lamp to run for 5 seconds and then turn off.

I felt great, that I had made something useful out of old junk lying around.

Any medium to low power appliances should run through this timer with no problem at all.
The microwave oven board that I used was rated at 800 watts, so I will be confident to use it for anything less than that. If you make one yourself and use a microwave oven rated at over 1000 watts, even many high power devices may also be okay.

However, I would encourage caution about using high power devices. I did say that common sense was required for building the project, and I'd say it applies to using it as well, the user should make their own decision as to what amounts of current can be drawn depending on the specs of the original microwave oven and the wires and cabling you have used to connect everything together.

Lastly, there are microwave ovens out there which control the microwave power through PWM.
You can tell these because the relay will emit a clicking noise when the timer is activated.
I had 2 microwave oven face plates in my junk pile, one made a clicking noise, and one didn't. So I simply used the one that didn't. But I have no idea how many of either kind are out in the real world.

The ones that click would probably be useless for this project, so it might be a good idea to safely find out which kind you have before attempting the same endeavour.