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)
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.