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
Screwdriver
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.

Comments

[pcooper2]

Interesting idea, but it may not be achievable with just any salvaged microwave oven, as many use a separate step-up secondary winding on the same transformer that supplies high voltage power to the magnetron tube to also provide low voltage power to the control electronics. The high voltage output is dangerous, and such multi-secondary transformers tend to be big and heavy. It might be necessary to purchase a small step-down transformer just for this project to reduce the mains voltage to the 6 to 12 volts needed by the electronics power supply.

[mrshow555]

Is there a way to simply remove the timer from the microwave as you've donw and convert it to battery power? I'm looking for a cheap way to get a countdown timer with a 7 segment display. It doesn't need to trigger an appliance, just dispaly a countdown...

[WolfVecho]

it honestly would be just as easy to go buy a $3 digital egg timer mrshow555 they even may have a beeper built in

[marquoise]

You should be able to run it from a battery, just find the 5V and GND lines on the board (see below in the comments for more). And of course this makes a great countdown timer. I have used it myself like that to practise an exercise that I wanted to be able to compete in 8 seconds. Just set the timer, it counts down and beeps when it is finished. If you try the battery thing, could you leave a comment on whether it worked or not for any other people interested in trying it.

[Derin]

i think so

[ongissim]

You should make a wall mount for it and tie it into a circuit of your home so you can time a TV or lamp when you go to sleep. Or, you could do the same thing but make a cover and lock it so you kids are forced to do their homework, instead of watching TV! Wait, I'm 13...

[Derin]

hm interesting but I think there is a buzzer to indicate the time is over...well desolder that and put a light display that says "DO HOMEWORK"in red...wouldnt need this here in turkey because kids in public schools they give 50 pages of hw each DAY and 1oo pages for weekend

[marquoise]

The circuit for the timer is a standard DC one, so far I haven't seen a microwave timer that runs on anything other than 5 Volts. Even though the board is powered by AC, essentially all that happens is there is a standard wall wart style circuit which is included right there on the board. You could run this off a battery or an external 5Volt source by simply soldering separate +5V and Gnd wires to the board on the right spots. The right spots will be somewhere between the big ass transformer and the microcontroller chip (the biggest one). The timer used in this instrctable has already been given away, but I checked on another microwave timer board that I have, and the 5Volt and GND points are clearly marked on the top of the circuit board.

[laxplayer_13]

How will they be marked? This may sound stupid, but i need the timer to turn something else on an off that runs off of 6 volts, is what your describing still capable of doing this or will bypassing some of the components cease this function?

[marquoise]

Look on the top side of the board (the side without the solder) for something marked +5V and GND. I haven't tried it yet, but running the board at 5V from an external source would not make any difference to the timers functioning. Switching something at 6V would be the same as connecting to mains electricity. Just connect your 6V wire to the big relay. Good Luck

[laxplayer_13]

Well, i understand circuits, but i am a little shaky in some of the finer point of electronics. I do not see anything marked +5V or GND would there be other symbols or markings used to mark these areas? The big relay is the boz with the two prongs? and originally in the microwave circuit this was also connected to where the power supply came in do i need to do this or will it automatically go channel my electricity out of that without me doing anything. I am attaching + wire for my batteries to the 5V and the - to the ground? Sorry for so many questions, but of this works it will be an immense help to a larger problem i am working on solving.

[laxplayer_13]

this is a blurred image of the board, I don't expect you to read it, but does it appear to be the same type of circuit that you used before?

[laxplayer_13]

.

[marquoise]

I have circled in red where I think the 5V and GND points are. You should probably power the board from the mains and check with a multimeter to be sure though. Just be careful not to zap yourself in the process. As you correctly guessed, the big black thing with the 2 prongs is the relay. The relay is essentially just a switch. You can imagine the 2 prongs as 2 wires, the relay will touch them together or pull them apart. Now that you've liberated all the wires from the microwave oven, those 2 prongs are completely separate from anything on the board. Here's what you gotta do, solder the + and - wires from the battery to those 2 wires in the red circle. If you don't know which one is which, don't worry. There is a 50-50 chance you'll get it right regardless. If you get it wrong, you'll fry the microcontroller and render the board completely useless. If that happens, all I can say is welcome to the world of hobby electronics. Most of what I know about semiconductors is from accidentally destroying them, and you'll have to get your cherry popped one day. However, if you happen to solder the points correctly, the timer should work with no problems. Then you use those 2 prongs as a simple switch for your larger project. I originally wrote this instructable as being an electronics project which was simple and uncomplicated, but looks like your project is turning out to be considerably less so. Good Luck.

[laxplayer_13]

I just got my microwave, so I will take it apart soon and see if its all as easy as its cracked up to be, thanks for the input.

[laxplayer_13]

Is it possible to run this off of DC or battery power and how many volts would be required? Is the circuit strictly set up for AC and would a power rectifier work to let it run off of DC or would the circuit need to fundamentally change in order to function under DC power?

[marquoise]

It turns out that the maximum time is actually 3 hours 20 minutes, as opposed to 1 hour and 40 mins. I'm not sure how many other microwaves out there will allow for the extra time, but it will give some extra breathing room for applications like battery charging.

[LasVegas]

Great job! I only have one recommendation. The Ground wire (green) should also be tied to one of the mounting bolts so the entire encosure is grounded. This is to protect the user in case any other wire finds its way to the enclosure.

[marquoise]

I live in Australia, I didn't realise that there are only a handful of countries that use ground with the mains plugs. If you don't have 3 prongs on your electrical plugs, then ignore the green wire in the diagram and pictures. The maximum time limit of 1 hour 40 minutes is because the microwave timer only allows 4 digits to be typed in. That could be changed if some clever person can tell us how to swap out the existing microcontrollers with a 40 pin PIC chip, or even better, simply reprogram the existing ones. But, for my purposes, I can accept the current limitation. I love the idea of swapping the relay with a SPDT and using a switch to choose between normally on and normally off.

[HamO]

Very nice, I'm sure you have inspired several of us to search for salvagable microwavea for this peoject. Thanks for a job well done!

[dmacrae]

Well now you got me looking for another piece of useful junk.

[Aeshir]

Hey I just noticed something...the pwer bar in the picture in the intro has diagnol holes where plug prongs go in...do you live in britain or something? Is that what they look like? xD i'm confused. And does it only run for 1 hour 40 minutes just because that's the limits of the timer that you have, or does it only go that long becuase it's not connected to the microwave or something?

[Crash2108]

Plug J Grounded: Australia, New Zealand, China, Argentina, throughout South Pacific.

[Aeshir]

OMG! This is awesome! I've been looking for something like this for a battery charger and now you've found one out for me! Thanks a lot! Holy exclamation mark batman! !!!!1!111!!! :D

[Myself]

Great idea! My father has an occasional need for just such a timer, not pressing enough to purchase one, but it would save him some sitting around with a stopwatch. Next time I see a junker microwave, I know where it's going! If you have a changeover (SPDT) relay, you could wire it along with a "3-way" switch on the front panel, giving you the choice between setting on-time for a normally off device, or off-time for a normally on device.

[marquoise]

You're absolutely right, the solder and insulate option would be a better idea for the longer term. The decision to replace the switch was an instant last minute thing and I was almost finished so took the easist option I could think of at the time. I never really considered soldering it, but I would definately have done it that way if not for the extra wire nut sitting in front of me at the time. Thanks Marq

[microman171]

Instead of a wire nut if you have a soldering iron then you could use that..... then afterwards to insulate either PVC tape or heatshrink :D Awsome Idea man!! Good Job