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Over the years, SILICON CHIP has presented many timers – they are amongst the most popular of school projects. Here’s how a reader “made” a timer for a darkroom enlarger – the parts are readily available from council cleanups or rubbish dumps! F or what they do – basically, turn an enlarger lamp on for a certain (controllable) time and back off again, darkroom timers are very expensive beasts. Perhaps part of the reason is that the popularity of do-it-yourself photo processing is nothing like it was in years gone by – hence less timers are made – but that doesn’t change the fact that they are expensive. Come to think of it, they have always been pretty exe! Yet timers per se are now made in (probably) the hundreds of millions every year. Just about every commercial 84  Silicon Chip appliance these days seems to have a timer of some sort. Just take a look around you. They’re everywhere! Which leads us directly into this article... One of our readers, Jess Benning, was asked by a workmate to make an economic enlarger timer for her daughter, who was studying photography at school. “Too easy,” he thought. “A microprocessor controlling a transistor switching a relay . . .” Then he added up the cost. By the time added a power supply and put it in a suitable box, it would probably be a good fifty dollars Words by Ross Tester From an idea (and photos) by Jess Benning or so. Surely there was a cheaper way? With that thought in the back of his mind, he forgot about the project for a few days; that is, until he happened to visit a recycling centre at the local tip. There, sitting on the shelf, were several microwave ovens, all with inbuilt timers. “What’s that? Timers? I wonder if . . .” People mainly throw out microwave ovens for two reasons. The obvious one is that they don’t work – or at least they don’t heat (more often than not it is a relatively simple fix but we won’t go into that – microwave ovens are lethal devices). www.siliconchip.com.au The less obvious reason is that the microwave oven still works perfectly – but they have broken the (usually glass or ceramic) platter. If you have ever tried to replace one of those as a spare part, you’ll know what we mean when we say it’s often more economic to buy a whole new microwave oven! Back to our story: if the microwave oven doesn’t heat, it’s usually the high voltage supply which has failed. Sometimes it’s the magnetron but that is much less likely. But the timer, powered by its own low voltage supply, usually still works. “The timer . . . still works?” You’re probably one jump ahead of us, right? Of course – use the microwave oven timer as an enlarger timer. All you have to do is find a suitable microwave oven! Wait! They’re dangerous! But hang on a sec: didn’t we say a minute ago that microwave ovens are lethal devices? Yes we did – and for that reason we are going to say very clearly: NEVER take the lid off a microwave oven and apply power (or work on a live one!). Even if the magnetron is not working, you have a high voltage mains transformer which can very easily kill (and indeed has done so in the past). Please read that last paragraph again, out aloud. Even qualified technicians don’t like working on microwave ovens because they know just how dangerous they can be – the 5000VDC or so high tension of a microwave is dramatically more dangerous than the 20-30,000V high tension of a colour TV set. The difference is the microwave high tension is designed to supply real current! Here’s a tip. If (as we are doing here) you want to “rat” the timer from a microwave oven, once you’ve established that the timer section still works (ie, the display works), unplug the oven and then cut the mains cord off. That way, you (or someone else) won’t be caught dead. Literally. Even then, a microwave oven is not 100% safe. We’ll look at other precautions you should take with a “dead” microwave oven shortly. What’s in a microwave oven? We’re getting a little ahead of ourselves here. Let’s go back and have a look at a typical microwave oven. www.siliconchip.com.au Here’s the pushbutton control pad as removed from the microwave oven. It is a pretty simple job as long as you are careful with the flexible keypad and ribbon cable. Almost hidden inside the box, alongside the transformer, is one of the two relays from the microwave oven: this switches power to the enlarger lamp supply. Inside, there are six main parts: 1. low voltage power supply 2. timing circuitry / display / keypad 3. high voltage power supply 4. door interlocks 5. magnetron 6. light, fan(s) and platter motor All that is needed for this project are parts 1 and 2 – and yes, they are quite easily distinguishable. Or more to the point, the high voltage components and magnetron are very easily distinguishable – you need what is left. Incidentally, if you cannot readily work out which bits are which in the microwave, you shouldn't be attempting to reproduce this project. The microwave oven was bought for $15.00 from “Revolve” (the recycling centre at the local tip. Pretty apt name for selling microwave ovens, eh?). A relatively late model Sharp was chosen because the timer gave a couple of nice features: a digital clock (of course!) which you could see in the dark and an “auto start” function which was intended to turn the oven on for one minute – a feature which would be very handy for focussing. Even dead microwaves can bite! The magnetron power supply basically consists of a high voltage transformer (circa 3000V AC), a diode and a capacitor, yielding about 5000V DC. Usually there is some form of control to vary the proportion of time the magnetron is turned on, thus varying the output power. It’s pretty simple. But there is a catch for young (and not-so-young) players. The capacitor can sometimes retain its charge for a very long time after turn-off. In many (most?) ovens there is a bleed resistor across the capacitor to help discharge it after turn-off but there is no guarantee that the resistor has done its job or is even still intact. If the bleed resistor is open circuit, (or non-existent) the high voltage capacitor could still easily have several thousand volts on it days, weeks or perhaps even months after its last use. Such a high level of charge could still be lethal, or at best give you a very nasty bite. It’s not just uncomfortable – the shock can make you suddenly jerk your hand away and possibly jag it on some close metal. Trust us, it happens. The moral of this part of the story is to never trust a microwave oven power supply, dead or alive. Again, let’s reiterate – NEVER poke around a live microwave oven and be extremely careful poking around a dead one. We would ALWAYS discharge the high voltage capacitor before working on a microwave. The most usual way to do this is to short out the terminals with a large, well insulated screwdriver (note we said the most usual way, not necessarily the best way!). Beware the risk of bits of molten metal flying off if August 2003  85 Fitting the timer into a case: this was nice and easy. A suitable cutout in the lid lets us read the display, while the pushbutton control pad was transferred to the case lid complete with its multi-way flexicable. The photo at right shows the finished project, complete with the (red) mini power outlet fitted to the side of the case. All you have to do is punch in the time required, hit “start” and voila! there is significant charge. Of course we would always wear insulated gloves and a pair of goggles doing this. And here’s yet another trap for young players: you’ve discharged the high voltage capacitor and then left the microwave for, say, a couple of days. You touch the capacitor terminals and get a real bite! What can happen is that the charge can build back up again over time – and it could be several hundred volts or so. To prevent this, a clip lead should be used to short out the capacitor terminals, once discharged. What to do now? OK, so you’ve made sure the oven is off, the power plug is disconnected and (preferably) the lead cut off. You have also made sure that the high voltage capacitor is discharged and cannot recharge or be touched. Now you have to identify the low-voltage supply and timer circuit. Fortunately, this is usually fairly easy: it Inside a Sharp microwave oven. The high voltage supply is under the white cover. The wanted timer and control circuitry is clearly separate on the left side. 86  Silicon Chip almost always has a separate (much smaller) transformer and power supply and you will probably find in most microwave ovens the timer circuitry is modular – often directly attached to the display/timer settings (but if not, certainly connected via a wiring loom or ribbon.) Inside the microwave, there are usually two relays – one controls the magnetron and the other the turntable motor and blower fan (if fitted). Because these are both controlled by the timer circuit, you should be able to use one of these relays virtually “as is” to control the enlarger lamp. It’s then simply a matter of connecting the relay contacts so they switch the enlarger lamp supply. Because the low-voltage supply is mains powered and in all probability the enlarger lamp supply is also run either directly off the mains or via a mains power supply, the timer/display/relay assembly should be mounted in a suitable case to make it completely safe. Our reader also chose to fit a small mains outlet to the side of the case to make it all self-contained – that is entirely up to you. Not all of the keypad is used – only the digits and “start” buttons are really needed. Other oven-specific keypad contacts which are not used can be left out. The oven selected had a semi-flexible keypad. When fitting the keypad, to be able roll the unused pads up, the layers of backing need to be removed. To make it able to be rolled up the stiff layer should be cut and removed. To stop the unused buttons from staying on permanently once you roll up the unused bit, the carbon conduction layer needs to be removed and some contact or tape put over the switch layer to stop it from shorting to anything else. If the LED display is too bright for your darkroom, it’s quite easy to layer the display with filter film and cut it back to an appropriate level. That is much easier than trying to dim the display. Of course, this timer (which can be set to 99 minutes and 99 seconds) doesn’t have to be used in the darkroom. It’s handy for a wide variety of mains-powered timing applications. www.siliconchip.com.au How does a microwave oven work? Most people are aware that microwave ovens cook or warm food very quickly. But just how do they do it? Let’s look at the easy part of the answer first. Microwave energy inside a microwave oven excites molecules of water and fat (which are present in practically all food). As the molecules get excited, they give off energy – in the form of heat. Excite the molecules enough and collectively they give off enough energy to warm or cook the food. Microwave energy doesn’t penetrate the food very deeply – and in many foods penetrates to different levels. That’s why you can get food cooked Basic arrangement of a typical microwave oven. Microwave on the outside but not on the inside. However, a output from the magnetron is stirred up and fed into the oven microwave oven normally does do a better job than chamber where it interracts with the water and fat molecules a standard oven which cooks from the outside in, in the food, result in heat and cooking. by conduction. Incidentally, the air inside a microwave oven is only Because microwaves tend to travel in straight lines, they marginally above room temperature. Therefore it plays no need to be “stirred” to ensure they cover every nook and part in the heating/cooking process (unlike a conventional cranny of the oven. This is done with either a fan in the oven, where the air is heated). microwave’s path, or by turning the food to be heated on a And microwave energy does not affect most plastics, turntable or in many ovens, both. glass, ceramics, etc (although some may have additives Finally, a system of door interlocks ensures that if the which are affected). That’s why you can usually use these door is opened while the microwave oven is on, power is materials in a microwave oven without their melting! immediately cut to the magnetron to avoid cooking anything else (or anyone else) in front of the door. How do the microwaves get there? Microwaves are a type of radio wave; a super high fre- Who invented the microwave oven? quency radio wave. They are generated by a special type The correct answer is no-one! of vacuum tube diode called a magnetron. The effect was discovered quite by accident just after In the tube, electrons are emitted by a heated cathode the second world war when a radar engineer at Raytheon and, being negatively charged, are repelled by the neg- Corporation, Dr Percy Spencer, noticed that a candy bar atively-polarised cathode. Instead of travelling straight in his pocket melted when he was experimenting with a towards the positively-polarised anrelatively new kind of vacuum tube (you ode (as in a normal diode tube) the guessed it, the magnetron). He tried electrons are deflected by the magplacing popcorn kernels near the tube netic fields of very powerful magnets – and they promptly started popping. around the device (hence its name, The magnetron, by the way, was magnetron). invented back in 1940 by two English They actually start to spiral, or spin, scientists as a major (and successful) towards the cathode. Now the anode component of the Allies’ radar system in a magnetron is not a plate, like a during the war. diode – it is in fact a number of high-Q Next day, Spencer was demonstrating resonant LC circuits, called cavities, this to a colleague by placing an egg effectively connected in parallel. near the tube. If you’ve ever placed an What happens when a resonant egg in a microwave oven, you’ll know LC circuit intersects the path of an what happened next: it exploded – electron flow? It generates an eleccooked – all over the colleague! tromagnetic field – radio waves, if you Dr Spencer then made a metal box, like – in this case, at microwave level into which he directed all of the micro(the actual frequency, usually about wave energy. Unable to escape, the 2.4GHz, is controlled by the cavities). A typical domestic oven magnetron. density of microwaves became even The magnetron has a small trans- The socket at right applies power; the greater and food placed in the box mitting antenna, designed to radiate cylinder at the top is the antenna. cooked quickly. The microwave oven the microwave energy from the caviwas “born”, even though it took some ties at maximum efficiency. But the microwaves aren’t allowed twenty years before the first domestic microwave oven was to get out into free space. They are collected by a waveguide released. and “piped” into the microwave oven, itself designed for Now you’d be hard-pressed to find too many homes in SC maximum efficiency at microwave frequencies. the developed world without one! www.siliconchip.com.au August 2003  87