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All sorts of scrapped and secondhand devices can be used to generate your own electricity. Car alternators are an obvious one if you want to charge 12 Volt batteries. Small permanent magnet motors such as radiator fan motors or cassette recorder motors are easy to use as they produce DC power directly without any external circuits like rectifiers or control boxes.
There is another type of electric motor worth considering - the small stepper motors used in old computer printers. They are quite small and aren't suitable for producing more than a few Watts, but there are good reasons for looking at them. For a start, the lunatic speed at which computer equipment goes obsolete means there are enormous numbers of them available free. Unlike small DC motors, steppers will generate power at very low rotation rates; typically only about 200 rpm for a good output which is ten or fifteen times slower than the rate for a DC motor. Small scale generators to run things like computer games or flashlights can be made without mechanical complications like gearing. Because of their small size they're obviously not suitable for charging large batteries.
Better applications would be pocket sized generators to convert things like Walkmans and MP3 players to wind-up power, saving the waste and pollution of chemical batteries. Another possibility is small wind generators as the low rpm needed means a propeller could be mounted directly on the motor shaft. (Actual gears in a wind generator are generally a disaster - the whining noise is amplified by the blades and spreads over a wide area because of the height).
The present generation of printer motors are admittedly not large, and in fact are getting smaller as the old daisywheel and dot matrix printers are replaced by inkjets and smaller lasers. It is definitely worth experimenting with them though, as it is likely that the next generation of domestic appliances will be heavily computerised, and so full of nice big steppers. Anyone who has acquired experience on the small ones will be able to make these into some really nice generators.

Selecting Suitable Motors
Old Dot Matrix computer printers (the larger and older the better) contain at least two steppers. Usually one drives the roller and another moves the print head back and forth. Daisywheel printers will also have one to turn the daisywheel which can be a bit inaccessible but worth the effort. Tiny steppers were also sometimes used to wind the ribbon and in colour printers another minute one moved a striped ribbon up and down. Disc drives tend to be a bit disappointing - often the motors are built into the drive hub and contain some electronics so you can't get easy access to the coil connections. Really old 5.25" floppy drives contain a nice motor used to move the reading head back and forth - it's a lot more useful than the one for turning the disc which was sometimes a DC motor on older ones and tangled into the circuit board on later models. Very old hard drives (on 286 or 386 computers and less than 100M) use a small stepper to move the head array. Modern hard drives use an analogue galvanometer instead; it contains a pair of amazingly strong magnets - mind your fingers if you extract them! Physically large motors like the single ones which drive laser printers are obviously more powerful than small ones; anything less than an inch in diameter is probably only suitable for running a few LED's. They're OK for educational purposes or making illuminated things for playing with at chill-outs. (See the page on making Hub Disc Twirly Things)

Steppers come with different resolutions. Virtually all steppers are either 1.8° or 7.5° per step; (200 steps or 48 steps per revolution) the difference can be felt easily if you turn the spindle by hand. The 1.8° ones are obviously better for generating at really low revs, but also 'top out' lower. The coils in steppers have a relatively large inductance, and beyond a certain speed the output frequency gets so high that the impedance of the coils starts to become significant and limits the current. When making a stepper based generator, you need to keep the motor speed to around a couple of hundred revs per minute - something like the normal speed of a bicycle wheel.
Apart from printers, plenty of other things contain steppers. Scanners, shredders, faxes and photocopiers are also worth checking out. Be careful with things like copiers and laser printers not to get toner all over your workshop, especially if it doubles as your living room! Don't vacuum clean toner as the particles are so small they'll go through the bag into the air. Wash it off with water or clean it up with a damp cloth.  Really large steppers are found in automated industrial equipment and the large tape drives used with old mainframe computers which you might still find at auctions. The next generation of highly automated washing machines and dishwashers, household robots etc. will contain some nice big steppers, and it won't be too long before they are superseded and start to turn up at the rubbish tips and car boot sales. There's already a nice example of this in New Zealand where Fisher and Paykel have been selling stepper-driven washing machines for some years, and scrapped ones have been made into neat hydro generators by a local company appropriately called Ecoinnovation. The 20cm diameter motor in the Smart Drive washing machine is an example of the nice big motors just around the corner.

What's Inside a Stepper Motor
In the early days of DIY renewable energy, it was popular to make small wind generators out of bicycle wheels containing Sturmey Archer Dynohubs. Now almost a museum piece, they were the predecessor of the bottle shaped rim 'dynamo'. (I don't know what was the matter with the people who named these things - they were both ALTERNATORS producing AC; the term dynamo is better used for generators incorporating a synchronised contact breaker turning the output into DC. Maybe it was something to do with marketing) Anyway, the Dynohub was a small multi-pole alternator in the hub of either the front or rear wheel with an internal resistance of 6 Ohms and capable of generating 6 Volts when turned at 60 rpm. The performance wasn't that good - the internal resistance means that if you took a current of half an Amp from it the voltage would have dropped to only 3 Volts. In spite of this, many people made wind generators out of them by sticking blades in the spokes, rectifying the AC with a bridge rectifier and putting them on the roof of their caravan or bus to trickle charge batteries.

Stepper motors are also a small multi-pole alternator, but being more modern they have four phases while the old Dynohub had only one. In use, the computer puts a pulse of current into each phase coil in turn, moving the shaft on one step. As with a DC permanent magnet motor, turning the motor's shaft makes it work backwards, causing pulses of current to come out of the windings. However, the current is AC, going plus as a magnet pole approaches a coil and then minus as it goes away again. Usually there are four phases at 90 degree intervals so when one comes down to zero, the next one has reached maximum. This is a benefit as it means the output can be rectified to produce much smoother DC with hardly any gaps, but it means they have a scarily large number of wires coming out. Luckily it's quite easy to figure out which way around they are using a resistance meter (preferably digital), and getting them the wrong way around won't do any damage. The most common type of stepper has six wires coming out. (There are also five, four and eight wire versions; I'll come to those later - they are easy to understand once you've sussed the six wire one) The six wire stepper is actually two motors on one shaft, so the six wires can immediately be separated into two groups of three. Each group will have some connection to each other, but no connection to any of the other group. In each group, one wire is the common and the other two are the opposite ends of a winding which will give out oppositely phased AC.
In terms of resistance, the reading from the common to either end will be half the reading across the two ends. Having found the common on one set, you can use the same process to find the common in the other one. All four windings will have almost exactly the same resistance.
The majority of steppers are six wire, but there are other varieties. Five wire ones are easy; the two commons on the six wire have already been connected together for you which makes things easier. Eight wire ones are just like a six wire but with all the windings separate, and four wire ones are half of an eight wire one (or half a six wire one with the two windings separate).

There's more than one way to wire up the stepper to get a DC output. Unlike the dynohub, you can't wire it up to a bulb and run it off AC as it's got four separate phases and connecting any two directly will cause a short and stall it. On the other hand, if you're bursting to generate some power, connecting a small light bulb, say 6V 100 mA from ONE of the live phases to the common and turning the spindle with your fingers should get a result. It's quite a good way to find out if you're going to get a useful amount of power out of it, but you'll only get a quarter of the possible power that way. The simplest way to wire it up is to link the two commons to the minus terminal and then connect each of the four live phases through a small diode to the plus one as shown. Here's what it looks like.

The four lives will each go positive (and then negative) one after the other like the cylinders of a car firing and the diodes collect together all the positive pulses and feed them out. Because of the overlapping phases, the rectified AC never goes down to zero like it would from a normal bridge rectifier. Putting the bulb across the output should give a stronger result than before and a DC voltmeter will show that the output voltage is more or less proportional to the rotation speed. This is normal for a permanent magnet alternator and you will need to use a regulator limit the voltage. Because the stepper is acting as an AC generator, it doesn't matter which way you turn it so designs in which it is turned alternately forward and back by a treadle or foot pedal are possible.

If the motor you've got is rated at 5V but you want to generate enough voltage to charge a 12V battery, you can often get away with just spinning it a bit faster. If that doesn't work, you may be better off using this voltage doubler circuit with two bridge rectifiers. I've built a pedal generator which can be switched between the two configurations, and there's less difference between them than you'd expect. The double voltage configuration gives a good voltage at lower speeds but has less current capability as there's twice the winding resistance. The normal four diode setup gives more current when driven faster, but not twice as much as the AC impedance of the windings has an effect due to the higher frequency.