> I've been thinking of building a Pedal Generator
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> Supposedly, Eddy Merckx was putting out 200 watts when he broke the hour record... 1 Hp =746 watts. 200 watts is less than a 1/3 hp.
A good reference power for a good cyclist for more than a sprint is about 0.25 horsepower (186 Watts). Although you can train (or drug) leg muscles pretty big, your lungs are only about 1/4 the size of a horse's lungs, so your sustained power is about 1/4HP. Merckx's 1/3HP (250W) for an hour is a very good performance, which is why he holds a record.
If you have ever run with a pack of good cyclists, averaging 100-150 Watts for miles, you know you would not want to work a computer at the same time. Even if you could hold your upper body and typing-finger steady while your legs spin madly, your body, brain, and fingers run at the edge of oxygen starvation and get slow and dull. Seems to me that a better output while doing office work is in the 50 to 100 Watt range. That's at the bicycle wheel; there will be additional loss in step-up gearing, winding resistance, generator field power, and whatever you use to turn this unstable power into steady power.
And be realistic: the utility company will sell you 100 steady reliable Watts for a penny an hour. Pedal 25 hours a week: You "make" $1 per month, $12 a year, $120 a decade. You need to be junk-rich to assemble an efficient pedal-generator that will break-even on economic grounds. And in strict economic terms, you have to count some of your food-costs as "engine fuel". (On the other hand, if you are paying a gym to loan you a bike, then you could break-even just on gym membership fees.)
The most obvious generator is a car alternator. However these are scaled for about 1,000 Watts output, so at 100 Watts output there may be more drag and loss than output. Also they are made for a vehicle with 100+HP (74,600+ Watts) of power on tap, so low-load efficiency is not important. A motorcycle dynamo may be a better scale, but they are not laying around most folk's garages and some of these are just weird. (My BSA just tapped the ignition magneto, and then wasted all excess voltage in a humongous Zener.)
Also, the car alternator scaled for 1,000 Watts output has a 25 Watt field coil. Until you get to 12V, the first 25W of whatever you pedal all go into the field coil, not the load. That's a lot of overhead for a 50W-100W system. A 100W alternator from a smaller vehicle (small motorcycle, lawn tractor) would have a smaller field coil so more of your leg-work ends up in the load.
Most dynamos work about the same as far as gearing. Take a car alternator for simplicity. It may not exceed 12V until engine speed hits 700-1,000RPM. The alternator is usually belted-up 1:2, so at minimum voltage it turns 1,500-2,000RPM. The bicycle pedals turn 70RPM for best leg performance. Bike gears vary, and can be variable; 48:16 or 1:3 speed is a typical one-gear ratio, 52:13 (1:4) to 28:32 (0.9) are common. So the rear wheel turns 210RPM (280-60RPM). Not fast enough to hang a standard small alternator on. You would get maybe 4 or 5 Volts flailing in top gear. We need a total 1:28 step-up gear ratio. Normally you do not want to exceed 1:10 in a single stage of gearing: either friction or size becomes large. However a 2-stage gear is size and friction too. 1:28 is a rather unhappy number. Oh well. Given the 200RPM wheel, we want another 1:8 or 1:10 speed step-up.
There is the "obvious" trick: the tire is about 27 inches diameter, so if we put a 3 inch wheel on the alternator and press it against the tire, we have the right gear ratio. Possible alterative is a belt around the wheel and a 3" pulley on the alternator: cog-tooth belts will run well on a small pulley and can also get good non-cogged grip on something as large as a bicycle wheel. But they are not cheap. A standard car fan-belt may actually work OK, if you can find a shallow flat-bottom one to ride the wheel well. Thinner is probably better: the heavy-duty belt for my ThunderBurd probably had a significant fraction of a horsepower of internal friction when bent around small pulleys. In a bike's higher gears, you are unlikely to be able to break any standard fan-belt 3/8" or wider. In office work, 1/4" might be strong enough, but won't fit car-size pulleys.
Motorcycle and other smaller dynamos usually like higher speeds. A cycle alternator might need a 2" wheel pressed against the tire; if you find a 100W DC motor (most DC motors work fine as generators) it may want a 1" wheel. Jamming a small wheel hard against a rubber tire gets into rubber-flex drag, so you don't want to go real far this way.
In principle, strapping magnets (old hard drive guts) to the wheel and coils (oversize guitar pickups) to the frame will give you electricity. To work at low RPM you need more magnet and copper than an "optimized" design, and you are likely to have a lot of leakage flux. And getting the voltage to come out right will be trial and error. And it will be variable frequency AC, which may need more conversion before use.
Honestly, in most offices, this is a dumb idea. Utility electricity is so much cheaper and better than a bike-dynamo that it is a waste of money. You might think what else you can do with spin directly, instead of making electric to do something. If your office is cool, just pedaling will warm you up, and adding a drag-brake will release 50W-100W of heat too. If your office is hot, a bunch of boards shoved in the spokes will fan a good breeze up your butt. If your coffee pot filter leaks sediment, strap your cup to the wheel and centrifuge the grit to the bottom. Heck, get a thick-wall insulated metal teapot and drag it on the tire, pedal hard: you could make boiling water without electricity in less than an hour.
