Back to Latest DIY_____

This project worked the opposite way around to my usual way of operating; normally I start off with at least one major component which is what inspires the idea. In this case I was actually asked to produce a generator which would power a small colour video screen for computer games for possible se in a TV programme. The original request had been to pedal power the game box itself but that wasn't possible as it contained a disc drive.

I decided to go for an exercise bike base as it should be less complicated than making a frame to safely hold a chopped up bicycle, but of course none turned up in spite of regular visits to the rubbish tip and I had to resort to buying one out of the free ads. However, I did much better with the motor which is the other main critical component. The trouble with exercise bike formats usually is getting a high enough gear up; a neat way out of that if you don't need a high power output is to use a 24 Volt motor to generate at 12 Volts. A suitable one did turn up in an electrically adjustable bed on the tip. Its 8mm spindle had originally been machined into a four tooth spiral gear, making fitting a friction wheel difficult The spindle was modified by filling in the grooves with MIG weld, one at a time to avoid cooking the front bearing. The resulting set of wobbly lumps was then trimmed back down to 8mm with an angle grinder while running the motor; a bit crude but it finished up a nice tight fit in a paper feed roller from a laser printer. It's all right up to 20 or 30 Watts, but anything higher wasted energy in the higher winding resistance and needed fast pedalling to compensate, not to mention tending to trash the printer roller. However 20 Watts should be adequate as the idea was for a child to be able to pedal it for long periods at an energy level comparable with brisk walking.
This particular type of exercise bike has a reasonably large pressed steel flywheel which is good for high gearing but is much lighter than the cast iron ones and doesn't have a freewheel. On the other hand the whole thing's lighter making it easier to move about. A possible problem is the lack of front frame which could make mounting the motor difficult.
Anyway, the first thing was to start making a cradle to hold the motor which will need to lean on the flywheel from a pivot. Some bicycle tube fitting nicely over a printer rod looks like the raw material for the pivot itself.
Next, the front leg tube was braced and extended using larger bicycle tube to provide something to mount the pivot on.
The bits for the motor mount and pivot were jigged up and welded though at this point it still wasn't obvious where it was finally going to go.
The motor was put back in the cradle and the mounting bar made from a piece of rectangular tube from an old table was balanced in place, using the entire length of the printer rod to get the pivot horizontal.
A pair of M6 nuts had been welded over holes in the centre section of the pivot tube so the printer rod could be locked in place. Washers on each side could be moved to adjust the final motor position to line up the roller with the centre of the flywheel.
A section of printer rod was cut to size and a first test carried out to check for any tendency for the roller to slip or bounce under load; luckily it seemed to work OK.
Having decided the mount position was all right, the long support bar needed bracing as it looked vulnerable to being bent by any impact on the front of the generator.
A suitable brace was made out of an otherwise useless looking piece of bicycle front fork.
As there still wasn't a lot of structure at the front, the support bar was also used to mount a glassfibre board and heat sink for the regulator by welding a couple of M6 bolts on to the side.
Apart from the lack of other positions, the heat sink would get the benefit of airflow from the flywheel.
It occurred to me that something would have to be done about protecting the open frame motor from stray metallic particles, though it would be a pity to completely hide the armature. While thinking about that, I went out of the workshop door and immediately found a plastic bottle on the ground - it turned out to be an exact fit on the motor. It's surprising how often that sort of thing happens.

The next thing to do was to make the circuit for the voltage regulator. It was more complicated than those on some other generators as the generator needed to be foolproof and easy to use and there was an extra requirement not to supply power at well under 12 Volts to avoid damage to the display screen. Apart from general power supply problems, LCD displays often use small fluorescent tubes for backlight and these are easily fried by running under voltage.

The circuit diagram finished up looking like this. The bits in blue had to be added on as modifications to make it do what it was meant to.
The big diode connected across the input normally doesn't do anything; it's for reverse polarity protection which is definitely needed as there is no freewheel on the bike. If the generator should get pedalled backwards, the diode limits minus voltage to less than 0.5V and the friction wheel slips due to the virtual short circuit. A shunt diode was used instead of a series one as it saves energy and keeps the pedalling speed as low as possible.
The circuit around the big 2N3773 transistor (which is mounted on the Heat Sink) is just a basic shunt regulator; at about 13.5V the 2N3773 starts to be turned on by current from the Zener diode and prevents the voltage exceeding about 14.5V by taking lots of current and dumping power into the Heat Sink. In addition, a small bit of the current coming down through the 13V Zener diode goes down the 180 Ohm resistor and turns on the small ZTX450 transistor as well, so a red flashing LED indicates that energy is starting to be wasted in the Heat Sink and the pedaller can slow down a bit.
The next part of the circuit is also quite basic. The (fairly) fixed base-emitter turn on voltage of the other ZTX450 transistor is use to sense the voltage available, so it is only applied to the output terminals via the relay if it's high enough. It's similar to the low voltage cutout circuits used in small wind and solar setups to turn the power off before a battery gets over-discharged.. The variable resistor is adjusted to give no output below about 11 Volts.
When testing the original version of the circuit, the relay tended to buzz, turning the output on and off rapidly anywhere close to the intended switch point. That was because the relatively high internal resistance of the motor caused the voltage to drop under load, turning the output off, but then immediately putting it back on again when the voltage sprang back up with the load removed. To cure this problem, I added a collection of secondhand power supply capacitors adding up to about 10000uF to reduce the sudden volt drop at the motor, and also the extra 68k resistor which makes the turn on point higher than the turn off one. After re-adjustment the output wouldn't turn on until 12.5V had been reached but then still stayed on until it dropped down to 11V. There was still a slight tendency to bounce at high current loads, but the pedaller would notice that happening much more than they would if it ran steadily under voltage.
Using the base-emitter voltage of the small transistor as a sensing standard isn't ideal as it will vary with temperature, but it's expected this generator will be used indoors where the temperature should be fairly constant.
The circuit fitted on to a piece of glassfibre board next to the Heat Sink; these were both recycled as were the reverse diode and all the capacitors.
The generator was reassembled and tested with a 8 inch monochrome video display.

However, the TV people then decided on a non renewable energy solution, and the generator was modified by the addition of a new removeable output box with screw terminals, a car lighter type socket and a Voltmeter. The Stanwell Woodcraft Folk group have now got it; they used it successfully at the Global Village camp in August to power an original design of binliner monster as well as charging mobile phones.