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When people are pedalling our generators, they expect more pedalling to produce more effect. To some extent this happens naturally; light bulbs get brighter, the inflatable binliner monster gets bigger and so on. However this doesn't work on everything and they also expect the volume on the iPod Booster to increase or more things to power up as they produce more energy.
This is quite a common requirement and I've been asked several times by pedal generator builders about good ways of doing it. There is a circuit for a basic progressive light bulb display in our Children's Pedal Generator DIY page, but it's relatively complicated to build and uses a lot of separate components.
Earlier this year I came up with a simpler and more versatile circuit. So far I've built three or four variants which all worked OK, and thrashed the original around various primary schools and Green events so it's still not exactly a fully proven design but if you do a copy it shouldn't need much debugging.
It uses the ULN2003A Darlington Driver chip which was originally designed to operate relays from the output of old TTL or CMOS logic circuits. It was also sometimes used to operate the pin driver coils in dot matrix printers as well as seven segment LED indicators - it conveniently has seven sections. It turns out that if you put an extra resistor in series with the input it can be made to work as crude voltage threshold switch. (If you download its data sheet and look at the circuit diagram it should be fairly obvious why this works). A resistor of 68k makes it turn on at about 10V and 100k gives 14.3V. A range of values in between will produce a corresponding range of turn on voltages suitable for use with a 12V pedal generator.
In my prototype I used eight 10 Watt car bulbs. The first one was wired straight across the input voltage and the remaining seven turned on progressively by 5 Amp relays switched by the seven sections of the ULN2003A chip. This is quite tidy as it exactly used all the sections of the chip, but you could either leave some unused or add in some of the sections of a second ULN2003A if you want more channels - they're cheap enough and available from Maplin as well as more industrial suppliers. You might notice from the picture that I actually used two different types of relays in the prototype, but they're all 12V 5A rated.
Sometime this design might find its way into the proper DIY design pages (probably when I finally get around to a major update...) 
The two 1N4003 diodes in the top left hand corner of the diagram are for polarity protection in case someone accidentally connects it the wrong way around - you wouldn't need to include them if you were building it into the pedal generator.
The capacitor at the bottom is 1000uF on mine. I put it in to reduce the effect of voltage fluctuations caused by the input voltage going up and down during crank revolutions; the value's not at all critical and the one I used was recycled like a lot of the parts I get. The diodes are similarly uncritical and almost any 1A rated diode will do. The lamps are standard 10W bayonet fitting car ones; I shoved them through 16mm holes in the metal front panel and soldered wires directly on to them rather than spend out on holders. The conductors shown as thick lines need to be rated at least 10 Amps as that's what the thing will take when someone manages to pedal hard enough to light all the lamps. This happens relatively rarely in primary schools even when the teachers have a go.
I got all the bought parts in mine from Farnell, but it looks like you could also get them from Maplin as well:

If you make one don't forget to wire pin 9 of the ULN2003A to the positive relay power as that's what protects the outputs from back emf from the relay coils. Our stuff is all reverse polarity protected so the kids can connect it to any generator themselves - if you connect it up the wrong way around only the bottom lamp will light up. I've also made a few of these for lighting a row of recycled christmas tree light LED's as a basic voltage indicator; if you do that you can leave out the relays as each output will switch up to 500mA but you'll need to put in appropriate series resistors for the LED's.
Obviously the ULN2003A is not a precision device and if you get one from a different production batch you might have to make slight changes to the input resistors to shift the voltage range up or down. There might also be a tendency for the voltage thresholds to be temperature dependent but we haven't noticed any change in the performance of ours outside or in heated halls.

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