If anybody is reading my posts, you may have noticed gaps between them. I have several ideas for topics, but so far I’ve had a surprising amount of trouble finding time to write entries.
Last week’s distraction was a quick little project that my brother Bill asked me to do. I didn’t catch all the details, but a couple of his colleagues at Frontier Associates were planning a weekend demonstration for a group of 5th-6th graders. The idea was to mount a generator on an exercise bike and use it to power an ordinary 100-Watt bulb. The kids would pedal both with and without the load to see how much actual work was required to create the electricity. This sounded like a fun little project, so naturally I took the job.
A quick search found a DC Bicycle Pedal Power Generator, from which I freely cribbed ideas. Fortunately my demo was to be simpler, so I discarded the battery and the charge controller. My major components were:
- An exercise cycle (from Bill’s attic).
- A 100W, 24V brushed DC motor.
- A 170W, 12V to 120V inverter.
- A 100W, 120V light bulb.
My first try for a generator was a nice 24V German Fan from American Science and Surplus. Ain’t surplus great? For less than $18 total I got a really nice blower. Alas, it seems to be a brushless or AC induction motor, ’cause it won’t generate at all! (Open up the back, and instead of brushes you see a very fancy and expensive-looking controller board. Totally useless.)
Punting, I ordered electricscooterparts.com‘s EMOT-100, little brother to my wheelchair’s two 500W motors. Not quite as cheap as the blower, but this one still cost less than $50 with priority mail shipping. The important part: it’s a simple DC motor—the kind with brushes, magnets, and windings. Windings moving in a magnetic field make electricity, so spinning a plain DC motor makes it a generator!
The pedals on Bill’s exercise bike drive a big “semi pneumatic” (solid-rubber) tire. For resistance, a small adjustable pinch roller drags on the tire. I thought it would be a simple matter to convert the pinch roller to a pick-up roller, driving the motor to generate current. But the setup turned out to be too complex to describe here; it involves some steel bits, a hose clamp, a 3″ bolt and other hardware, spare skateboard bearings (blankdecks.com), some chain, and a #25 sprocket (McMaster-Carr). If you really want details, post a comment to say so!
The motor puts out 12-24V DC, but the plan was to drive a 100W household light bulb at 100-120 volts. AC really isn’t necessary, but I figure a DC to AC inverter is the most practical way to get 120V from 12V anyway. I found one on clearance that looks like an XPower Inverter 175 Plus produced under a former brand, and it works well.
This is a modified sine wave (MSW) inverter. That’s code for “square wave.” MSW offends audio systems and the like, but most appliances don’t care—especially not a light bulb. A socket-to-plug adapter plugs the light bulb directly into the inverter’s outlet. Since I don’t have any regulation or storage in the circuit, the inverter beeps to complain about low voltage as soon as you start pedaling, but then it goes quiet and a second later the bulb lights. It’s kind of clunky, but I’m told the kids enjoyed regulating the voltage themselves by watching the attached digital multimeter ($3 on sale from Harbor Freight).
Just in case, I mounted four 12V bulbs in a spare project box to make a 12-24V load. I used two bulbs each of 23W and 13W. One pair was wired in parallel, the other in series. This creates a sort of “bar graph” effect, with one bulb lighting first, then each bulb brightening in turn as you pedal faster. I suspected the kids might pedal fast enough to burn out a parallel-wired bulb, and sure enough, they did. I might go back and put a voltage limiter in the circuit, but apparently the kids preferred the inverter demo anyway.
This setup is not without worries. The 12V bulbs can get pretty hot, the sprockets are exposed, and the 120V bulb should really be in a cage. But I’m told the event went off without a hitch, and that the generator bike was a huge success. They’re even planning to take it to other educational events.
Not a bad distraction, at that.