Better DC-DC converters

I went back to using bipolar transistors and managed to make a circuit that could get up to 12 volts. I changed the output circuit to a darlington network to get more gain (hopefully.) The circuit was operating at an input voltage of 2.7 volts.

I tried switching to the 5-volt supply and I could comfortably get 12 volts into 220 ohms — 54 mA for a total power of 0.65 watts; with 51 ohms, I could get 8 volts or 1.3 watts. I'm pretty sure I'm up against the output capacity of the transistor at this point, so I tried a heat sink but it didn't help.

Anyway, in the process of digging around, I found this high-power NPN transistor. I hooked it up on the DC-DC converter and managed to drop 15 volts across a 51-ohm load with 5 volts in — a total of 4.4 watts. That's getting there.


Current limiting the new LED's

I originally bought resistors to use with the RGB but I realized the voltages and currents might be different. Indeed, the 3mm red/blue and 3mm green LED's require different values. I calculated the results with either a 4-volt or 5-volt supply (which I'll probably use.)





Desired Resistor


2.2 V

20 mA

2.8 V
3.8 V

140 ohms
190 ohms


3.3 V

20 mA

1.7 V
2.7 V

85 ohms
135 ohms


3.2 V

20 mA

1.8 V
2.8 V

90 ohms
140 ohms

Anyway, I tested using 150 ohm resistors for blue and green and a 180 ohm resistor on red. This balanced things so each LED was driven at about 30mA with a 7.5V source. The 150 ohm resistors dissipated 0.14 watts and the 180 ohm dissipated 0.17 watts, but with 20 mA, the figures are 0.06W and 0.08W respectively, allowing me to use 1/8W resistors. I also noted that I can use two layers of Scotch tape to diffuse the light nicely. I checked the circuit with 5 volts — green was 18mA, blue was 20mA, and red was 22mA — very close to ideal and easy to fix in software, so I went ahead and bought packs of 150-ohm and 180-ohm resistors.


More people in the back yard

I showed off the Bike With 2 Brains to my friend Sondra who had bought me a ticket to go to Burning Man in 2004 for my birthday. She was really impressed that I actually got as far as I have. Plus, it's insanely fun to ride. Later on, some new friends Elliotte and Sarah got to try it and they liked it as well. Sarah slipped off the pedal and hurt her foot a little, so I think I'm going to find some other pedals.


Building a better, stronger, faster fork

One of the forks broke during a test on July 4. The wheel got stuck against a bump in the grass and wouldn't turn. When more force was applied at the front wheels, the axle slipped out of the slot in the fork causing all the force to be applied to one leg of the fork and the wheel axle. It bent both parts pretty good.

I noticed that I had used forks for 26" wheels so I did the same for this one. I created two more bent forks so I'd have a spare.

Nothing like spotting poop in your yard to get you to slip the nut off the side of the fork and bend it to uselessness.


Independence Day picnic

I got around to getting the vehicle out of the car and put together the front wheels. Now that I can see it in good light, the paint job is pretty dinged up — I'm not too happy that it chips as easily as it does. Anyway, I got to let some friends of mine try it out and they really liked it.

I think I damaged the back wheel when I was on the back end while Jan and Todd were trying to drive, but we kept going. Jan and I finally broke it in when Jan spied dog poop in my yard — and I don't even have a dog. Oh, but anyway, the cheaper-forked back wheel slipped out of the fork halfway, bending both the axle and the fork irrepairably.

Nothing like spotting poop in your yard to get you to slip the nut off the side of the fork and bend it to uselessness.