kwartzlab makerspace

Sep
7

LED bicycle light refit

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I ride my bike almost every day between April and November, so a good bike light is a must. This is an LED conversion of my ancient CatEye incandescent bulb. Finished this last spring. Click thru for deets…


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This light clamps to the seat post and originally ran on 2 pair of AA batts for 3V. Issues: it was heavy and power hungry (pulling several hundred milliamps, 600mA I think).

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Design goals were:

  • reduce weight (not me, the light)
  • reduce power
  • improve visibility by flashing the light
  • maintain the same water-tight case

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This is really a simple circuit, breadboarded here for proof-of-concept. These are high-brightness LEDs from Digikey coupled with five 2N7000 N-channel FETs to pull down on the five strings of LEDs.

A Microchip PIC 12F675 8-bit microcontroller flips the FETs on and off in sequence to establish a flashing pattern. The pattern is stored in EEPROM and can be updated by dumping a new table into the chip.

That’s right: no current-limiting resistors. To save parts and keep the size small, I used software-based PWM on the FET gates. I just did cut-and-try to figure out a duty cycle that would give good brightness, but keep the average current low enough to not smoke the LEDs.

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I cut a piece of proto board so that it fit inside the original lens. Then I poked the LEDs through in a symmetrically random way until I filled the board.

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Tried to be a little strategic with the LED placement so there would be minimal mods to the proto board traces required.

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The original case had small side windows with light pipes to the bulb, so I added these to side-facing LEDs, lined up with those windows.

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I’m big on unit-testing (you think debugging software is hard? try hardware), so I just plugged this rig into the same breadboard. Wanted to ensure it was bright enough with the lens on and figure out total average current draw. It was something crazy-low like 40mA. It’s a fairly slow flash rate.

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I transferred the microcontroller and fets to their own protoboard which I fabbed to fit in one of the battery slots. I dropped 2 of the 4 AA’s to make room for this gear in the case, but still running at 3V.

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It all fits. This is more wire than was probably necessary, but I had the room and didn’t have to wrestle with it too much to get the whole thing in the case that way.

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Boxed back up. All design goals were met:

  • much lighter having dropped the 2 batts
  • power consumption is a mere fraction of the old bulb
  • excellent visibility – these LEDs rock
  • it all fits back in the case

I put a fresh pair of AA’s in this in the spring and they easily take me through the whole season.

I really enjoyed this project because of its utility and because I was able to build exactly what I wanted. I like the feature of being able to upload different flash patterns and have considered using the LEDs as device input in future versions to allow contactless uploading of patterns. The bill of materials on this conversion is in the neighbourhood of $6.

Just ping me if you’re interested in the code.

Happy making,
DW

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