I want to share with you my experience in building a tail light for my bicycle.
I live in Ukraine and I like to cycle to my work but commuting by bicycle is not really popular in here and we don't have bike lanes on roads. So, I have to ride on the public roads along with cars.
I'm really concerned about my safety so I decided to build a really bright tail light. So, every car driver would notice me on the road.
I wanted to build a small and compact tail light so I decided to use the case from the cheap Chinese tail light which I bough on DX (SKU:110252)
The description to this light says that it has 3w led. It's obviously not true. I disassembled it and found out that it has 40mA forward current PLCC led.
But the case was fine. It had the lens which perfectly matches with a 1W led on a star which I bought on ebay (1W red LED 40-45lm).
To power the device I decided to use two 10440 Li-ion batteries connected in parallel. Each of the batteries is rated to 600mAh but the real capacity is around 300-350mAh. So the total capacity of two batteries is around 600hAh.
The batteries fitted the original battery holder perfectly. Only minor soldering was needed to connect them in parallel.
To control the led I used the led driver board which I also bought on DX (SKU: 6190).
It has Atmel Attiny13A microcontroller on board and that is great because I've had some experience in programming them.
The led is rated for forward current 700mAh but the control board provided 1000mAh current. So I removed one of the 7135 chips to lower the output current.
I also soldered an ISP programming connector to flash a new firmware.
Here are some pictures of the process:
The original board from the tail light
The new control board
I decided to leave the programming connector for future firmware update.
And glue it back.
Main features of my firmware:
- Two flashing modes. (I decided that flashing modes are more than enough for the bicycle light)
- Low battery warning. Changes flashing mode and reduces led power to minimum.
- Turn off at critical battery charge.
- Check battery charge. Flashes one to five times depending on the battery charge.
The source code is open and everybody is free to modify it.
The total cost of build is:
Tail light case: $5.30
Control board: $3.19
Total: $ 14.79
I understand that It just a little cheaper than the original Planet Superflash tail light. But mine has 1W led and the Planet has only 0.5W
Also I got a satisfaction from building something with my hands and you can't put a price on that
Unfortunately I don't have the original Planet light so I can't compare them. I can only say that my light is bright, very bright.