Mod a Bike Flasher

I am never happy with standard devices. Everything (OK, nearly everything) has to be changed, improved, altered. So when I got my bike flasher about seven years ago (1999!) it had almost invisible green LEDs in it. At that time those hi brite blue LEDs came out. I wanted that! But there was a problem: as the flasher is powered by two AAA batteries there is not enough voltage to light the LEDs, which require 3.7 V. So I poked around with my scope and found that the LED current is actually pulsed at 150 kHz. Opportunity loomed! When you use an inductor to store energy it can be used to increase the voltage. Using a PNP transistor (any universal device will do) I fed the pulsed output of the flasher chip (securely hidden beneath a blob of protective plastic) to the base, switching it on and off. It worked like a charm! After better LEDs and also white ones became available I upgraded them.

Then disaster struck! My flasher got betweeen my spokes and the coil was wrecked. I had no idea of its value, and an unoperative flasher. So I decided to perform a mini study to figure it out. I found a coil, I guessed it was 22 µH and atached it to the circuit. It turned out that (unfortunately not documented) this was not enough: the coil saturated quickly because it was physically too small and it did also not store enough energy. My scope showed that after only 2 µs voltage dropped to zero, as the coil became saturated. The current rose too fast because the value was too low, and ended up being 0.18 A way too soon. This was the maximum current the transistor could supply. A FET would not have been handy because of the voltage needed to fully drive a P-FET and also there would not be much of current limiting, something the base current of my trusty BC557B would do for me. So I decided to find a bigger coil. As I have lots of power supplies of defunct satellite digiboxes this was easy. I settled for a 47 µH coil which was exactly right. It does not saturate and reaches the maximum current of the PNP just before turnoff. On the scope can be seen that the voltage swings negative (hence the reverse polarization of the LEDs). When the energy in the coil is depleted the coil starts to resonate. The amplitude is below 5 volts so this is harmless.

So now I dumped twice the energy in the coil and it did not saturate on me! I was rewarded with a higher light output from the LEDs and a halved power consumption, which had dropped from 90 to 50 mA at 3.0 V. One more note: also not photographed is the 0805 SMD base resistor. I had used a leaded 0.25 W carbon resistor but it broke off and damaged the PCB tracks due to bad bicycle paths...


schematic

top_side_switch

coil

finished1

finished2

scope



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