Saturday 26 October 2013

Make dynamo powered led taillight for your bicycle that remains on when stopped @ crossings & traffic lights

If you bought a bicycle in the last ten years, chances are that you have a hub dynamo for the front light and that your tail light runs on batteries. Besides being silly (why would you want to pay for batteries), it is also impractical (you need to remember to switch it on and replace these batteries).

My tail-light has an extra feature: it automatically switches off when you least expect it (e.g. a little bump on the road)!

If you're like me, you will want to have a tail light that is always on (hub dynamos don't require much power from your legs) and that does not require batteries. This post will investigate how this can be done.

Hub dynamo specifications

I just took a random specs sheet for a Shimano hub dynamo and found this:
  • Rated for 6v and either 2.4w (no tail light) or 3w (tail light installed)
  • No over-voltage protection (German regulations require one, but I live in Belgium)
So, I double checked mine (it's a Shimano DH-2N35) just to make sure... and it is a 2.4w 6v hub dynamo and has no over-voltage protection. No big surprise here.
Tail light circuit diagram

Finding a decent led circuit and choosing the right components

Since I want to make the new light fit inside the existing tail-light housing, the new circuit will be a compromise between its efficiency and space.
Instead of re-inventing the wheel, I had a quick look at what the Internet has to offer and found this site suggesting tons of circuits for bicycle lights as well as this post.

After looking a little longer, I found an elegant solution from Atoomnet where the light would remain on even during a (not so) short stop at a crossing/traffic light.

ZZZThe circuit is designed in two distinct parts (see diagram above):
  • Charge pump: its role is to increase the voltage with the use of diodes and capacitors (left side of the diagram)
  • 5v DC power supply: converting the increased voltage in a stable 5v (DC) (right side of the diagram)
The following sections will explain how these circuits work.

Charge pump

6v AC is provided by the dynamo on the left side of the circuit. There is no guarantee the voltage will not reach higher values.
  • When the dynamo's pin 1 is negative (compared to pin 2):
    • Current flows through D2 and charges C1.
  • Since we're working with AC, the dynamo's pin 1 now becomes positive (+6v, and pin 2 is 0v)
    • C2 has +6v on its negative side and its charge gives it 6v extra on its positive side. We therefore have 12v.
    • D2 is blocking (blocks anything below 20v, see specs)
    • D1 lets current flow provides 12v to IC1 and charges C2
  • When pin 1 of the dynamo becomes negative again
    • C2 cannot discharge through D1 (blocking <20v) and continues discharging its 12v through IC1.
    • and we start over: current flows through D2 and charges C1.

A 5v power supply

The 7805 voltage regulator gets something between 0v and 12v (theoratically) from the charge pump and stabilises the the output voltage (on the right) to 5V. This 5V will charge the supercapacitor (C3).

The output of the 7805 charges C3 (which is our super capacitor) up to 5v (which is the max voltage IC1 provides), never reaching the 5.5v maximum for our capacitor.

Final result
(photo & integration by Fran├žois)
Since we are using a voltage regulator, if we're below 5v on the charge pump (its left), the charge accumulated in the supercapacitor will not flow back. According to its specs, it requires as input voltage between 7 and 25v and can provide a current up to 500mA (which is limited by the capacity of C1 and C2 anyway).

When the charge of C3 reaches approximately 2v, our red diode starts letting current flow though it and our taillight shines !

Since C3 is a supercap, you can stop for a (not so) short while at a traffic light and your tail light will remain on (for about 30min).

The circuit soldered & ready to be fitted in the tail-light
(photo & circuit by Fran├žois)