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A friend of mine would like to convert her bottle dynamo (bike generator) to run a LED light. The light needs to stay on for maybe 90 secs when the bike is stopped, and needs to blink (using a particular pattern).

The generator is labelled as 6V, though it has been difficult to establish what that actually means in practice: the available information suggests that peak voltage varies with speed (see here). On the assumption that the "dynamo" is producing higher voltage than is needed by the uC, Here is what I have in mind for the solution:

enter image description here

The idea for two capacitors as following:

  • it is difficult to find an affordable supercap that is rated for high voltage, while 5.5 V 1F caps are cheaply available
  • C1 smooths out the input to the Vreg
  • C2 is what the LED/uC runs on while stopped. The LED might not be as bright (controlled by R1), but at least it is on.

I am concerned about what happens when the voltage generated by the "dynamo" is not sufficient to supply the uC power, and about wasting power in the regulator. Do you have suggestions for mitigating these problems, and is there anything else I am missing?

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    \$\begingroup\$ With regards to the super caps, why not place two in series? \$\endgroup\$ – secretformula Dec 28 '14 at 23:43
  • \$\begingroup\$ @secretformula nice idea, thanks. I think to be safe you might need three or four, in case the rider goes does a long hill with the dynamo engaged. Really need to find out what the max voltage is, but form what I understand it can go pretty high... \$\endgroup\$ – angelatlarge Dec 28 '14 at 23:49
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    \$\begingroup\$ A few ideas: 1) Boost/Buck converter between the rectifier/small smoothing cap and main storage cap? 2) uC controls LED brightness according to available power/voltage? 3) use as few diodes and BJTs as possible. Use a MOSFET for Q1, save another 0.6 V. 4) uC could be the regulator itself, with another MOSFET to turn off the generator input when the capacitor is full (near maximum V). Just make sure the circuit will charge the cap in the state before the uC wakes up at 3.3V \$\endgroup\$ – tomnexus Dec 29 '14 at 0:15
  • \$\begingroup\$ Very useful ideas. Do you want to make this into an answer and expand on making the uC a vreg? \$\endgroup\$ – angelatlarge Dec 29 '14 at 2:00
  • \$\begingroup\$ Related: electronics.stackexchange.com/q/228258/568 \$\endgroup\$ – hlovdal Apr 15 '16 at 12:37

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