Automatic light switching from a hub dynamo...

There are various commercial lights which turn on when it gets dark.

I've not got one of those, I've used a switch up until now, but would like to automate my front light (my rear light is always on, the rectifier, for USB charging, is independently switched) as I keep breaking the switch.

Has anyone built an appropriate system (i.e. one that doesn't require a battery)

Maplins; sub-miniature toggle switch with the water-resistant cover. Been through the mill on my commute but solid as you like. Failing that, and to answer your question, look up the use of a photo-resistor and a pair of transistors.

Shimano has one: the SW-NX30. Should be available online.

I use the subminature toggle for my rectifier, but that's safely tucked away.
The switch I keep kicking is on top of the dérailleur post (recumbent trike) - the toggle switch would be rather vulnerable up there.


The shimano one turns up at 16 euros (although that looks like a wholesale site) to £119 (retail).
If I could find it in the UK for that sort of money then I'd adapt it to fit - but I haven't been able to yet...

I can't imagine the electronics inside are impossible to make with a few small components - then I could fit it into the dérailleur post for a truly automatic lighting system.

I searched and got results from Germany, but they won't ship internationally. What I also found were results about the extremely overbuilt circuit that limits voltage to 6V (instead of some 7V like in standard headlights), and has a tendency to turn off the light without reason, meaning you have to stop to get the light working again. Not recommended.

A recumbent trike. I like it.



You shouldn't be kicking 'owt unless there's something sticking out sideways. By derailleur post do you mean the one shifting cogs at the front? The light on this is a Lumicycle halogen mounted on an aluminium chain-guard for compliance with BHPC regs for racing. It has a sub-miniature on the aft end and seems solid enough.

And where's your hub dynamo? Is it a Brompton 20" one?

But for using an LDR, try this: http://www.reuk.co.uk/Light-Dependent-Resistor.htm although I'd swap the single transistor for a darlington pair, give you firmer switching. They also offer an op-amp version which seems unnecessarily complex.