I'm trying to add turn indicators to my bicycle, driven by the bicycle dynamo (nominal 6 VAC, but voltage does vary; between 6-12 V in all practical driving conditions). The input voltage is rectified and smoothed with a lot of caps. The indicator lights are pretty much off-the-shelf, but I'd like to get some feedback on the handlebar end on whether they are actually blinking or not. (The blinking logic is in the lights themselves, so all I see is whether they draw current or not).

I've got a 1 ohm sense resistor wired up between the indicators and ground, so what I'd like to do is turn on a LED whenever the sense voltage exceeds e.g. 40 mV*.

My current idea is using LM358AMX as a comparator (since I have those readily at hand), and then have a voltage regulator, current limiting resistor and the LED in series on the output. But since it's a dual op-amp, can I use the second op-amp to do the regulation for me? Or, if I setup the op-amp as a 100x amplifier, somehow limit its output to 3 V?

(*) Not very exact. If I use a resistor divider, the value will depend on VCC anyway, but then again, so does the indicator current.

  • \$\begingroup\$ (Congrats on not supposing you need to stabilise the supply voltage. You'll need a reference voltage of sorts. To not damage the (no pun intended) indicator LED, limit current, not voltage.) \$\endgroup\$
    – greybeard
    Mar 30 at 6:28


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.