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
    Commented Mar 30, 2023 at 6:28


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