Robust 24V alternator input circuit

Question

I need a bit of advice on the following proposals for interfacing an industrial mining truck's 24VAC alternator signal to a micro. The frequency is DC to 1khz. The voltage varies between 5 and 24V depending on the charging. Its a very dirty signal and the return path is via the chassis ground of the truck. I have 3 ideas but need some input on which one will work the best - robust is the name of the game here.

Any other ideas or advice would be much appreciated.

If you measure a signal on low-side so it’s tied/connected with Gnd (not a Vcc) you should use a npn transitor with emitter on Gnd. An optocoupler is a good idea. — Michal Podmanický, Mar 12 '24 at 15:41
At what voltage level of input signal you want to switch an opto? — Michal Podmanický, Mar 12 '24 at 15:46

score 0 · Answer 1 · answered Mar 12 '24 at 18:03

My recommendation is to start simple and add complexity where needed.

My first proposal is therefore to use an optocoupler with a single resistor on the input side, i.e. the optocoupler LED will be powered by the alternator voltage through the resistor. Maybe you need to limit the voltage with a Zener.

Robust 24V alternator input circuit

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