[iltheo84]

Hello, I need to put together a small circuit to convert a switched 12V input to 0-5V input for my Speeduino.
I want to make it the most robust and realiable I can (without overengineering it), and I was planning to use an optocoupler with a suitable series resistor to achieve a forward current of 5mA @12V (a little more @ 14V); the output of the optocoupler would then pull the Speeduino input to ground when active.

If it were you, would you add anything else to the 12V part of the circuit to handle spikes, fluctuations and such?

[dazq]

On my opto inputs I have a zener diode to protect against overvolt pulses, there is a thread on input circuits although mainly meant for trigger use it will give some pointers for GP use too.
It is possible to make a 12v input into 5v using just a single diode and pullup or you could make a potential divider to drop it also.
A opto does provide the ultimate in isolation though.

[JHolland]

Opto-couplers really aren't required because you have a common ground, I don't recall ever seeing one in an OEM ECU.
It may not be necessary to add the zener if the forward current during the surges does not exceed the maximum rating but it may give sufficient reverse voltage protection.

[iltheo84]

Opto-couplers really aren't required because you have a common ground, I don't recall ever seeing one in an OEM ECU.
It may not be necessary to add the zener if the forward current during the surges does not exceed the maximum rating but it may give sufficient reverse voltage protection.

I have a few optos (Vishay VO615A) laying around so I thought I could put one in good use