EE question: convert RS232 voltages to RC servo

[EricVanWyk]

Originally Posted by Ether

If I put 12 volts reverse voltage across 2 diodes in series, won't I have 6V reverse voltage across each one? That's awful close to the max allowed for the H11L1M.

Shouldn't there be a resistor R2 across the H11L1M inputs to prevent this?

This is where things get painfully handwavy, because we are outside of what the datasheet specs. Also, you can't really assume that 12V/2 = 6V. You can think of diodes in series as a resistor divider where the resistors are extremely non-linear and suffer huge part to part and thermal variation. The math gets ugly and full of conflating variables. Make conservative assumptions and eliminate as much noise as possible, expect an answer with >=1 significant figure

Take the 1N914, selected by sorting digikey by price and choosing the first one with a decent datasheet. Figures 1 and 2 show reverse bias current curves, and are in the 20nA range for 12V. I'm confident the LED can handle this 20nA of reverse bias because at 3V in reverse it is leaking up to 10uA. The 6V number on the H11L1M sheet means that 6V @ zero ohms is being applied and survived forever in a worst case manufacturing / temperature scenario.

Another option is to put the diode antiparallel across the LED. The disadvantage is that it conducts in both directions. The advantage is that it is easy to prove from the datasheets that everything will be ok.