>> A 3v9 zener diode + a small series resistor?
> 
> So the zener would drop down my higher voltage to 3.9V, and the
> resistor would act as a current limiter, both on the remote end. That
> way I get to use a higher voltage on the sending unit side, correct?

No, at least I don't think you think the way I think.

Put the small resistor in series with LED, zener diode in parallel with
this combo. Zener shorts a reverse voltage, and limits (by drawing
currunt, hence energy) the 'normal polarity' voltage. The resistor
limits the (peak) current when the zener is limiting the voltage. In
normal use the zener is not active, and the resistor adds a small
voltage drop (assuming a constant current driver).

Wouter van Ooijen

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