EE question: convert RS232 voltages to RC servo

[EricVanWyk]

I suggest a fully series circuit: +/-12V -> diode -> resistor -> PWM input -> return.

Lets take a look at the jaguar because we have a schematic available for it. Its PWM input stage is a H11L1M in series with 150 Ohms.

The H11L1M has a threshold current of 1.6mA, and can handle up to 60mA continuous. It specs forward voltages at 10mA in the 1.2 to 1.5V range. It can only handle up to 6V in reverse.

Lets target 3mA. The forward voltage is in the 1 to 1.3V range (Figure 6 plus slop), and lets say that our series diode's forward voltage is .3V. Total silicon drop is somewhere between 1.3V and 1.6V, lets call it 1.5V. Our resistance should be (12V - 1.5V) / 3 mA = 3.5kOhm

Zener-ing it can be made to work, but that is more appropriate for a voltage-mode input. The Jaguar and Victor both have current mode inputs.