In another threaddiscussing SSC-NG, a question came up related to interfacing a 3.3v microcontroller to RC servos. I didn’t want to confuse that thread, so continuing discussion here…
As stated in the other thread, it seems that a 3.3 volt PWM signal is sufficient for the servo control line. I’ve done my own testing using a Propellor chip (3.3v on all IO pins). In my test, I’ve connected the servo control line directly to an IO pin on the micro, with only a 1k resistor in series. The servo power is from a 5v line, the micro power from a 3.3v line, and everything shares common ground. I’ve tested with various servo brands, including Tower Hobbies, JR, and Hitec. I only have analog servos currently, but all of them have worked.
So my first question…does anyone know of a servo specification, “standard”, or even a data-sheet, that says a 3.3v control signal is sufficient ? (If there was such a spec, I’m guessing it would state that the servo control signal is TTL compliant.)
My second question is related to the recommended resistor on the IO line. I’m new to electronics and trying to understand basics still. My question is why is the resistor recommended? (And normally, it’s recommended to use a 4.7k resistor…I simply didn’t have one laying around so used a 1k resistor.) Is this what is referred to as a “pull-down resistor” and it is recommended only so that the IO pin does float or go randomly high due to some small “backfeed” from the servo?