MoTeC Global Forum

I want to control an RC servo using an aux output on an ADL3. The RC servo requires a duty cycle control with fixed frequency (I think). Square wave, 3-5V. Power for the RC servo will be from a battery.

An aux out cycles the ground signal. I need to cycle the voltage. How can I do this? (If I had an ECU I think this would be simple as they have voltage outputs).

Thanks in advance.

Leigh

You need to wire a pull-up resistor between the Aux output and a 5V supply on your dash. Use a 1k, 4k7, or 10k resistor. This will feed 5V to the servo which is pulled to ground by the Aux output, giving the required square wave.

Most RC servos work on a small pulse width between 1mS and 2mS in length for their full range of movement. The repetition rate (or frequency) is not critical, just the pulse width. You can set an Aux output to Duty Cycle Control at 50Hz , and use a look up table to generate the correct duty cycle.

Effectively you need a minimum duty cycle at 50Hz of 5% (ie 1mS) and a maximum duty cycle at 50Hz of 10% (ie 2mS) for the full range of movement, so a look up table will help ensure that you don't run outside of these duty cycle limits. Most RC servos drop out if the pulse width is wrong.

Thanks Duane, that should do the trick!

Some servos like older Futaba 9k series with metal gears are frequency sensitive. If the frequency is to high (over 70Hz) it will start to rotate and HW will be damaged. 20ms pulse to pulse or 50Hz will be just fine. Some servos will need 1.6ms pulse for center position.