# Controlling a 4 wire servo

I'm trying to control a 4 wire servo that was used in an RC car for the steering. It has two power connections, but I can't make sense of the other ones. If there is power on the red and black wires, the motor turns into one position and stays there, no matter what I put on the other wires. I also tried to vary the voltage of the power, no difference. How can I control its position?

The board it was connected to: https://drive.google.com/file/d/0B8Ox1li7YRxoREt1Z3RNcXp4OU0/view?usp=sharing

Usually RC servos work on a pulse principle. The input idles low. A pulse (TTL-level) of width 1.5ms puts the servo at mid-position (using feedback from an internal pot), 1ms brings it to one extreme and 2ms to the other.

The pulses need to repeat at some frequency, 50Hz for example, the exact frequency is not too important. The same receiver can supply, in turn, pulses to several servos since the maximum pulse time of 2ms is much less than the 20ms period.

• But which one is the input, and what's the other one for? I already tried to send PWM signals via an Arduino on both of them, without any results. Commented Aug 31, 2017 at 22:45
• If red goes to 1 & 14, and black to 4, I would guess that red and black are power based on this only. If you reverse polarity on the power you will likely destroy the chip (and it looks a bit like that has happened in your photo). Commented Aug 31, 2017 at 23:41
• The motor is still turning though. But what would the grey and the white wires be for? Commented Sep 1, 2017 at 7:57

I'd suggest from the photo that Grey/White are the DC motor power, and in an RC car will most likely be 7.2 V (a 2S Li-Ion).

The Black/Red wire is the signal wire.

You should find that one wire of each pair is ground ......measure from Grey to Red and Black ...then White to Red and Black. Whichever pair are shorted those represent the ground. That will then leave you with two wires ....one of the Grey/White will be the 7.2 V motor power and one of the Red/Black (most likely Red) will be the input signal.

As Sphero pointed out the signal is a repeating frame at 50 Hz with 1.5 ms period.

• I measured all the pairs you mentionend, but it seems like none of them are shorted Commented Sep 1, 2017 at 7:43
• Take a photo of the back of the PCB Commented Sep 1, 2017 at 16:21

So I figured it out, all I had to do is connect grey and white to a power source, and depending on the polarity the motor turns either left or right, and centers itself otherwise. But it has to be an external power source, connecting them to the same 6V source in parallel didn't do anything.