What is inside an RC servo motor?

To drive an RC servo, a PWM signal* is sent to the motor. The motor interprets the signal and sets its angular position accordingly. What is the internal demodulator/drive circuit to do this?

Most of the work is done by a dedicated IC. One I found from a web search, and the other I found by taking apart a servo (that I ruined with too high of a supply voltage).

 

The motor is driven by a half-H bridge (two NPN transistors); for the smaller model that I opened, the transistors are integrated within the IC. Part is Agamem AA51880. Mitsubishi also makes a similar IC M51660L with external transistors.

Interconnections to pc board.

Darren Sawicz discusses servo operation further here, including all of the common 3-wire color codes and a way to make your own servo.


* Servo position for pulse widths: 1ms LEFT    1.5ms MIDDLE    2ms RIGHT
 Total time must be roughly 40ms. (10 is too low and 70 is too high.)