Sorry, Larry perhaps I wasn't clear. I think that is what I said.
My idea of using a brushed controller was to put it only in the motor leads not the retract servo supply. Driven from a separate channel to control the effective voltage to the motor, how is that different in principle from a pot in the motor supply? Advantage is that if it is a proper controller and not just a variable resistance it supplies time sliced full voltage rather than reduced voltage to the motor and keeps the torque up. It acts only as a speed controller in the motor supply circuit and has nothing to do with the logic of the servo retract function.
The gear channel pulse width controls when the servo retracts and extends as normal, the other controls the speed of the motor.
Agree the TI chip driven by an external pulse source is not straightforward. On further reading, they don't in fact respond to a standard RC PPM pulse width but are a simple bi-directional controller driven by steering logic which would need to be switched at the correct intervals to alter the speed.
It may not all work in practice but it would be fun to try.
UPDATE: But it won't work on thinking about it a bit more since the retract is supplying a bidirectional voltage to the motor and any motor controller will only work in one direction - oh well, that's the big advantage of a pot. A resistor doesn't care which way the current flows.
Originally Posted by Lnagel
Using that TI integrated circuit, which is essentially a small DC motor speed control, or an aircraft low amperage ESC would not work. They essentially put out a DC voltage that varies in amplitude as the PWM input from the receiver varies in width. The output of those speed controls is incompatible with the input requirements of the gear assembly. The logic in the gear assembly is such that it looks at the length of the incoming PWM pulse from the receiver and decides if its wider than or narrower than 1.5 msec. It then uses that pulse width to determine the polarity of the voltage applied to the motor. Regardless of the length of the input pulse the logic applies a DC voltage of the same amplitude to the motor. As such the speed of the gear motor is always the same and there is no effective way to control it remotely.
The same goes for the servo speed function of a transmitter. What that does is control the rate at which the PWM changes from minimum width to maximum width and vice versa. Unlike a servo, the gear logic doesn't care about the rate at which the PWM changes, it only cares about the absolute width of the pulse as compared to 1.5 msec.
What could be done is to put a potentiometer in series between the gear motor and its power supply. The potentiometer could then be adjusted manually to set the desired gear operating speed.