Potentiometers for a radio control


I'm building a typical remote control airplane and would like some advise on something.

I have a pair of ps2 joysticks I am using that are electrically just potentiometers.
I noticed that all decent remote controls have trim in addition to the joysticks for fine control.

I was wondering if it is possible to connect the trim pot and the joystick pot to the same analog input.
I guess there are two possible ways:

  1. In series so that the resistance of both are added - So if the joystick was 10k and the trim 2k, the trim would basically add a smaller amount to the value of the joystick
  2. In parallel - this would follow the standard 1/ x+ ... formula.

I assume 1 is the better way to go, but does any one have hints or suggestions on this?
How would it all be connected?

Why not use a PS2 controller as it is? There are libraries to interface directly
to the PS2 controller as it is basically SPI.

I have the joysticks - not the actual controller.

Maybe I should just find a full controller.

I had a better plan though:
Add 5k resistors to both ends of the trip pot and then connect that in parallel to the joystick pot.

That way, the trim will count far less than the joystick and also have a minimum and maximum effect.

The circuit you describe may not work exactly as you imagine since the parallel pot will affect linearity of the analog output. You'll be introducing what's known as “exponential”, which makes servo movement less pronounced when the stick is near center and greater at the extremes of stick deflection. In fact, I've modeled an expo function after the same type of circuit. In your case, I expect the expo rate will not be balanced unless the trim pot is centered, which negates the effect you want. The 5K fixed resistors should reduce the nonlinearity and imbalance, but it may still be noticeable.


Thanks for that info Jon
I was afraid of that happening because of the 1/ aspect of resistors in parallel.
I guess the best way forward is each pot on its own analog in - then I can control the balance etc in software.