Joystick reproduction (RC transmitter)

Tony Duell ard at
Sat Jun 13 14:24:38 CDT 2009

> Tony Duell wrote:
> > Not being a Cromemcop person, I don't have scheamtics.. 
> There's a PDF on Harte's S-100 site.
> > How are the pots used?
> +12V on one end, -12V on the other and the center tap goes
> back to the A to D card.  The test procedure specifies +2V
> and -2V at the full deflection of the stick.

Assuming there are no typos there, this sounds like one of those joystick 
mechanisms that only uses the middle bit of the rotation of a normal 
potentiomete (hence the swing to +/-2V, not +/-12V). A lot of joysticks 
I've seen have special pots with a shortend resistive track so you get an 
output swing essentially the same as the supply voltage -- this is not 
what you want here.

Howeer, assuming the input inmpedance of the ADC card is fairly high, the 
actual value of the potentiometer resistance is not likely to be critical.

If you have _any_ analogue joystick, you can probably convert it to work. 
If it's a 3-terminal potentiometer type, fiddle the voltages applied to 
the ends of the track to get the right swing on the slider (e.g. if the 
slider goes from one end of the track to the other, you could supply it 
from +/-2V, and have a +/-2V output). If necessary (if the potentiometer 
resistance is too high), you can buffer the output from the wiper using 
an op-amp wired as a unity gain buffer.

If you have a 2-terminal joystick (a variable resistor), like most old PC 
joysticks, it's a bit more work. WHat I did wa make a current sink 
(op-amp + transistor + R's) and connect the variable resistor between the 
top of that and +5V (I kept it going to +5V, as in a PC, so that any 
autofire circuits in the joystick would still work). Then the voltage 
drop across the variable reisstance is proportional to the resistance 
(constant current, remember). I tapped that off, and used an op-amp or 
two to get the right output swing and offset. 


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