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el cheapo rotor controller



In the eyes of a college student everything should be cheaper... why not 
rotor controllers?

The PC gameport has four analog inputs [well sorta -- I'll explain in a 
second] and four digital inputs.  It seems to me that the ubitquitious 
and cheap nature of this port could be put to good use controlling 
antenna rotors.  The "analog" inputs do not work like a conventional 
ADC, instead the port expects a variable current as the input.  The port 
measures how long it takes for this current to charge a capacitor to a 
known voltage and reports this time as an 8 bit value.  Most rotors that 
I've come across use a potentiometer for position feedback and thus 
provide an excellent variable current output.  Otherwise, if the 
position output is a true voltage source a simple resistor in series 
between the voltage output and the analog input should create the 
necessary variable current.  The parallel/serial port could be used to 
output the appropriate motor control signals.  Between these two ports 
and a simple relay board we have a very low cost 4-axis rotor 
controller.  It seems to me that this would go along very well with 
Bob's simple APRS tracking system which I believe is open loop.  Even if 
your "legacy free" computer does not have a joystick port I'll bet you 
can pick one up on a cheap sound card for <$10.  I'll have to see what 
kind of code I can churn up to make this work...
73s,
David Carr
KD5QGR
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