Paul Kimbrel's Musing

I’ve been tooling around with this project for a bit now, doing research, getting materials, etc., so here’s a quick update of how I got to today…

First of all… references. I was posting a question on QRZ around rotors and I got several great references. The best one was Doug Braun’s website:

http://www.dougbraun.com/rotor_mod.html

From here, I’ve gotten some great ideas and information on how to create my az-el rotor. First, I needed an azimuth rotor. That was easy. When we had our house re-roofed, I had the guys take our old TV antenna down. It had a perfectly good rotor on it.

I started looking for software to control the rotor and found this website:

http://ludens.cl/Electron/fodtrack/fodtrack.html

Now, I’m not entirely interested in the software (since I’m not running DOS or Unix), but I am interested in the hardware described that FOD Track controls. Several other software packages can control the same hardware and it’s pretty straight forward.

It works like this… An interface is hooked up to the computer’s parallel port and a couple of 8-bit signals are sent to the interface to control the rotor. The interface takes the 8-bit signals and runs them through a digital-to-analog converter (DAC) to get a voltage from 0-1 volts (so, for example, a digital number of 128 would be .5V).

The analog signal from the DAC indicates the desired position. Now comes the tricky part. Both the azimuth and elevation rotors are assumed to have potentiometers (variable resistors) on them that can be used to indicate their position. The higher or lower the resistance, the farther the rotor is in one direction or the other.

Then, the voltage drop across the potentiometers is compared to the voltage coming from the DAC. If they are not the same, the rotor is turned in the right direction to make them same. When the voltages match - the rotor stops. Some calibration is involved to get the right values to align.

The problem with my azimuth rotor is that it does not have said potentiometer on it. So… I’m going to add one!

Cue Doug’s site. Doug has done just that with a cheapo Radio Shack rotor. I’m going to do one with a Magnavox rotor (from Menard’s). I may have to deviate from the design a bit (the chassis are not quite the same), but I intend to chronicle this adventure on this blog.

Of course, once I get the azimuth rotor figured out, I’ll have to figure out the elevation rotor. But one challenge at a time!