[TroyO]

I understand that the POT leads on the ubiquitous KBIC and similar 90v dc motor controllers are "hot" and you can't just control them directly with a digital signal. (Otherwise you let out the magic smoke.) You need some kind of isolation between the digital circuit and the "hot" mains level voltage pins the POT hooks to.

I was looking around and came across an "idea just so crazy it might just work", LOL.

In concept.... wire a LED to a PWM controlled pin on the parallel port and use appropriate current limiting resistors.

Stuff it in a tube, and on the other end connect a photoresistor and wire it between P2 and P3, and use a ~10k resistor between P1 and P2 to provide a pull to the low side.

As the PWM LED gets brighter, resistance drops and the motor goes faster.

Things that may make it "wonky" are the non-linearity of the photoresistor and the response lag? For $5 or so it may just be worth a shot, LOL.

Is there such a device already packaged in an IC? "Opto-isolators" always seem to be digital or to have a built in transistor that outputs voltage?

[Al_The_Man]

Here's one you could adapt maybe, it is analogue isolation, but might be converted, also other RuggedCircuits here may help.
Circuit #24 : Analog Optoisolation
Also KB's isolator is board SI-5.
Al.

[TroyO]

The KB isolator board is $75.... kind of steep. CNC4PC has something that also may do the trick, but it's $40. The circuit listed might also work but looks like it would need a ~5V supply. (Which could probably be vampired off the POT high side output?) I'm not sure it would work right since I think what KB has done is ride the "0-10v" pot poewr lines over the AC input signal so whatever output you have would need to ride the AC "wave" too.

Many ways to skin the cat, as they say. I want the $5 boxcutter option, LOL. ;-)

To that end, I may have found something..... Excelitas Technologies Sensors - VTL5C2 - Optoelectronics & Lighting - Optocouplers/Optoisolators - Allied Electronics

I ordered 2... what the heck, worth a try.

[jalessi]

TroyO,

This looks interesting.

Serial Motor Controller

Jeff...

[Al_The_Man]

Many ways to skin the cat, as they say. I want the $5 boxcutter option, LOL. ;-)

To that end, I may have found something..... Excelitas Technologies Sensors - VTL5C2 - Optoelectronics & Lighting - Optocouplers/Optoisolators - Allied Electronics

I ordered 2... what the heck, worth a try.

These are a little cheaper
Invalid Request
Al.

[TroyO]

Hmmm, the "Variable Isolated Attenuator" circuit in the datasheet looks like just the ticket. I'll try the ones I already ordered first but I bet you are right, that could be used too. The key is basically outputting a resistive load that can ride "over" the AC signal. I'm still an electronics newb but I *think* what the POT leads on those controllers are doing is they are riding an AC signal, so if you were to read each with a multimeter you'd see a ~70v signal on the "low" pin, an ~80V signal on the "High" pin and the wiper would sweep the ~10V differential...... I think, LOL. Anyway, just trying to drop a voltage output on to a ~70v signal relative to gnd would exceed the voltage specs on most devices. By using a resistive output I'm thinking it can just "float" along with the rest of the circuit while mantaining isolation. The "Isolated attenuator" would accomplish the same goal.... being able to "float" on the high voltage pot lines so the only voltage it "sees" is the differential.

Attention anyone listening in, I am a newb so take anything I say....errr, type with a block of salt.

[TroyO]

So, I got to playing around with this even more and set up a breadboard for testing and sure enough, it seem sto work just fine. I will note this is with a lightbulb but I think it will behave similarly when really hooked up to a motor.

While not exactly a precision control, it should be able to give you ballpark RPM control from your PC.

Cost was $8 shipped for the Vactrol and lets say $0.10 for the resistors. Not bad bang for the buck, I think.

Note that the Vactrol is providing the "isolation" so be sure there's no connections between the two sides.

CoC Schematic and quick demo vid attached. (I hope.)

VID 20130910 00006 - YouTube