Nuts and Volts - October 2010

>>>YOUR ELECTRONICS QUESTIONS ANSWERED HERE BY N&V READERS

Send all questions and answers by email to forum@nutsvolts.com Check at www.nutsvolts.com for tips and info on submitting to the forum.

more. The best device would have a floppy drive 34-pin connector on one end, a USB plug on the other, and some controller in the middle. The current USB external floppy drives are quite different from anything I could use for this project.

#1 The best way to approach this and probably the least problem-filled is to get an older PC — one that will run your adjustment software (lots of PIIIs running XP out there that you can buy for a song). Get a long 34-pin floppy cable, as well as a long power cable extension, open the PC up and connect the floppy cable to the motherboard, connect the power cable extension, pop one of the front covers off of the PC, snake the cables through the opening, then seal the PC back up. Most PC motherboards still make a provision for a floppy drive connection so that isn't a problem. A search on the Internet for the cables (try Cyberguys - www.cyberguys.com or Electronix - www.electronix.com) shouldn't be difficult. Now you have a test PC with the necessary cables hanging out the front of the unit. Unfortunately, they do not make an external floppy adapter like they do for making external IDE or SATA devices that use USB connectivity. What makes this setup nice is that if you can connect to the Internet with the PC, not only do you have a nice test jig you also have a PC to surf the Internet for your repair business.

Ralph J. Kurtz Old Forge, PA

USB floppy that isn't "integrated" and that consists of a USB to floppy controller with a standard 34-pin interface.

Several places sell these in the $40-$60 range. Try www.pcrush.com/ product/Floppy-Disk-Drives/590364/ Buslink-FDD1-Floppy-Drive or at Amazon Market place from www1. shopping.com/xPO-Universal-Bus link-FDD-1.

Barry Cole Camas, WA

[#6102 - June 2010]

AC Motor Control

I'm building an alt-az antenna rotor controller from scratch, but I'm having trouble with the circuit for controlling the AC motor in the RadioShack rotator. I've tried using a TRIAC with little success. I believe they are my answer but I don't understand how to use them. What I need is a circuit that will accept one direction bit and one enable bit that will control a motor’s direction of spin. The original controller acheived this by feeding one or the other non-neutral leads of the motor with ~22V AC; the third lead is common neutral.

Don't forget to put diodes across the coils so the inductive kick doesn't hurt the driver or microprocessor output when the relay de-energizes.

If the problem is simply isolation or ground referencing, consider using opto-coupled TRIACs. They work just like the mechanical relay. You put their terminals across the wires that went to the pushbuttons. Their internal LED is the gate drive, so ground referencing is not a factor. They can also be driven from microprocessor outputs. Use a series resistor to limit the current to 20 mA or so. Either solution would be cheap — buy the parts for both and see what works best.

David Sarraf Elizabethtown, PA

#2 Since you didn't supply information about the power requirements of the rotators, I just winged it. The schematic in Figure 2 should be helpful.

Digi-Key sells a high power opto-coupler TRIAC for about $1.23 USD (751-1490-5-ND). If your rotators use less than 1A, this should do it for you.

Daniel De Jager Edgewood, WA

#2 You did not specify 3. 5, 5. 25, or 8" floppy drive. Here is something that should work: The Buslink FDD1 3. 5" USB floppy drive IS a standard 3. 5" floppy drive with an easily accessible 34-pin ribbon cable in a case with a USB to floppy controller. Will it work with a 5. 25" drive? I don't know yet, but it's the first time we have found a 3. 5"

#1 You may be having problems with ground referencing the gate signal or with the TRIAC eating up too much of the supply voltage (they do have a small voltage drop when they are ON).

One option that would fix both problems would be a pair of mechanical relays. They would exactly duplicate the pushbuttons in the rotor controller, so the rotor will be none the wiser and should work exactly as it always has. You would control the relay coils with a microprocessor and possibly a driver transistor. Figure 2