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If you know of a retail catalog that have such things I'd appreciate a pointer. I can find plenty of Linear Postion Potentiometers - but they cost in the hunderds of $$. Need something much cheaper. I know I could devise a 3:1 lever ratio or some such thing and use one of the cheap (but mechanically range restricted) linear potentiometers for audio circuits, but would rather not.
Any particular reason why? You're probably not going to get a 5-6" device any cheaper than what you're already finding, because they're such specialty items. But the reducer-arm design is so simple it's at least worth reconsidering. The hardest part is finding a mounting location, but you must have one in mind for your linear actuator. Put a screw into a block of wood, through an oversized hole in a piece of scrap plastic, with two coat hangers for control rods. You're done. You could make it slicker, but that'd be a start.
All Electronics has a 2" device, 2 for a buck:
http://www.allelectronics.com/cgi-bin/category.cgi?category=470020&item=LSP-50KL&type=store
You might contact these folks and see if they'll sell you a single-quantity unit for "prototype" purposes of their 7.3" motorized slide pot. It's $9.80 (probably more for a single quantity) and I bet you could easily remove the motorized function and just use the pot:
http://www.73.com/a/0179.shtml
If you want to get hackish there are lots of ways around this. Steal a wiper from some dump-bound device - plated battery contacts work well since they're made of spring stock anyway. Wrap insulated nichrome other high-resistance wire evenly around a block of wood - a flat surface is better than a dowel/rod. Sand the face to remove the insulation. Put it in a plastic project box, and cut two holes to pass in a coat hanger. Solder/attach the wiper to the coat hanger. If you sand the coat hanger and coat it with solder you'll help reduce the resistance of the rod. Obviously, one wire goes to the coat hanger, and the other to one end of the nichrome wire.
Your solution won't be too accurate but it will work for what you need and you can't get much cheaper than this. I've done this before and it works OK as long as you're careful with how you build it.
For what it's worth, carbon-strip linear potentiometers may not be good for the engine compartment anyway - the vibration can cause the device's wiper to jump off the surface, and unless you use an analog voltmeter it may be tough to get a good reading right when you want it.
Regards,
Chad
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