Hi Chris, Remember how large the minute hand on your watch is compared to what's making it spin. =] Gears, gears, gears-- low backlash gears, if possible. However,. having used potentiometer-feedback motors (RC servos), there is a potentially very dangerous problem with using many (cheaper) potentiometers for a metrologic system. The problem arises with potentiometers that move a conductive element along a track with a resistive coating. The problem is, if there are particular points on that potentiometer that you stop at a lot, there is some static friction when you start moving again. This additional friction causes the potentiometer to wear out faster at these frequent stops, increasing noise at (and very near) these points. If you are exercising a strain gauge, for example, you might be going from vented to 3000 PSI or so very often. Eventually, the very points you want to measure most accurately at-- your test points-- will be the noisiest points on the pot. However, there's another way to do this: optical. There are at least two ways to hack a dial gauge to do this: One (digital): Rotary Encoder Add a "zero" button, and measure the number of pulses you've seen since zero. Pros: 1. With a little calibration, you might be more accurate than the original dial gauge was in the first place! 2. Most rotary encoders don't need much torque. 3. Rotary encoders are cheap as hell and can be yanked out of dead serial mice that are otherwise only suitable for melting down into lawn gnomes. Cons: 1. Pressure must change slowly enough that the encoder can't be outrun. Using gears, your tradeoff will be between resolution and maximum rate of change. 2. If using gears, backlash will increase measurement error. Two (analog): Variable-Density Transparent Disc Acquire or make a disc of transparent material with a variable-opacity coating, such that at the minimum angle the disc is almost completely transparent, and at the maximum angle the disc is nearly opaque. Such discs are available commercially; the best ones quickly available to you will probably be the glass ones used in certain photography enlargers. Find a trashed enlarger nobody wants anymore and save the variable density discs! Alternately, if you want higher-quality parts, see if Edmund Scientific Optics has something appropriate; I seem to remember them having something close. Allow this disk to rotate on the meter arm (which you can leave intact if you use light plastic instead of glass). Place either an LED or a laser diode (yes, you finally have a good reason to use one for something =] ) behind an opaque plate with a pinhole in it (or two opaque plates with pinholes in them, about 2-3mm apart, if using an LED) so that you get a sharp pencil of light which, you guessed it, needs to fall onto a phototransistor or cadmium sulfide (CdS) cell. Make the whole thing light-tight, amplify as appropriate, and feed into PIC A/D converter. Consider temperature compensation if appropriate. Incidently, if you want to make your own variable-transmissivity disc, the setup involves less effort than you might think. I'm planning to put a page up with instructions, if you're interested. Cheers, Matt Heck Metrologic Software Development Crystal Engineering Corporation "Pressure is Our Business!" http://www.crystalengineering.net PS: My company makes some great handheld digital pressure calibrators if you want something to calibrate your gadget against when you're done. =] ----- Original Message ----- From: "Chris Loiacono" To: Sent: Monday, April 15, 2002 9:01 AM Subject: Low-torque potentiometers > Steve Baldwin gave me a great idea for measuring high-pressure a while > back - that of dissecting or hacking a mechanical gauge mechanism, coupling > it to a pot, then into an A/D pin. > Now I have a very small shaft (which the needle-type dial indicator was once > pressed on to) that I want to couple to a pot shaft. I have checked the > vaiety of pots in my parts drawers and toy box, and they all seem to require > more rotational torque than I would like to apply with the tiny mechanism. I > want to find a type of pot that is easy to turn. > > I have searched the catalogs hoping to find some kind of designation that > might apply - to no avail. > > Does anyone have any clues that may help me along with this??? > > TIA, > Chris > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body