From gstopp@fibermux.comTue Jun 27 14:04:56 1995
Date: Tue, 27 Jun 95 13:59:56 PST
From: gstopp@fibermux.com
To: analogue@hyperreal.com
Subject: Home-brew Ribbon Controller


     How I Made a Home-Made Ribbon Controller
     
                - OR -
     
     Everything You Always Wanted to Know About Conductive Plastic But Were 
     Afraid To Ask (for fear of getting blank stares)
     
     Okay, I know I said that the parts are easily available - I hope I was 
     right. As you may know a ribbon controller has a metal band (the 
     ribbon) that is suspended just above a big long piece of resistive 
     material so that you can press on the ribbon at any point along the 
     length of the ribbon and get a continous range of voltage from the low 
     end to the high end (like a fretless bass).
     
     For the long piece of resistive material I used an antistatic bag. Now 
     there are several types and you need to use the right one. You need to 
     use the opaque jet-black rubbery-type antistatic bag. This kind has an 
     actual measureable resistance of about 10K per inch depending on how 
     much contact you make with your DVM probes. Try to get a brand new 
     un-wrinkled bag, and cut it up such that you get a strip about 1" wide 
     and 30" long. Don't use any bag that you can't measure resistance on - 
     generally if you can see through it then it won't work.
     
     For the metal ribbon I used a metal strap that is used to remove 
     solder from surface-mount IC's. It's kinda like a solder-wick in 
     theory except it's solid steel - a couple mils thick, a quarter-inch 
     wide, and it comes on a roll a few feet long. I don't know how easy it 
     is to get this for those of you that don't work at an electronics 
     related company.
     
     Okay. Glue the long conductive rubber strip to a piece of plexiglass 
     or plexiglass-covered wood or something with infinity ohms (I'll talk 
     about impedances later). Suspend the ribbon using some kind of 
     standoff at each end, with a hard anchor at one end and a moving one 
     that's spring loaded at the other (I used a metal picture hook, bent 
     flat, with some random spring from my junk drawer). Coat the exposed 
     face of the metal ribbon (the side your finger will touch) with 
     masking tape trimmed to the width of the ribbon. This will insulate 
     the metal ribbon from your finger.
     
     Now for the electronics: ground the low end of the rubber strip and 
     put +12 volts at the top of the rubber strip through a 10 or 100 ohm 
     resistor. This resistor will prevent any possible two-fingered 
     shorting out of the strip between +12 and ground. Use higher wattages 
     for lower ohmage, as usual. That's the ribbon part.
     
     The ribbon itself goes to a buffered capacitor to provide the CV. You 
     need a real good buffer here cuz the cap has to be small, since the 
     resistance of the ribbon is fairly high and if yer cap's too big yer 
     gonna get unwanted portamento. Experiment time. Also if you were to 
     glue the rubber strip directly to a piece of wood the conductivity of 
     the wood will slowly discharge the holding cap, so that's why you 
     should use plexi.
     
     Put a pushbotton somewhere to send a gate or trigger or whatever you 
     need. Put a range pot and an amount pot with a summing amp if you 
     want. Heck, add an LFO while yer at it.
     
     Well there ya go, let me know if this is enough to get you guys going.