Hi All,

I've been working on a project building a sensor system to read the keys on a conventional manual typewriter (like but differently than usbtypewriter.com). I am specifically building for one model of typewriter, unsupported by usbtypewriter.

I had a great idea (we all know how that goes) at one point. It didn't work, but it's been nagging at me that if I'd known more it might have. The idea: If I simply built a strip with some resistance material and slipped it under the key bars, I could read the resistance (with one analog input) anywhere along the strip when a key bar struck it. The Teensy will happily output a discrete number reflecting the voltage read at any point along the resistance strip. This works. I wrote a sketch (my first dive into Arduino) that took the outputted reading and dropped it through a filter that returned a character equivalent (i.e. if the number equaled +-255 then send "A"). I gave each reading a range to capture to account for bouncy readings.

I initially tested this using a pot as a substitute for the resistance strip. It worked a treat. I got nice, relatively stable readings from the pot that my code translated into characters and sent out. I could "type", albeit slowly, with a potentiometer.
In actual use I never managed to stabilize the readings enough to be useful. I tried "debouncing" in the code, but never got to reliability.

I tried putting a thin layer of foam below the resistance material, thinking that the key bar would bottom out and I could capture the peak reading. No go. It would still throw out readings well above and below the range I was targeting.

I'm not by any means an Arduino programmer or electrical engineer. However, I concluded two things: Analog readings are subject to noise, and because I was targeting a very specific range of numbers for any given key I couldn't get past the spurious readings. Two, the fast mechanical action of the key bar connecting/disconnecting on the resistance strip made for a pretty inefficient switch and just wasn't reliable enough (for an analog signal). In testing, watching the readings as the keys were depressed, I would see the target number but nearly as many spurious numbers well out of any range I could code for.

This seemed like such a nice, simple elegant solution. The resistance strip is electrically isolated from the typewriter, one end is fed ground and the other 3.2 volts from the Teensy. The analog input pin is then grounded to the typewriter body -- and that connection, through all the metal bits -- is reliable. So it works just like a potentiometer -- or, rather, it is a potentiometer. Such a solution could work for any typewriter -- just cut to length, add contacts at both ends and run a program to capture key strokes and calibrate. Alas, it's beyond me.

Here's are the bare bones of the code, minus the various debounce routines I tried. Remember, I'm brand new to this stuff! I've since moved on to building a sensor bar with cascading multiplexers and 43 contact points so it's all digital (no analog) and I know that this will work.

Code for reading resistance strip:

#define cellPin A0

const float mvpc = 4.55;
char myVariable;
float counts = 0;
float mv = 0;
#include<Keypad.h>
#include<keyboard.h>

void setup() {
// put your setup code here, to run ounce;
Serial.begin(115200);
}

void loop() {
// put your main code here, to run repeatedly:

counts = analogRead(cellPin);


Serial.println(counts);

if (counts >0&& counts <28) Serial.println("A");
if (counts >41&& counts <51) Serial.println ("b");
if (counts >64&& counts <74) Serial.println ("c");
if (counts >87&& counts <97) Serial.println ("d");
if (counts >110&& counts <120) Serial.println ("e");
if (counts >133&& counts <143) Serial.println ("f");
if (counts >156&& counts <166) Serial.println ("g");
if (counts >179&& counts <189) Serial.println ("h");
if (counts >202&& counts <212) Serial.println ("i");
if (counts >225&& counts <235) Serial.println ("j");
if (counts >248&& counts <258) Serial.println ("k");
if (counts >271&& counts <281) Serial.println ("l");
if (counts >294&& counts <304) Serial.println ("m");
if (counts >317&& counts <327) Serial.println ("n");
if (counts >340&& counts <350) Serial.println ("o");
if (counts >363&& counts <373) Serial.println ("p");
if (counts >386&& counts <396) Serial.println ("q");
if (counts >409&& counts <419) Serial.println ("r");
if (counts >432&& counts <442) Serial.println ("s");
if (counts >455&& counts <465) Serial.println ("t");
if (counts >478&& counts <488) Serial.println ("u");
if (counts >501&& counts <511) Serial.println ("v");
if (counts >524&& counts <534) Serial.println ("w");
if (counts >547&& counts <557) Serial.println ("x");
if (counts >570&& counts <580) Serial.println ("y");
if (counts >593&& counts <603) Serial.println ("z");
if (counts >616&& counts <626) Serial.println ("1");
if (counts >639&& counts <649) Serial.println ("2");
if (counts >662&& counts <672) Serial.println ("3");
if (counts >685&& counts <695) Serial.println ("4");
if (counts >708&& counts <718) Serial.println ("5");
if (counts >731&& counts <741) Serial.println ("6");
if (counts >754&& counts <764) Serial.println ("7");
if (counts >777&& counts <787) Serial.println ("8");
if (counts >800&& counts <810) Serial.println ("9");
if (counts >823&& counts <833) Serial.println ("10");
if (counts >846&& counts <856) Serial.println ("11");
if (counts >869&& counts <879) Serial.println ("12");
if (counts >892&& counts <902) Serial.println ("13");
if (counts >915&& counts <925) Serial.println ("14");
if (counts >938&& counts <948) Serial.println ("15");
if (counts >961&& counts <971) Serial.println ("16");
if (counts >984&& counts <994) Serial.println ("17");

Serial.println(counts);



delay(500);
}