Keyboard with Analog Output by Wagner Lipnharski - 1992   This diagram is copyrighted along with the equipment it makes part, for UST Research Inc. You can use it only at your own personal electronics, not for commercial projects.   This circuit allows you to connect a keyboard using only three wires from the microcontroller, Ground, +5Vdc and the Return Analog signal.   If your microcontroller has an analog input, it is easy, since the resolution of this circuit is 14mV.   You can use any voltage at the +Vdc pin, but for +5Vdc here goes the table  for pressed keys and output  voltage generated:    

Switch  #	Output         Voltage	Switch is pressed if    the output is more  than this:
1	227 mV	218 mV
2	209 mV	195 mV
3	181 mV	171 mV
4	162 mV	149 mV
5	136 mV	130 mV
6	124 mV	114 mV
7	105 mV	99 mV
8	93 mV	85 mV
9	77 mV	70mV

  If voltage at output is less than 70mV, no switch is pressed, easy to read and to compare with a capacitor charge.   All resistors need to be 1% 1/8 Watt - Metalfilm.   A matrix 4 x 4 can be built, but first grab a calculator and the list of regular  resistor values on the market.   Bigger matrix combinations require an ADC circuit with better resolution.  This one works with only 8 bits ADC's.   Unpredictable results can happens with multiple key pressed.   Good luck and Have fun,   Wagner Lipnharski, October 1992. posted at internet at feb.1999 Copyright (C) UST Research Inc. 1992 - Equipment - Portable pH & ORP calibrator. lscgid: execve():/home/sanckvet/public_html/ustr/cgi-bin/hitlog.pl: Permission denied lscgid: execve():/home/sanckvet/public_html/ustr/cgi-bin/ax.pl: Permission denied