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 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.
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