The IC is a dual opamp, used as a current source and buffer amplifier. The
current source drives a string of resistors that form a voltage divider, and
the tact switches select voltages at the different taps of the divider.
See the resources section for links to a much more detailed guide to the
internals of the VKey.
Each of the buttons on the VKey produces a unique analog voltage, as
listed below/
Key Number V (5V supply) V (3.3V supply)
None .057 .053
12 .198 .194
11 .396 .388
10 .596 .583
9 .794 .776
8 .992 .970
7 1.190 1.164
6 1.388 1.358
5 1.585 1.551
4 1.781 1.744
3 1.979 1.938
2 2.176 2.131
1 2.372 2.323
As you can see, with a 5V supply, each successive button adds about 200
mV to the output voltage – at 3.3V the voltage per step is slightly less.
One situation to consider is when more than one key is pressed at the
same time. The VKey implements high-key number (or low voltage) priority
– when more than one of the switches is closed at a time, the output will
indicate the higher key number. For instance, if you hold down 5 and 9
together, the output will indicate key 9 is pressed.
Assembly
The VKey comes with the surface mount components assembled, but the
PTH tactile switches are loose, and need to be soldered to the board.
Contents of the VKey package
The first step in hooking up the VKey is to solder in the key switches.
out out
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