> How about operating a keypad with nothing more than a
> single PIC A/D input pin?

Dave, I was thinking along similar lines last night for multiple
toggle switches, so thanks for bringing it up

http://home.clear.net.nz/pages/joecolquitt/mixer.html

One function of the PIC is to produce two pairs of antiphase PWM,
the properties of which are set by the pots and switches

As you can see, there are just four switches as part of a voltage
divider. Although the number of switches can be increased, the
resolution of the ADC would eventually need to be increased
also. The reason is because this is slighly different to the 4x4
question, as I need to detect any combination of switches from
"all off" to "all on", ie 16 clear and distinct values (as David
Harris has mentioned). If they were 0x10 apart coming from the
ADC that would be wonderful. I've done similar work before so
not expecting any major problems

Although it's not needed for this job, I did wonder how many
switches (or keys) could be used if that spare 4051 input was
employed. I may have put this up as a challenge at some time,
so now seems a good opportunity. As yet I've not tested any
values for the 5 resistors in the circuit

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