Keypad Implementation using A/D

[rrb011270]

Mabuhay,

Anyone from the community who have done keypad implementation on a PIC using Analog-to-digital conversion?

Any info, materials and/or a sample snippet ...

Thank u.

[kypec]

Hi,

if you want I will send you an Excel sheet which I used
for calculation of resistor values in ladder configuration.
The values were originally calculated for PIC with internal
comparator module but it can be also used for PIC with
A/D converter module of course.
All you need to do is just to type in your voltages, number of scanned keys and then obviously write some C functions for measuring
the analog input (Vx) and evaluating obtained value.

good luck with your project,

kypec

[mechwu_2.0]

I calculated as below for 3x4 and 4x4 keypad but never tested.
Let us know if it really works.

best wishes.

Ref. see Tip #7 of this AP notes.
http://www.microchip.com/1010/pline/picmicro/category/embctrl/8kbytes/devices/12f675/9827/index.htm
----------------------------------------------------------------

[mechwu_2.0]

1. The contact resistance of keypad is not considered in my design.

2. The contact resistance of keypad might get rusted after years ...

[rrb011270]

[rrb011270]

[mechwu_2.0]

1. For 10-bit A/D, simply multiply my numbers by 4.

2. The contact resistance of my keypad is 20 ~ 100 ohm depends on how hard it is pressed, measureded with a multimeter. check it out with your keypad.

Best wishes

[rrb-pic]

[rrb011270.]

[Guest]

For 4x3 keypad with 8 bit-A/D value 0 ~ 255, I said:

if (y<120) key = '?'
else if (y<130) key = '1'
else if (y<134) key = '2'
....
else key = '#'

For 4x3 keypad with 10 bit-A/D value 0 ~ 1023, it should be:

if (y<480) key = '?'
else if (y<520) key = '1'
else if (y<536) key = '2'
else if (y<556) key = '2'
....
else key = '#'

You can recompute the numbers based on the resistor network for better performance. for example, key '1' gives roughly 1023*(50k)/(10k+40k+50k) = 511.5, key '2' gives roughly 1023*(50k)/(6.7k+40k+50k)=529, so, the decision point should better be (511.5+529)/2 = 520, well, still the same as 130 x 4 in this case, so,

if (y<480) key = '?'
else if (y<520) key = '1'
...

should work

Best wishes

[Guest]

Hello,

Can this method of keypad implementation using a one channel A/D say, AN0 incorporates with #int_ad be possible?

need your comments....

thanx

[Guest]

[mechwu_2.0]

I did my design by trial and error to equalize the gap between buttons. Considering 8 bit A/D with 5 V for 0 ~ 255, each step is 5000 mV / 255 ~ 20 mV, and my design gives a minimal gap of 4, which means roughly 80 mV, so, it should work under a noise less than 10 mV, which shall not be difficult. Anyway, I never tested, so, that'a all I can say.

Good luck