# An analog keypad. Will it work?

Someone was asking for a solution to sensing 48 buttons. I thought since I might need a solution to a similar problem, I mocked up a diagram that uses 1 analog pin, 8 digital pins, and 7 resistors. Will it work?

The digital pins will be pulled to GND for the active column and left in tri-state for non-active columns.

Thanks.

Yes, you must scan allong the pins to see which row has a pressed button.

something like this (not tested)

``````int keypress()
{
int x;
int row = -1;
int key;
for (int i=0; i< 8; i++)
{
digitalWrite(pin[i], HIGH);
digitalWrite(pin[i], LOW);
if (x  < THRESHOLD)
{
row = i;
break;
}
}

if (row > -1)
{
return 8*row + lookup(x);  // to do ;)
}
return -1;
}
``````

you could also use a shift register serial in parallel out (needs 3 lines IIRC) - http://arduino.cc/en/Tutorial/ShiftOut -

shift in 1 bit high and then clock it from bit 0 to 7 -

don't use shiftOut() but do the clocking in your own pace!

there are some nice videos too ;)

In theory you could get away with no more than two analog pins, I believe...

in theory with one analog pin and 48 very precize resistors, say 100 ohm + 5240 ohm

would give 48 steps from 0 to 500 in analogRead => so diff between 2 values is about 10 - just larger than noise (~2 bits)

advantage is that the SW would become very simple: button = analogRead(A0) / 10;

However in practice ....

I had not even considered that. I like that approach.... :-)

Rob,

Thanks for the link. There’s no code on the site though. I was a bit concerned when driving with a port extender but since this extender has an open-drain mode, there won’t be problems. Still one needs to use it so inactive columns are not driven to opposite polarity. It would short if you drive column 0 to high and 1 to low and hold two buttons one from column 0 and one from 1. If I use arduino pins under tri-state mode, the problem is solved.

I think I will give my own schematic a try with a membrane matrix keypad. I have a bunch at home. Next time I order from digikey, I’ll grab a few of those port extenders.

it is not visible as a hyperlink, just click on the filename and you fly to - http://www.spikenzielabs.com/SpikenzieLabs/Project_64_files/Button64v4.pde -

Thanks Rob!