3x4 Keypad with MM74C922 Encoder

[Persia]

Hi
I have been through some posts and I realised that there is a common problem interfacing 4x4 keypad with 74C922 encoder. But I couldnt find the solution using 3x4 keypad. Well becouse this keypad hast got one of the column there is a problem with the left out pin in the encoder. Can you please tell me how you make this enocder from 1-16 to 1-12 . I mean how do you deal with that extra pin ? here is what I have tried but the problem is that the numbers are 1 digit ahead. So when I presss 1 it gives zero , 2 gives 1 , 3 gives 2 , 4/5/6 are ok then 7 gives 8 , 8 gives 9 .
https://www.electro-tech-online.com/attachments/keypad-jpg.17043/

 

[DigiTan]

You can leave it floating. That pin is actually the output of a One-Hot encoder and would float anyway when no one's touching the keypad.

 

[Persia]

I did try leave it disconnected , but it didnt make any difference.

 

[nart_88]

try using instead of a decoder a microcontroller, example: pic16f84 and now that you know the outputs just plug them in the microcontroller inputs and let it display tha value -1

 

[ericgibbs]

I did try leave it disconnected , but it didnt make any difference.

hi,
That original circuit isnt wired to give the ASCII format.

Look at this truth table.

You will get the 'wrong' code from the pad.

Post an image of the front of your keyapd.

 

[Persia]

Hi
The problem was solved when using a microcontroller . I used different reading process where I set all the rows 1 and checked for pressed column and vice versa . The keypad is one of those old telephone 3x4 keypad which there isnt any data sheet for it so you have to find out the Xs and Ys yourself using multimeter. Thanks for the replies.