# 4x4 keypad without microcontrollers ?

Discussion in 'Homework Help' started by natrix, Mar 24, 2009.

1. ### natrix Thread Starter New Member

Mar 24, 2009
7
0
I am working on a project for (digital design) , that requires me to use a 4x4 keypad the problem is that the keypad has only 7 lines like the following sketch

There is obviously 16 key on it , the question is how to take there output on 16 lines ? using digital gates and wires only !!!

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
How do you only count 7 lines? Or is your actual keypad different from the illustration?

Make a truth table based on which key has been pressed, and which lines are connected by the key. Add pullup resistors to the rows and ground to the columns, and there you are electrically.

3. ### lightingman Senior Member

Apr 19, 2007
374
22
Look at the 74HC922. It wil scan and decode a 4 x 4 keypad and give 4 bit binary output and a key-down output.

Daniel.

4. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,349
731
The process is pulling a column high, with the other three low, and check for a high on any of the row wires. Then shift the high column left/right by one, check for any high outputs, and repeat.

When a high output is found, note the column: A, B, C, D (4321 in your diagram), and which row, 1,2,3,4 (4321 in your diagram) that had the high voltage on the scan for which key is down. Debouncing is the same, check if 3 "scans" show the same key down, the scanning of the other columns provides the delay.

5. ### natrix Thread Starter New Member

Mar 24, 2009
7
0
sorry I ment 8 lines anyway thanks for all of you