Hello

I am having trouble designing a circuit for a 4 by 4 keypad. I am using a ring counter for my logic and connecting my rows and columns to a 74LS273 Latch to generate a key code. But I am not sure how to set up the key press detect part, I was going to use 4 AND Gates but I am not sure how to connect them to my circuit. Can anyone offer any suggestions

Thanks

 

Hi.

Take a look at the 74HC922. It will scan a 4 x 4 keypad, give a 4 bit binary output and a key-down output. A great little chip if you can get them. If you put the binary output into a latch (CD4042), then using the key-down output through a monostable (CD4047) to trigger the latch. It will hold the data at the output of the latch until the next key is pressed.

Daniel.

 

Digikey carries the MM74C922:
http://www.digikey.com/scripts/DkSearch/dksus.dll?PName?Name=MM74C922N-ND

 

If my assumptions about how you are using these parts is correct, Use a 4 input OR or NOR gate,with the four inputs going to the output of the matrix. Whenever an output of the matrix is goes high, (one or more buttons is down), and the output can be used to latch the output of the matrix. This will not detect any buttons with a key code of 00.

 

Thanks for the suggestions I will give the 74HC922 a try, havent used that before.

 

Thanks for the suggestions I will give the 74HC922 a try, havent used that before.

You'll have a hard time finding it.
The MM74C922 has the same functionality though, and Digikey has it in stock.
See my prior post.

 

Hi again.

No need to OR the outputs. There is a key-down output available (da output) for this from the 922.

Daniel.