Keyboard Matrix with a Twist

[k3v1nP]

A conventional keyboard matrix can be drawn as rows and columns of wires, with a switch where each row wire crosses a column wire.
You scan the matrix by (for example) having pull-up resistors on each of the column wires. You drive one of the row wires low, and see which of the column wires have gone low in response - and you conclude that the switches on the junctions between the driven row and the responding column(s) are closed (i.e. pressed).

This is fine until you have several switches closed.
Suppose you have the switches that join row A with column 2, row A with column 3 and row B with column 3 all closed.
When you pull row B low, column 3 will be pulled down.
The problem is that this will pull row A low. And this will pull column 2 low so you think you have the switch between row B and column 2 closed as well.
(Sorry I'm no good at ascii art).

There is a fix:
Put a diode in series with each switch.

Hope this helps.
- Danish

[mister_e]

appart from having an existing zillion of these in stock, could you explain WHY you want to use closed-type switches???

Sure it's all doable...

[k3v1nP]

I'm trying to make my own alarm system.

[Archangel]

Welcome back Steve.

[aratti]

You can solve the problem with a 16 channels mux (74hc4067)

You connect one input pin of your pic to pin 1 of the mux.

Ground pin 12 and pin 15 of mux (Enable)

Connect pin 24 to + 5V

Connect Pins 10; 11; 13 and 14 to 4 output pins of your pic.

Pullup all 16 mux inputs with a 10k resistor

Ground all 16 mux input with n.o. switches

Write a code to scan the mux. Read input pin. If pin=0 then switch is close.
If pin=1 then switch is open.
Scan position will tell you wich swicth has been activated.

Al.

[k3v1nP]

You can solve the problem with a 16 channels mux (74hc4067)

Ground all 16 mux input with n.o. switches

Al.

Can this be used with normally closed switches?

[aratti]

Yes I made the mistake to write n.o. intending n.c. Sorry for that!

Al.

[skimask]

Can this be used with normally closed switches?

What's up with the switches?
Are they what you have on hand and want to use them up?
Are you short of pins on your present project, yet still need to hook up the switches?
Just trying to get a feel for a good solution. The '4067 isn't a bad solution, for an N.O. switch. But I'm thinking, if you're going to go that route, may as well whip up another PIC to do the tough work for you...