12 Button Keypad (Matrix Encoded)

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Tommy volts
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12 Button Keypad (Matrix Encoded)

Post by Tommy volts » Fri Aug 20, 2004 10:07 am

While getting some stuff I needed online from All Electronics i also bought a 12 Button Matrix Encoded Keypad (about $5). A very simple keypad.<p>The problem is that it has 12 buttons but only 7 pins (7 pin header). I now know that the keypad is designed for a microcontroller (for example the EDE1144 Keypad Encoder IC).<p>Is there any other way to use the keypad without having to use a 20 pin microcontroller?.<p>For example can a wire it to a 7 Segment LED display? or some other simple IC that will out put 4 bit binary?

poseidonx
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Re: 12 Button Keypad (Matrix Encoded)

Post by poseidonx » Fri Aug 20, 2004 12:00 pm


Tommy volts
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Re: 12 Button Keypad (Matrix Encoded)

Post by Tommy volts » Mon Aug 23, 2004 9:00 am

poseidonx,<p>Maybe the only way to use this keypad is with a microcontroller. Thank you for the link to the notes for using the PIC 16C54 for the interface. I will try using the PIC 16F84 for the interface( I built a programmer for the PIC 16F84 several years ago).

rshayes
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Re: 12 Button Keypad (Matrix Encoded)

Post by rshayes » Mon Aug 23, 2004 8:21 pm

If that is a telephone keypad, it is probably arranged as 3 columns and 4 rows. This is consistent with 7 connections.<p>Each button will connect one column line to one row line.<p>The array can be scanned by sequentially applying a voltage to the 3 column lines using a divide by 3 counter, and looking for an output on one of the 4 row lines. If an output is detected, a 4 line to 2 line encoder will supplly 2 bits indicating which key, and the divide by 3 counter can supply the other two bits. Three or four standard ICs should be able to do it. If more complex devices, such as a programmable logic array or a microcontroller are available, then they can also be used for this purpose.

Tommy volts
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Re: 12 Button Keypad (Matrix Encoded)

Post by Tommy volts » Wed Aug 25, 2004 3:06 pm

Stephen,<p>The three or 4 counter IC's sound like a great way to go, I'd really rather avoid having to program a microcontroller.Do you know where I might find a schematic design for this?

rshayes
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Re: 12 Button Keypad (Matrix Encoded)

Post by rshayes » Wed Aug 25, 2004 7:14 pm

Actually, there is an IC that does this, and suprisingly, it has not been discontinued.<p>Look on the Fairchild web site (www.fairchildsemi.com) for the data sheet on the MM74C922 or MM74C923. That has a fairly detailed block diagram in it.<p>For a twelve switch keypad, I would drive the 4 row lines and read the three column lines. A divide by 4 shift counter can be built with a dual D type flip-flop. This can be decoded to four signals using a quad NAND gate package. These would give sequential low pulses.<p>Sensing the columns would be a little more complicated. The column lines can be pulled high with resistors. A switch closure would pull one line low. A 3 input NAND gate would give a pulse when any key was pressed. This would be the strobe pulse to latch the state of the counter and the column lines. Some encoding would still be necessary to designate which column had the pressed switch.

Tommy volts
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Re: 12 Button Keypad (Matrix Encoded)

Post by Tommy volts » Thu Aug 26, 2004 9:02 am

Stephen,<p>The Fairchild MM74C923 data sheet has everything I need to know. After I build the encoder I will post the results.<p>thank you.<p>"Why?...because its there"

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