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# Matrix keyboard

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#### Spasm

##### New Member
Hey all...
Does anyone know how to implement a matrix keyboard that contains 40 plus characters but uses less than 14 I/O pins?
I'm not talking bout using serial connection here.

Thanx

if you only push a single button at a time, then 2 converters from 8 to 3 will do great.
you can wire them in a matrix and get 7*7 = 49 characters and only have to use 6 pins.
i'll think about the schematic a bit.

Hey...
Can you tell me which converters are suitable for this ?
I've tried searching but there mostly there are 3 to 8 rather than 8 to 3 converters.

Hi,

This is about the simplest way i can see
to matrix up a small keyboard for 42 buttons.

I have shown PNPs so as its positive logic output.
5k is a bit higher impedance than usual,
thats cos its on the bases too,
but it should operate most TTL ok.

John

#### Attachments

• 6x7matrix.jpg
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Hi john...
Hrmm...is there any way to lessen the I/O pins?
Lets say less than 10?

Hi Spasm,

Yes, you can get it the number of output points down quite low.
But it will increase the complexity of the unit you have to build.
The circuit shown above is very simple and fairly easy to construct.

By using chips you can reduce each side of the matrix to binary form,
so you have two binary numbers, which refer to each side of the
matrixed keyboard.

These two are usually called the 'Rows' and the Columns'

Pressing a button would produce a response from a particular column
and a particular row.

As Bogdanfirst has already mentioned, using two 8 to 3 way
multiplexer chips is probably the best way if you want to use chips.

Use one chip for the columns and another for the rows.

This would give you a three bit output from each chip, plus a bit for
KP which means 'Key pressed' thats seven in all.

If you want to reduce the output points still further, you would be
getting into specialist areas, like chips to make a given voltage
and other chips to give binary output for given voltages, or chips
that respond to values of resistor placed across each button or some
such complex arrangement, or circuits that would give a certain
length of pulse - which is almost like a serial setup.

I will have a bit of a look around to see if i can find a suitable
8way to 3bit multiplexer chip.
That would allow you to expand your keyboard to eight rows & columns
which would cope with 64 buttons, but it would of course be fine
with the 40 you want.

Cheers, John

ok, here are some multiplexers.
now, i don't think that 8 to 3 will give you 64, unliess you will use an extra 7th line as 'keypressed', lije John1 said.
here are some multiplexors......
74148(8 to 3)
74147(10 to 4)
4532(8 to 3)
40147(10 to 4)
that's all i can find for now....

i found this, its a page of info about the 74148 8to3 way
multiplexer, also called an Octal priority encoder.

If Your processor use multiplex-display, the MPX outputs can drive the rows, and need only inputs for columns.

Hi all.
Thanx for the reply guys. I will try and use the multiplexer chips and see how it goes.
Btw Sebi, what do u mean by "processor use multiplex-display"?
I'm using the mc68hc11 MicroP.

Thanx

Yeah, i never learn english... i think if you need a keyboard, need also a display for proc. Most of case the display driving from proc. is multiplex type.(all segments parallel drived from port (8pins) and digit select ( for each digit))

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