foot pedal design

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Hey guys,

I have this keypad with 12 functions (ie 12 micro switches). It needs to
be hard wired with a cable length of 2 meters. RF/IR is out of the
question. The logic signals will go to a micro controller and will
perform functions depending on which button was pressed.

This means that my cable (connecting the keypad to the micro controller)
will need to be 14 core (including power and ground).

This is really not what, because to terminate 14 cores is really hard
work... (I'm lazy...)- not to mention the cost of a 14 pin DIN style
connector...

I'm thinking is it possible, somehow to encode the 12 lines of logic
down to maybe BCD or even 2 wire protocols. This would significantly
reduce my cable core count and makes it easier to assemble.

What do you think?

A) is there a 12 line encoder? (I've seen 4-16 decoders, but not 16-4
encoders)

B) if i could use 2 wire protocols, what would be the way to do it? i2c,
rs232/485, others?

C) dedicated micro-controller? -yuk...

thanks for any suggestion...

Re: foot pedal design


yopu want to relace 50c woth of cable and connectors with WHAT??

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Re: foot pedal design


There are lots of ways but the one that comes to mind immediately is a
simple string of resistors fed back into the micros A/D. You can feed a
current through the string and measure the average voltage to determine
which pedal is pressed (just assuming one at a time at present). Of
course, in this arrangement you would need different value resistors.

Even without an A/D you can add a cap and just measure the RC time constant.

Oh yeah, and of course there are all the other methods using shift
registers, encoders, I2C, micros etc etc. But the resistor approach
means that you can implement a simple 2 wire daisy chain with each pedal
shorting out the it's resistor in the chain. That way you can tell if
the loop is intact and which pedal is pressed.


*Peter*
http://www.pbjtech.com /


J D wrote:
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Re: foot pedal design


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An A/D ladder!! yeah.. why didn't I think of that!... nice and simple...
let's see...

1. power line
2. channel
3. ground...

wow.. only 3 wires... I like that...

Re: foot pedal design


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Actually you only need 2: power and the signal wire. You pull the signal
wire low with a resistor so the A/D sees ground normally, and the various
switches pull the line towards the supply voltage via different resistances.
The voltage seen is based on the ratio between the pull-down resistor and
the switch's pull-up resistor.




Re: foot pedal design


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"priority encoder"  you may need to couple two 8->3 prioroity encoders.
or use two 4->2 encoders one for row and one for column.

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depends what's easiest to interface with the micro.
have you considred DTMF ?
how about a 555 that the buttons effect the timing of?

A few hunderedths of a second isn't going to be a big deal, is it?

Bye.
   Jasen

Re: foot pedal design



Appreciate the help and advice... i really like the ADC version...

Anyway, I found this link which is pretty good for keypad designs - just
in case someone does a search on this topic...


http://www.discovercircuits.com/K/keypad.htm

cheers all
john

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