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A/D keyboard

P

Phil

Jan 1, 1970
0
I want to implement a keyboard with 7 switches
and need to use an A to D. Seems staightforward
except I need to detect if more than 1 switch is
pressed at the same time. So I will need a ladder
with 0.5,1,2,4,8,16,32 for the resistor values, I plan
to wire 7 resistors in series across each swiches and
feed the A to D from RX/R1 junction.
Has anyone done this before and any idea what values
to use for RX and RY with R1 = 500, R2 = 1K ...R7
= 32K ?

VCC--RX--R1--R2 --R3--R4--R5--R6--R7--RY --VSS

Phil
 
J

Jim Thompson

Jan 1, 1970
0
I want to implement a keyboard with 7 switches
and need to use an A to D. Seems staightforward
except I need to detect if more than 1 switch is
pressed at the same time. So I will need a ladder
with 0.5,1,2,4,8,16,32 for the resistor values, I plan
to wire 7 resistors in series across each swiches and
feed the A to D from RX/R1 junction.
Has anyone done this before and any idea what values
to use for RX and RY with R1 = 500, R2 = 1K ...R7
= 32K ?

VCC--RX--R1--R2 --R3--R4--R5--R6--R7--RY --VSS

Phil

Sounds like you are trying to make a D-to-A. Look up R-2R Ladder.

...Jim Thompson
 
M

mike

Jan 1, 1970
0
Phil said:
I want to implement a keyboard with 7 switches
and need to use an A to D. Seems staightforward
except I need to detect if more than 1 switch is
pressed at the same time. So I will need a ladder
with 0.5,1,2,4,8,16,32 for the resistor values, I plan
to wire 7 resistors in series across each swiches and
feed the A to D from RX/R1 junction.
Has anyone done this before and any idea what values
to use for RX and RY with R1 = 500, R2 = 1K ...R7
= 32K ?

VCC--RX--R1--R2 --R3--R4--R5--R6--R7--RY --VSS

Phil

This MIGHT be a bad idea. Do the math. Look at the
real resolution/accuracy of your a/d (not just the number of bits),
resistor tolerances, noise
immunity requirements, definition of "at the same time",
what happens if the keys are misinterpreted? Also look at how much
overhead you'll incur polling the A/D.
My gut tells me you want a different implementation.
A one-wire-type interface with a OTP microcontroller
might be cheaper than the parts you need to make the a/d scheme work.

Now, if you had two a/d inputs and could arrange the switches so
the simultaneousness is only two at a time and always spread across two
inputs, it becomes
a slam dunk for tolerances and simultaneousness.
mike


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J

James Meyer

Jan 1, 1970
0
Sounds like you are trying to make a D-to-A. Look up R-2R Ladder.

...Jim Thompson

Good call Jim. I recommend that Phil gets 25 or so 100K resistors of at
least 1% accuracy. The R-2R can be made with all one value of resistor and if
they all come from one batch then they will be very likely to track one another
in value. It's only the ratio of R to 2R that really matters and not the actual
R value. Any value from 1K to 1Meg would work. I picked 100K to make things
easier on the reference supply and still allow reasonable bias currents for the
following op-amp.

Jim
 
J

Jim Thompson

Jan 1, 1970
0
Good call Jim. I recommend that Phil gets 25 or so 100K resistors of at
least 1% accuracy. The R-2R can be made with all one value of resistor and if
they all come from one batch then they will be very likely to track one another
in value. It's only the ratio of R to 2R that really matters and not the actual
R value. Any value from 1K to 1Meg would work. I picked 100K to make things
easier on the reference supply and still allow reasonable bias currents for the
following op-amp.

Jim

In the past year I've gone bananas over potentiometric DACs... at the
IC level who cares that it takes tons of resistors... I've even made a
two-stage 14-bit one that's been reduced to practice.

How are your ribs ?:)

...Jim Thompson
 
G

GPG

Jan 1, 1970
0
7 switchesWhy? Be more specific about application.
 
A

Alan

Jan 1, 1970
0
I want to implement a keyboard with 7 switches
and need to use an A to D. Seems staightforward
except I need to detect if more than 1 switch is
pressed at the same time. So I will need a ladder
with 0.5,1,2,4,8,16,32 for the resistor values, I plan
to wire 7 resistors in series across each swiches and
feed the A to D from RX/R1 junction.
Has anyone done this before and any idea what values
to use for RX and RY with R1 = 500, R2 = 1K ...R7
= 32K ?

VCC--RX--R1--R2 --R3--R4--R5--R6--R7--RY --VSS

Phil
Have a look at:
http://www.st.com/stonline/books/pdf/docs/2484.pdf


HTH
Alan


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J

James Meyer

Jan 1, 1970
0
In the past year I've gone bananas over potentiometric DACs... at the
IC level who cares that it takes tons of resistors... I've even made a
two-stage 14-bit one that's been reduced to practice.

How are your ribs ?:)

...Jim Thompson

Ribs are almost no problem now. Best estimate I've gotten so far is
that ribs take two months to heal 100%. That makes mine about 50% well.

I found out last week that my arm was broken too. They told me at the
ER right after it happened that I had one broken rib and a sprained arm. After
looking again at the X-rays later, they said *two* ribs and a chipped bone
somewhere in my arm.

All things considered, I can think of several better ways to spend a
holiday.

Jim "on the mend" Meyer
 
P

Phil

Jan 1, 1970
0
Alan
I need to read more than one switch and
and the ST solution is only for a single keypress.
But thanks for your suggestion.

Phil

----- Original Message -----
From: "Alan" <[email protected]>
Newsgroups: sci.electronics.design
Sent: Sunday, January 11, 2004 3:39 AM
Subject: Re: A/D keyboard
 
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