Subject:
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Re: sensor input mux calculations
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 29 Dec 1998 19:10:47 GMT
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Original-From:
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John A. Tamplin <jat@*SayNoToSpam*traveller.com>
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Viewed:
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3892 times
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On Tue, 29 Dec 1998 MwalimuB@aol.com wrote:
> No. The 10k resistor that pulls up the input line and a resister that is put
> across the connection form a voltage divider. The formula given calculates
> the fraction of the voltage that will be at the junction of the two resisters.
> That voltage is represented by the value returned by the port. Two 10k
> resistors return one half ( 10000/20000= 1/2) of whatever you're reading, so
> you see 512 for raw values or 50 if it's percent.
Here is my understanding of the circuit:
AVcc
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Rp 10k
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A/D input -----------------------+--------------------
| | | | |
| S0 S1 S2 S3
Rc | | | |
| R0 R1 R2 R3
| | | | |
GND +----+----+----+---- GND
Where Rp is the pullup resistor and Rc is the impedence of the converter
itself. I couldn't find any spec on it, but I assume it is very large
(10M or higher) to avoid affecting the input signal. What is measured is
the voltage drop across Rc. This is a voltage divider circuit and the
voltage drop across R (the combined parallel circuit of Rc, R0, ... R3)
is AVcc*R/(R+Rp). I ignored Rc for the calculation below, treating it
as effectively infinite.
R is just the resistance of a parallel circuit, and the effective
resistance of a series of parallel resistors is 1/sum(1/Ri). If a switch
is open, that resistor is not part of the circuit and if no switches are
closed you get infinite resistance (1/0).
Combining all of this gets what I posted before. I'm not sure why you said
I was wrong, since it is essentially the same as what you did with a bit
more generalization and devising a way to analytically determine which
switches were pressed. Please explain where I have gone wrong if you
still think I have.
> I don't know about the other details, but I have built a prototype sensor and
> it works with the values I predicted (with some allowance for varying
> resistance due to defects, temperature, ect.).
What kind of variance do you get?
John A. Tamplin Traveller Information Services
jat@Traveller.COM 2104 West Ferry Way
256/705-7007 - FAX 256/705-7100 Huntsville, AL 35801
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message is in Reply To:
 | | Re: (no subject)
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| No. The 10k resistor that pulls up the input line and a resister that is put across the connection form a voltage divider. The formula given calculates the fraction of the voltage that will be at the junction of the two resisters. That voltage is (...) (28 years ago, 29-Dec-98, to lugnet.robotics)
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