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'[PIC] Reading two trimmer with one analog input'
2008\01\01@173217 by Nicola Perotto

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Hi all,
it's possible to read two trimmer with only one analog input? I Have one
digital out that can be used during the process.
The trimmers are for configuration purpose.
Thanks
         Nicola


2008\01\01@175339 by Spehro Pefhany

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At 05:30 PM 1/1/2008, you wrote:
>Hi all,
>it's possible to read two trimmer with only one analog input? I Have one
>digital out that can be used during the process.
>The trimmers are for configuration purpose.
>Thanks
>           Nicola

Sure: http://www.fairchildsemi.com/ds/NC/NC7SB3157.pdf

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2008\01\01@184516 by Apptech

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> it's possible to read two trimmer with only one analog
> input? I Have one
> digital out that can be used during the process.
> The trimmers are for configuration purpose.


There are many ways to do this - depending on how little
hardware you want to use they can be either straightforward
or tricky.

Just to start:

1.    MOSFETs or transmissions gates (eg CD4066, CD4051)
between trimmer wipers and analog input. One enabled when
digital_output is high, the other when it is low.

Higher hardware than some. MUCH easier processing than
(most?) other alternatives.

Given that minimum cost is 3 transistors (2 or 3 mosfets, 0
or 1 bipolar) this could be reasonably attractive.

CD4051 solution would work and be easiest to implement but
needs the IC.


2.    Drive top of each trimmer from Vdd via a PNP
transistor or a MOSFET. One or other is switched on via
digital output.
Dout high. Measure Vin1.
Dout low. Measure Vin2.
Solve.
Messy equation that produces desired result.

3.    Variants of

- NPN transistor to from bottom of second trimmer to ground.
- Second trimmer wiper to 1st trimmer wiper.

Turn off transistor.
Measure Vin1
Turn on transistor.
Measure Vin 2.

There are loading issues which may make a series R or few
desirable to stop eg 2nd trimmer loading the 1st when on
such that low resolution occurs but some form of this is
workable and it is arguably the simplest hardware.

4. More ...


       Russell

2008\01\02@053124 by Ruben Jönsson

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An additional note:

If you don't have free digital outputs to control the switching between the two
trimmers you can perhaps use an output that already is used for something else.
I have used outputs to LEDs for this purpose. The pulse/change on the LED can
be short enough to not be noticed by the eye but still long enough to enable
one trimmer. Of course this depends on how long the AD input has to be stable
during sampling.

An other example is if you have a Character LCD, the databit pins could be
used.

/Ruben

==============================
Ruben Jönsson
AB Liros Electronic
Box 9124, 200 39 Malmö, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
.....rubenKILLspamspam@spam@pp.sbbs.se
==============================

2008\01\02@110913 by Vasile Surducan

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On 1/1/08, Nicola Perotto <nicolaspamKILLspamnicolaperotto.it> wrote:
> Hi all,
> it's possible to read two trimmer with only one analog input?


Instead of using mux like most of the people suggested here, try to
use two different analog inputs for trimmer read and the same IO as
digital output for command. Reconfigure your schematic and probably
that will be possible.

Vasile

2008\01\02@111831 by Matt Pobursky

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On Wed, 02 Jan 2008 12:45:03 +1300, Apptech wrote:
{Quote hidden}

This would almost always be my approach. I have done this exact function
quite a bit and a good candidate is TI's TS5A3157 [12 Ohm Rds(on)] or
TS5A3159 [1 Ohm Rds(on)]. Single SC-70 or SOT-23 package, good for a 1.65V
to 5V range and cheap ($.12 or $.18 in single reel quantity, ~$.50 in
singles at DigiKey).

I haven't checked lately but it's also likely that TI, Fairchild, Toshiba,
et. al. probably make make several other flavors of very small pin-count
"microgate" analog switches. I just happened to know of the TI parts since
I've used them in several designs before.

Matt Pobursky
Maximum Performance Systems

2008\01\02@112727 by Bob Axtell

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Vasile Surducan wrote:
> On 1/1/08, Nicola Perotto <.....nicolaKILLspamspam.....nicolaperotto.it> wrote:
>  
>> Hi all,
>> it's possible to read two trimmer with only one analog input?
>>    
>
>
> Instead of using mux like most of the people suggested here, try to
> use two different analog inputs for trimmer read and the same IO as
> digital output for command. Reconfigure your schematic and probably
> that will be possible.
>
> Vasile
>  
Didja get this? Vasile means to read one of the pots infrequently, then
the rest of the time, run a
DIGITAL OUTPUT from that pin. Good idea, one I have used many times.

This would work well for two POTS that should only be read while
powering up the product,
which is 99% of that application anyway (almost nobody requires pot
adjustments with
the power ON). The CMOS input that is normally connected to the digital
OUTPUT will rarely
be affected by the fact that a POT is hung on it, as long as the POT
value  is high and the
pot is NOT adjusted near ONE END, which will adversely affect a digital
input.


--Bob

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