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PICList Thread
'Info on BCD switches'
1998\11\24@153735 by Kevin McCusker

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Hello all,


Can anybody help me with the following:
1: How to use a multiple BCD thumbwheel switches (They will used as
setpoint for voltage and current measurements)
2:How best to read voltages inthe range of 24VAC to 480VAC and currents
from 5Amps to 200 Amps
3:I know I can buy them but then where is the fun, I want to make a
differential pressure sensor to monitor filters in a HVAC system (When a
pressure difference is detected the filters need cleaning)

This is a senior college project and any information would be very, very
helpful.
Thanks
Kevin McCusker

1998\11\24@170340 by paulb

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Kevin McCusker wrote:

> Can anybody help me with the following:
> 1: How to use a multiple BCD thumbwheel switches (They will used as
> setpoint for voltage and current measurements)

 We've looked at this before, it often gets into multiplexing, diodes
etc.  Consider PISO shift registers, (HC165, CH597 - whatever is cheap),
one per eight switches, mounted adjacent to the switches with a SIP
resistor pack for pullup.

 Board layout it thus dead simple.  Only three lines (load, clock,
data) to the PIC, at least two may be shared with other functions, no
matter *how many* switches.
--
 Cheers,
       Paul B.

1998\11\25@125359 by Harold Hallikainen

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On Wed, 25 Nov 1998 09:00:39 +1000 "Paul B. Webster VK2BZC"
<spam_OUTpaulbTakeThisOuTspammidcoast.com.au> writes:

>  We've looked at this before, it often gets into multiplexing, diodes
>etc.  Consider PISO shift registers, (HC165, CH597 - whatever is
>cheap), one per eight switches, mounted adjacent to the switches with a
SIP
>resistor pack for pullup.
>
>  Board layout it thus dead simple.  Only three lines (load, clock,
>data) to the PIC, at least two may be shared with other functions, no
>matter *how many* switches.


       I agree that this is a "dead simple" way of going.  However, I
believe that "The ideal design has zero parts", so I TRY to not add any
extra chips or pull-ups to a design.  A fair number of my designs have
three BCD switches, so I use port B with internal pull-ups and 4 bits of
another port with external pull-ups.  This is all on a 16c74a.  Of
course, on smaller chips, or designs that need all those pins for
something else, you're stuck either multiplexing with a tone of diodes or
doing the shift register trick.
       I wonder how cost works out between using a PIC with enough pins
versus adding a shift register with a smaller PIC.  Another approach that
has been mentioned on the list is to use the thumbwheel switches with a
resistor network to provide a variety of analog voltages to an A/D input
of the PIC.  Anyone have ideas on a SIMPLE encoding scheme for this,
possibly using a standard resistor network?

Harold




Harold Hallikainen
.....haroldKILLspamspam@spam@hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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