digital dashboard

tom cloud cloud at hagar.ph.utexas.edu
Mon Sep 30 18:50:49 GMT 1996


>
>> Look in Radio-Electronics, July, 1990 for an article on building
>> a digital dashboard.  I personally like the Intersil ICL-7106, ICL-7116
>> chips.  They are 40-pin, but require a minimal amount of external
>> components and drive the LCD (or LED -- see ICL-7107 or 7117)
>> display directly.  Hosfelt Electronics (800-524-6464) has these chips
>> already on a circuit board with the display for about $16.  The
>> circuit is the standard circuit described from Intersil, and is a
>> differential input.
>> The sensors and scaling their outputs to the DPM are another problem.
>> As long as the outputs are linear, simple op-amp circuits will
>> suffice (LM-324 is a quad op-amp and allows single supply operation).
>
>
>Tom,  
>
>Thanks for the help on getting me started.  The only other question
>I have is how to handle the display.  Do you have any ideas as 
>to how I would display the proper units for each measurement?  
>For example:   Suppose a .5V signal corresponds to an inlet air temp of
>150 F.  How would I get the LCD to display the 150 F temperature?
>Thanks in advance for any thoughts.  
>
>Chuck Thigpen

The National Semiconductor Linear Applications books are great for
circuit ideas.  Making a display read in the units you want is
called "scaling" and can get real tricky.  Of course, the easiest
is when all you have to do is divide a voltage down using two resistors.
The hairiest can be when you have to add an offset voltage and maybe
linearize a logarithmic sensor output (e.g. a thermocouple).

For your specific example: you gotta know both ends of the spectrum
and whether it's a straight line (linear).  But let's just take your
specific example:  if you have a meter that reads 200 mV (actually
199.9 mV for a 3-1/2 digit meter), .5 volts is too large a voltage
for it.  Divide the .5 volts to where it is .15 volts and your
DPM will read 150.0 (you can usually set the decimal point wherever
you want it).  This would probably be done using a couple of resistors
and a pot for fine adjustment / calibration.

        input o--- R1 --- POT --- R2 --- GND
                           ^
                           |
                        output

Sample values:  if you want to vary the output say +/- .05 volts
around the .15 volts:  choose a pot (there are fewer values available).
Say 1 k.  Then (.05)*2 = .1 and .1 volt / 1k = 100 micro-amps.
Since the voltage across the pot is from .1 to .2, the voltage across
R1 = .5 - .2 = .3 volts and across R2 = .1 volts.  Then the values
for R1 = .3 / 100 microamps = 3 k; and R2 = .1 / 100 microA = 10 k.

The "load" your network puts on the sensor is the sum of all the R's
and is 3 + 1 + 10 = 14 k.  If your sensor requires less load (higher
R, you can multiply each value by the same amount and boost it to
any value.

Hope this helps -- you'll probably have to do some studying.  Radio-
Electronics and other electronic hobbyist mags can be very helpful.
Tom Cloud <cloud at peaches.ph.utexas>
I have no idea where any of the preceeding stuff came from ...





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