Barometer

Discussion in 'The Projects Forum' started by azngeek, Sep 10, 2008.

Jul 22, 2008
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Hi I'm interested in designing a barometer. Am planning to use a Motorola pressure sensor in conjunction with an 8 bit AD converter.(Already done signal processing level shifting, filter, and amplified). I used an instrumentation amplifier for level shifting and amplfication. Filtering just a simple passive capacitor.

I am interested in measuring from 50.0 kPa to 100.0 kPa, which the pressure sensor is able to do. So if I have 8 bits digital output from the AD converter, how do I drive 4 seven segment displays. As I'm only interested in 0.5 kPa increments, I was thinking of using the LSB to toggle bettween 0.0 and 0.5 kPa. So I have the remainding 7 bits to control the remainding 3 seven segment displays. Though the MSB doesn't really tell you anything as it is always 0. Using 100.0 kPa = 0000 0000 out of the AD and 50.0 =0110 0100. With decreasing pressure increasing bit count.
I was thinking of using an SN5446A BCD driver for the seven segment display, I just was thinking I might get some input on using 7 bits to control 3 seven segment displays.

Apr 5, 2008
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Hello,

You are saying that the pressuresensor has an analog output.
Why don't you use a 3.5 digit lcd or led panelmeter?
That has a range from -199.9 to 199.9 mVolts.

Greetings,
Bertus

Jul 22, 2008
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Hello Bertus,

Many thanks for the reply. I just had a look at one of the 3.5 digit lcds. That would definitely solve my problem, except for this project we are required to produce a digital solution, for the barometer. That was one of the criteria for the project. But still appreciate the input.

So basically right now I've got digital output of

0000 0000 for 100.0kPa
and for every decrement of 0.5 kPa, increase of 0000 0001.
So, for 50.0 kPa there's a digital output from the ADC of 0110 0100. (I'm only interested to a pressure of 50.0 kPa)

The 0.5 kPa toggle on the 7 segment display makes sense. Just for the remainding 7 bits I need to control another 3 segment displays, one of the other segment dispalys will only be on at 100.0 kPa. so that's not a problem either. The remainding 2 7 segment displays, that's the tricky part.

Thank you again Bertus! NEver knew of 3.5 lcd panels before you told me. Will keep that in mind for future projects.

Jul 22, 2008
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I think I've got it figured out : ) Thanks guys : ) though I think I am going to have a play around with that analog single input 3.5 lcd. : P That would make so much more sense... but sadly does not fulfill the design criteria.

5. SgtWookie Expert

Jul 17, 2007
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It sure would make your solution a lot easier if you inverted your inputs and re-scaled the output of your instrumentation amplifier. That way you could run the output of the ADC through three 74185 binary to BDC converters, and from there it would be duck soup.

Jul 22, 2008
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Thank you Sgt. Wookie. And here I was thinking of using a decoder to select the BCD driver accordingly for the 1s and 10s. I still think you do need to do the same for your solution, and you would still need the same amount of BCDs, but it would make a lot more sense. Thanks for the suggestion

7. SgtWookie Expert

Jul 17, 2007
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Well, unless the output of your ADC is BCD, you'll have to convert it to BCD in order to have the correct display. For an 8-bit binary input, one way to accomplish the conversion is to use 74185 IC's. Those are getting pretty scarce nowadays. DM8899's could also be used, if you handle the 3-state outputs properly.

Another way to completely sidestep the issue would be to use an ADC that has BCD outputs. Microchip sells a couple of ADCs that do exactly that; however they're only available in PLCC packages.

Jul 22, 2008
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Hi sgtwookie. My ADC that I selected had digital output that was in binary. I implemented logic gates (very primitive) to do it. I used a multiplexer to select between 4 bcd drivers for the outputs of the 1s. And I used one BCD for the 10s. So A total of 5 BCD drivers for the seven segment display. Thanks for the suggestion though

Jul 22, 2008
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Oh hi, me again. Just wondering, if I'm controlling the ones with a decoder and 4 bcd to seven segment displays, the one tenths with one bcd to seven segment display and one bcd to seven segment display for the 10s and hundreds each, would it be recommended to use decoders as intermediates, for the one tenths, tens, and hundreds just for timing purposes?