# digital air pressure gauge

#### smokeygolf

Joined Oct 23, 2006
12
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#### beenthere

Joined Apr 20, 2004
15,819
That might be difficult with the voltage requirement. My Digi-Key catalog shows Honeywell pressure sensors are the only ones that run in that range, and they all take higher supply voltages. The ASCX150DN (part # 287-1007) goes to 150 PSI, but needs 4.5 - 16 volts. The output is .25 - 4.5 volts. Price is $66.46. You can get an LCD 3 1/2 digit panel meter for$21.66. It's part #227-1060. Full scale is 200 mv, so you will also need an op amp with a negative supply to offset the .25 volt sensor offset, and scaling resistors to reduce the full scale voltage. You would want the 150 PSI voltage to be 150 mv to make the meter read directly in PSI. It takes 9 VDC.

There could be other ways to skin that cat, but this will work, with the addition of a pressure tap and an enclosure plus a small pc board. The easy way out would be two 9 volt batteries for power.

#### kender

Joined Jan 17, 2007
264
I would agree with the previous poster that the single +3V supply rail is a big constraint for the parts selsection. I would add a step-up converter and boost the supply voltage to +5V. I would look at the regulated capacitive charge pump, if low cost is desired. I would look at the inductive boost converter, if higer efficiency is desired.

#### smokeygolf

Joined Oct 23, 2006
12
hi, would there be any chance of you drawing a rough circuit diagram of exactly what you mean? thanks

#### kender

Joined Jan 17, 2007
264
hi, would there be any chance of you drawing a rough circuit diagram of exactly what you mean? thanks
Here are some application notes:

Maxim had recently published a very nice app note "An Introduction to Switch-Mode Power Supplies" http://www.maxim-ic.com/appnotes.cfm/appnote_number/4087 About 1/3 of this note is dedicate to step-up converters (if you havent worked with switchers, you should read it all).
Charge pumps http://www.maxim-ic.com/appnotes.cfm/an_pk/669
Heres a neat charge pump + LDO topology: http://www.maxim-ic.com/appnotes.cfm/an_pk/3202

#### smokeygolf

Joined Oct 23, 2006
12
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#### kender

Joined Jan 17, 2007
264
Based on the parameters you've posted, the battery needs to be 12 amp-hours (Ah) in order for you device to run for 2000 hours. Typical 9V alkaline battery has a capacity of 0.6 to 0.7 Ah.

#### smokeygolf

Joined Oct 23, 2006
12
hi kender, how exactly did you work it out? also if i still chose a 9V battery how many hours would i get? Thanks

#### kender

Joined Jan 17, 2007
264
What you want to estimate is the capacity of the battery (Q), which is the integral of average current consumed (I) over time. Battery capacity is measured in ampere-hours (Ah). The battery with a capacity of 1Ah can deliver 1A for 1 hour. Ampere-hour is an engineering unit, the proper physical unit would be Coulomb (which would be equivalent to ampere-second). Notice a caveat - battery capacity doesn't take voltage into consideration at all.

Your circuit consumes constant current I=6mA. To find out how long can you run from a battery - divide the capacity by consumption (i.e. current). For example: 625mAh battery with 6mA consumption will run for 104 hours.

P.S. Batteries are "nonlinear function of everything" (to quote one of my friends). If you want to learn about batteries in-depth, you should read through this web site http://www.batteryuniversity.com

#### smokeygolf

Joined Oct 23, 2006
12
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#### smokeygolf

Joined Oct 23, 2006
12
anyone able to help?

#### kender

Joined Jan 17, 2007
264
the output from the pressure sensor is 30mv/psi, i was wondering if i used a 741cn op-amp as an attenuator, could i reduce this output to 1mv/psi to correspond to my digital display?
I would suggest to try a voltage divider.