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[on:]

Here's the datasheet: http://www.components.omron.com/components/web/PDFLIB.nsf/0/DEFC0948FE2A5C91862577A7005A8099/$file/2SMPP_1112.pdf I have the 2SMPP-03 model.

I picked up a few of these sensors for a breathalyzer project I'm putting together. I'm using these to detect if the user is blowing into the breathalyzer.

I think this sensor acts like a wheatstone bridge but I'm not quite sure. If someone could verify my assumption, I would appreciate it.

Also, I have no idea how much voltage I can supply this with, I plan on using 5 volts. Will I be ok with this voltage?

The last thing I'm not sure about is the drive current. Do I have to limit the amount of current going to this sensor?

Thanks in advance!
Bill

[Reply #1 on:]

hi,
can't help you with reading the datasheet. But I also like to know how this sensor works, how it can mesure air.
I hope someone will be able to answer you. 

Sobbat

[Reply #2 on:]

I picked up a few of these sensors for a breathalyzer project I'm putting together. I'm using these to detect if the user is blowing into the breathalyzer.

It seems that this sensor doesn't like side effects of a human blowing directly into it.
You could use some kind of membrane to separate the human's gasses from the sensor.

I think this sensor acts like a wheatstone bridge but I'm not quite sure. If someone could verify my assumption, I would appreciate it.

The sensor is drawn as a bridge.
And so is its example application.

Also, I have no idea how much voltage I can supply this with, I plan on using 5 volts. Will I be ok with this voltage?

You do not supply a voltage, you supply a current.
As you have 5 volts available for your Arduino, you can probably use that as your source.

Do I have to limit the amount of current going to this sensor?

Yes and no.
You do not really limit the current, you set the current.
As the current will stay at it's value, the voltage has to change upon sensing a changed pressure.

[Reply #3 on:]

Yes it is a Wheatstone bridge circuit. A little complex to interface to a arduino as you need to drive the bridge with a constant current source of just 100 microamps, and then use a differential instrumentation op-amp to amplify it to a usable single ended voltage output source suitable for measurement with an analog input pin. Page 40 of the datasheet gives you the general circuitry required, but without specific component values.

Lefty

[Reply #4 on:]

Would connecting this to 5 volts through a 50k resistor be considered a constant current source?

I was planning on using just the +Vout and running that through a non-inverting op amp to get a usable voltage out of this.

[Reply #5 on:]

Would connecting this to 5 volts through a 50k resistor be considered a constant current source?

I was planning on using just the +Vout and running that through a non-inverting op amp to get a usable voltage out of this.

Worth playing with maybe, but as the output is such a small millivolt variation Vs pressure variation, any change of Vcc is sure to effect the 'calibration' of the output voltage generated. Even changing from USB power to external DC power on an arduino will change the actual boards Vcc value by several tenths of a volt up or down in a lot of cases. There is a reason one drives a Wheatstone bridge with a constant current source rather then a voltage source, and that is to elimination one possible source of variation.

Lefty

[Reply #6 on:]

Would connecting this to 5 volts through a 50k resistor be considered a constant current source?

The device has a resistance of 20K +/- 2K, so you actually need a series resistance of about 30K. If you don't drive it with a constant current source, then the output will be ratiometric with the supply voltage - which is exactly what you want if you are using the default analog reference voltage (i.e. supply voltage) in the Arduino.

I was planning on using just the +Vout and running that through a non-inverting op amp to get a usable voltage out of this.

No, the output will be too small if you do that. Use an instrumentation amplifier such as INA126 instead.