Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Multiple flowmeters read through mic port of an Android device

  1. Mar 20, 2015 #1
    I am attempting to read two hall-effect flowmeters through the audio-in (mic) port of an Android device. I want to be able to read both flowmeters independently. The audio-in port is a standard headset port so it only has one channel for the mic.

    So far I have been testing with my desktop PC. I wrote a Java app that can detect pulses (interrupts?) from the audio-in (mic) port. I am able to hook up a single flowmeter by connecting the +5v from the audio-in port to the +5v of the flowmeter and the ground from the audio-in port to the pulse line of the flowmeter. I don't know if this is the proper way, but it works in that I get a signal from the flowmeter and I can see the pulses like so:

    AudioInterruptDetectorSC.png



    The first problem is the +5v provided from the audio-in port does not have enough amperage (I think?) to run both flowmeters. I figured I could splice into the USB cable that would be powering/charging the android device and use its +5v and ground to power the flowmeters.

    The second problem is knowing which pulse came from which flowmeter. I was suggested using an R-2R to mix the pulses into one signal but with voltages (and thus amplitudes) at 25% and 50%. That way, when I read a pulse with my app I know if the pulse has an amplitude of 25% it was from flowmeter A, 50% is flowmeter B, 75% is both.

    I tried to put together a circuit that used both of these but I was unable to figure it out. It looks like what I need is for the voltage output from the R-2R to control the voltage from the audio-in port's +5v to ground. But how?

    Here is what I put together on my own. I am very new to all of this.
    http://www.digikey.com/schemeit/#1fwi

    Thanks,
    - Mike
     
  2. jcsd
  3. Mar 21, 2015 #2
    Some more information:

    Here are the flowmermeters I have:
    http://www.adafruit.com/products/828

    I used my multimeter and the current from the audio-in port is 5.15v and 2.17mA.
     
  4. Mar 22, 2015 #3
    OK, did some more research and I think I need to implement a "voltage follower". I have updated my circuit to use an op-amp so I can control the voltage to the audio-in with the pulses from the flowmeters.

    http://www.digikey.com/schemeit/#1fwi

    Are there any specifications I should look for when choosing an op-amp? Can anyone recommend one?

    Thanks,
    - Mike
     
  5. Mar 31, 2015 #4
    Well, I got all my parts in and my circuit does not work.

    I discovered my flow meters change between an "on" and "off" state depending on the location of the wheel and do not send out a single pulse like I thought. When they are "on" +5v can be read from the pulse pin to the ground. When they are "off" it is 0v.

    The R-2R seems to work. When neither of the flow meters are "on" I read +0.02v from the output of the R-2R (combined pulses) to ground. When only flow meter 1 is "on" I read +0.95v. When only flow meter 2 is "on" I read +1.95v. When both are "on" I read +3.24v. While this will work, I expected to get +1.25v (25%), 2.5v (50%), and 3.75 (75%) respectively. Why don't I?

    The op-amp does not do what I want at all. When the op-amp's VCC is +3.24v (both flow meters are on) reading between the op-amp's OUT and the mic's ground gives me -1.2v. Why this negative voltage? I thought that the op-amp would change the voltage from IN+ (mic's +5v) to OUT (mic's ground) to match the voltage from VCC to GND. What am I missing here?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Multiple flowmeters read through mic port of an Android device
  1. Mic circuit problem (Replies: 8)

Loading...