Multiple flowmeters read through mic port of an Android device

[yo1dog]

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:

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

 

[yo1dog]

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.

 

[yo1dog]

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

 

[yo1dog]

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?