So for the past few months I've been working on a simple circuit which I've (very poorly - please excuse it! I'm not very familiar with this particular CAD software or professional schematic drawing in general) drawn a schematic of and attached. I should point out that in the schematic I've drawn each resistor as having its own separate ground, when this is not the case - all resistors are wired to the same ground.
More or less, it's an optics device which we (my group) are using for biological studies by measuring the transmittance of IR light through a liquid.
It's a pretty basic setup: 6 IR LED emitters and phototransistor pairs, 6 300 ohm resistors and a voltmeter measuring the voltage drop across each resistor. We're using that measured voltage and correlating it with the optical density of the medium. I should also point out that we're using a power supply to ensure that the current across each emitter stays constant at 60 mA.
The problem I've been having is that they're unsteady measurements. For the last 24 hours I've been measuring the voltage drop across all 6 resistors using water as the liquid. While R2, R3, R5, and R6 are all pretty steady and the differences in voltage are normally distributed about a value, R1 and R4 are steadily decreasing in measured voltage, to the point where I can't attribute it to chance or random distribution. I've seen this happen on multiple occasions, leading me to believe something's going wrong.
I don't think it's a problem with the voltmeters - there's only two voltmeters each measuring three resistors, and it seems to take measurements fine for all but R1 and R4.
I don't think there's anything in the liquid causing this either - it's plain water and there's nothing to suggest that it's been contaminated.
I'm open to all suggestions - I'm just not sure what could be causing this besides possibly a faulty detector, though this behavior seems to suggest something else to me.
Thanks!
More or less, it's an optics device which we (my group) are using for biological studies by measuring the transmittance of IR light through a liquid.
It's a pretty basic setup: 6 IR LED emitters and phototransistor pairs, 6 300 ohm resistors and a voltmeter measuring the voltage drop across each resistor. We're using that measured voltage and correlating it with the optical density of the medium. I should also point out that we're using a power supply to ensure that the current across each emitter stays constant at 60 mA.
The problem I've been having is that they're unsteady measurements. For the last 24 hours I've been measuring the voltage drop across all 6 resistors using water as the liquid. While R2, R3, R5, and R6 are all pretty steady and the differences in voltage are normally distributed about a value, R1 and R4 are steadily decreasing in measured voltage, to the point where I can't attribute it to chance or random distribution. I've seen this happen on multiple occasions, leading me to believe something's going wrong.
I don't think it's a problem with the voltmeters - there's only two voltmeters each measuring three resistors, and it seems to take measurements fine for all but R1 and R4.
I don't think there's anything in the liquid causing this either - it's plain water and there's nothing to suggest that it's been contaminated.
I'm open to all suggestions - I'm just not sure what could be causing this besides possibly a faulty detector, though this behavior seems to suggest something else to me.
Thanks!
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