In Pulse monitor using log amplifier, I talked about the problems I was having with using wooden blocks for holding an LED and phototransistor side-by-side, because the wood was too transparent, and the rather clumsy test I made using electrical tape:
I suggested in that post that I would buy a chunk of black ABS plastic for $10 and try making the finger cradles out of that. I realized later that I already have some black ABS plastic, in the form of Lego bricks. If I could use them, that would save me a lot of trouble, and provide a more easily duplicated block for others to use.
I ended up trying a black 1×2 Technic brick (which would cost about 1¢–3¢ each on Bricklink), drilling two ⅛” (~3mm) holes in the faces on either side of the axle hole. The axle hole in the Technic brick provides a light barrier between the two optoelectronic components:
I had hoped to be able to insert the 3mm LED and phototransistor from the bottom of the brick, but there was not enough clearance to do so easily, so I inserted them from the opposite face of the brick.
I tried recording the light levels with the front face taped over with black electrical tape, and with a finger covering it. The difference in voltage was large, indicating that the light through the finger was much more than the light leakage around the axle-hole light barrier. I was using an LTR-4206 phototransistor and a 1N914 diode followed by a unity-gain buffer.
I got around 394mV with the finger and 275–284mV with the holes taped. The variation on any given recording run with the holes taped was only about 0.3mV, but different runs, with different amounts of sunlight falling on the brick gave different levels. The minimum difference between the finger and the taped block is about 110mV, which translates to a 19.8dB difference in light (or a factor of 9.8).
But I was not able to get a pulse measurement with cis IR illumination. I could get a signal of about 1mV peak-to-peak with ambient (shaded sunlight) illumination, which corresponds to about 0.18dB, or 2% fluctuation in finger opacity, but that depended critically on the pressure on my fingertip—if it wasn’t just right, I got no visible pulse signal, just noise. I could get a bit more consistent results by putting the Lego block between my index and middle fingers and using a rubber band to clamp the fingers together. If the squeeze was just tight enough to throb, then I got fairly clean results, and I could get them fairly consistently, but I’m not sure whether others will be able to get similarly consistent results.
I also tried taping up the block with no IR illumination, to measure the dark current. I got 67mV, which should correspond to about 10nA, which is pretty good, since the spec for the LTR-4206 phototransistor give the max dark current as 100nA.
Bottom line: I think that the Lego bricks work about as well as the old trans-illumination wooden blocks that I’ve been using for a few years, and they are certainly much easier to make, requiring only drilling two ⅛” holes.
I’ll want to play around with different illumination, also. Lots of cis-illumination pulse monitor kits seem to use green LEDs, for example. Do those work better?