In Lego as LED holder, I wrote
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?
Since then, I’ve played around with a couple of different approaches for using Lego. The first, and most obvious, move was to use separate bricks for the LED and the phototransistor. If the phototransistor brick is clicked onto the top of the LED brick, then there is the ABS of the brick top between them, and very little light leakage. I tried both 1×1 and 1×2 bricks, which cost about the same at about 3¢ each on the used market.
Because I was too lazy to wire leads onto all my different LEDs, I also tried just sticking the LED in the breadboard and resting my finger on the bricks—it isn’t very sturdy, but for quick testing it is not bad:
I tried using several different LEDs. I got good results with 700nm and 607nm peak LEDs, but nothing but DC drift with green (565nm) LEDs. I would have tried a yellow LED, but I only had ones in 5mm packages, which is too big even for the axle holes, so the poor results there may have been due to mechanical, rather than optical difficulties (some signal was visible).
Here are the results with a 700nm red LED:
So, I have (at least) three choices for how to do cis illumination with Lego bricks, all of which I like better than using wooden blocks:
Drilling the bricks is very easy, and it would be even easier, if I made a jig that aligned the brick, rather than having to fiddle with the drill-press vice. I might even have the students drill their own bricks. I did not put Lego bricks on the parts list, but if I have to buy $5 worth of Lego bricks for this Winter’s class, it is no big deal.