I have some possibly really good news for those of us lamenting the obsolescence of the TSL1410 optical sensors.
Now that I'm finally past the IR project, I've been able to get back to finding a new plunger sensor. The first one on my list, which I'm furthest along on testing, is looking awfully promising. It's still in the prototype phase at this point, but it's a pretty complete prototype. If anyone wants to be a beta tester, it's far enough along that I can help you get one going.
The basic technology is an optical quadrature sensor, which is the same type of sensor they put in inkjet printers to keep track of where the print head is located along its track. It's a mature technology that's used in lots of applications that need high-precision position sensing. There are lots of these sensors on the market, too, so the basic approach is a little less dependent on a single source than the TSL1410 approach was. That said, the concrete design I have is all based on a particular device, so it would take some rework to adapt it to other devices.
A quadrature sensor is basically a little two-pixel optical sensor that moves along a "scale", which is a set of alternating black/white bars. The sensor picks up the edges as it moves across the bars. By counting the bars it passes, it can figure its relative position - "I'm 64 bars from the starting point". The two-pixel setup lets it determine which direction it's going, too, so it counts up or down as it moves back and forth.
I actually wasn't holding out much hope for this sensor. My big concern is that quadrature is an inherently relative sensing system, so I was worried that the sensor would occasionally miss bars and make the count drift over time. And I wasn't sure the sensor could keep up with the high speeds involved. But in my initial testing, it's looking incredibly solid - it looks like it's even better than the TSL1410 in terms of stability and precision. The sensor I'm using has a 75 LPI scale, which translates to 300 dpi. The TSL1410 is nominally 400 dpi for the sensor itself, but the practical resolution is about 100 dpi since we have to interpolate a shadow edge that's never as fine-grained as the pixel pitch. But we get the full 300 dpi out of the quadrature sensor, and it already feels smoother to me than the TSL1410.
The other good part is that the sensor I'm working with is pretty cheap - about $8 from Mouser.
Here's what the assembled system looks like (in my lab-bench plunger testing box):
The little white plastic part at the tip of the plunger snaps on between the spring and the e-clip. It holds the circuit board with the sensor, and it's a 3D-printed part, as you might guess. The white part at the base of the plunger is another 3D-printed part that holds the "scale" in place. The scale is the brownish rectangular part; it's actually a piece of mirrored acrylic, but you're looking at the back of the mirror here. That piece is stationary; the sensor carrier slides along it as you move the plunger.
Taking it apart so you can see the insides:
And the shiny side of the scale:
Those are the black/white bars you're looking at. They're actually printed on transparency film so that the mirror reflects in the "white" parts, and the transparency film is then stuck onto the acrylic with Scotch tape. (That part is hokey, but the tape is at the ends where there's no wear, so I think it'll actually hold up.)
A closer look at the sensor:
The ribbon cable matches the Pinscape expansion board plunger connector, naturally, so you just plug the other end into your expansion board port, select AEDR-8300 from the plunger type menu in the config tool (yes, I've already added it in my working version 
), calibrate, and you're set.
Now we get to the big downside. The overall system is a lot more complicated to build than the TSL1410 setup was. It consists of:
- two 3D printed plastic parts
- a laser-cut mirrored acrylic part
- a laser-printed-on-transparency-film overlay
- a small (1" square) custom circuit board (for the sensor)
All of these pieces are of course in electronic format ready for manufacturing, but the logistics are complex, since it involves ordering parts from Mouser, a 3D print service, a laser cutter service, and a board maker. And then of course you have to assemble the circuit board - which as you can see is a tiny and simple board, except that it has one rather intimidating part, which is the sensor itself. The sensor is a tiny surface-mount part with the pads on the bottom. I've assembled two of them so far; it was much easier than it looked, and both worked on the first attempt, but the smallness makes it intimidating anyway.
So, is anyone interested in taking a stab at being the first beta tester? It's a little bit of a risk, in that I only just got the prototype working, so it's not certain yet if the sensor will prove to be as wonderful in actual use as it looks to be in my test setup. But I do think at this point that the sensor will work; the beta testing is more for the process of assembling it, to see how hard it really is.
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