Looking inside a laser measurer


Tape measures are right up there with uncooperative-coiling (and -uncoiling) extension cords and garden hoses on the list of “things guaranteed to raise my blood pressure”. They don’t work reliably (thanks to gravity) beyond my arm span unless there’s something flat underneath them for the entire distance they’re measuring. Metal ones don’t do well with curved surfaces, while fabric ones are even more gravity-vulnerable. Speaking of which, the only way to keep a fabric one neatly spooled when not in use is with a rubber band, which will inevitably slip off and leave a mess in whatever drawer you’re storing it in. And when metal ones auto-spool post-use, they inevitably slap, scratch, or otherwise maim your hand (or some other body part) enroute.

All of which explains why, when I saw Dremel’s HSLM-01 3-in-1 Digital Measurement Tool on sale at Woot! for $19.99 late last October, I jumped for joy and jumped on the deal. I ended up buying three of ‘em: one for my brother-in-law as a Christmas present, another for me, and the third one for teardown for all of you:

The labeling in this additional stock photo might be helpful in explaining what you just saw:

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Here’s a more meaningful-info example of the base unit’s display in action:

The default laser configuration is claimed to work reliably for more than five dozen feet, with +/- 1/8-inch accuracy:

while the Wheel Adapter enables measuring curved surfaces:

and the Tape Adapter (yes, I didn’t completely escape tape, but at least it’s optional and still makes sense in some situations) is more accurate for assessing round-trip circumference (and yes, they spelled “circumference” wrong):

I mean…look how happy this guy is with his!

Apologies: I dilly-dally and digress. Let’s get to tearing down, shall we? Here’s our victim, beginning with the obligatory outer box shots:

And here’s what the inside stuff looks like:

Here’s part of the literature suite, along with the included two AAA batteries which I’ll put to good use elsewhere:

Technology licensed from Arm and STMicroelectronics? Now that’s intriguing! Hold that thought.

Here’s the remainder of the paper:

And here’s the laser measurer and its two-accessory posse:

This snapshot of the top of the device, as usual accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes:

is as good a time as any to conceptually explain how these devices work. Wikipedia more generally refers to them as laser rangefinders:

A laser rangefinder, also known as a laser telemeter, is a rangefinder that uses a laser beam to determine the distance to an object. The most common form of laser rangefinder operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimeter measurements, where triangulation and other techniques are often used. It is a type of scannerless lidar.

The basic principle employed, as noted in the previous paragraph, is known as “time of flight” (ToF), one of the three most common approaches (along with stereopsis, which is employed by the human visual system, and structured light, used by the original Microsoft Kinect) to discerning depth in computer vision and other applications. In the previous photo, the laser illumination emitter (Class 2 and <1mW) is at the right, with the image sensor receptor at left. Yes, I’m guessing that this explains the earlier STMicroelectronics licensing reveal. And the three metal contacts mate with matching pins you’ll soon see on the no-laser-necessary adapters.

The bottom is admittedly less exciting:

As are the textured and rubberized (for firm user grip) left and right sides (the two-hole structure at the bottom of the left side is presumably for a not-included “leash”):

I intentionally shot the front a bit off-center to eliminate reflections-of-self from bouncing off the glossy display and case finish:

The duller-finish backside presented no such reflectance concerns:

I have no idea what that white rectangular thing was inside the battery compartment, and I wasn’t brave enough to cut it open for a more thorough inspection (an RFID tracking tag, maybe, readers?):

This closeup of the back label does double-duty as a pictorial explanation of my initial disassembly step:

Screws underneath, just as I suspected!

You know what comes next…

Liftoff!

We can already see an overview of the laser transmitter function block (complete with a heatsink) at upper right and the receptor counterpart at upper left. Turns out, in fact, that the entire inner assembly lifts right out with no further unscrew, unglue, etc. effort at this point:

From an orientation standpoint, you’re now looking at the inside of the front portion of the outer case. Note the metal extensions of the three earlier noted topside metal contacts, which likely press against matching (flex? likely) contacts on the PCB itself. Again, hold that thought.

Now we can flip over and see the (even more bare) other side of the PCB for the first time:

This is a perspective you’ve already seen, this time absent the case, however:

Three more views from different angles:

And as you may have already guessed, the display isn’t attached to the PCB other than via the flex cable you see, so it’s easy to flip 180°:

Speaking of flipping, let’s turn the entire PCB back over to its back side, now unencumbered by the case that previously held it in place:

Again, some more views from different angles:

See those two screws? Removing them didn’t by itself get us any further along from a disassembly standpoint:

But unscrewing the two other ones up top did the trick:

Flipping the PCB back over and inserting a “wedge” (small flat head screwdriver) between the PCB and ToF subassembly popped the latter off straightaway:

Here’s the now-exposed underside of the ToF module:

and the seen-before frontside and end, this time absent the PCB:

Newly exposed, previously underneath the ToF module, is the system processor, a STMicrolectronics (surprise!…not, if you recall the earlier licensing literature…) STM32F051R8T7 based on an Arm Cortex-M0:

And also newly revealed is the laser at left which feeds the same-side ToF module optics, along with the image sensor at right which is fed by the optics in the other half of the module (keep in mind that in this orientation, the PCB is upside-down from its normal-operation configuration):

I almost stopped at this point. But those three metal contacts at the top rim of the base unit intrigued me:

There must be matching electrical circuitry in the adapters, right? I figured I might as well satisfy my curiosity and see. In no particular order, I started with my longstanding measurement-media nemesis, the Tape Adapter, first. Front view:

Top view:

Bottom view, revealing the previously foreshadowed pins:

Left and right sides, the latter giving our first glimpse at the end-of-tape tip:

And two more tip perspectives from the back:

Peeling off the label worked last time, so why not try again, right?

Revealed were two plastic tabs, which I unwisely-in-retrospect immediately forgot about (stay tuned). Because, after all, that seam along the top looked mighty enticing, right?

It admittedly was an effective move:

Here’s the inside of the top lid. That groove you see in the middle mates up with the end of the “spring” side of the spool, which you’ll see shortly:

And here’s the inside of the bottom bulk of the outer case. See what looks like an IC at the bottom of that circular hole in the center? Hmmm…

Now for the spool normally in-between those two. Here’s a top view first. That coiled metal spring normally fits completely inside the plastic piece, with its end fitting into the previously seen groove inside the top lid:

The bottom side. Hey, at least the tape isn’t flesh-mangling metal:

A side view, oriented as when it’s installed in the adapter and in use:

And by the way, about the spindle that fits into that round hole…it’s metallic. Again, hold that thought (and remember my earlier comment about using a rubber band to keep a fabric tape measure neat and tidy?):

Here’s the part where I elaborate on my earlier “forgot about the plastic tabs” comment. At first things were going fine:

But at this point I was stuck; I couldn’t muscle the inner assembly out any more. So, I jammed the earlier seen flat head screwdriver in one side and wedged it the rest of the way out:

Unfortunately, mangling one of the ICs on the PCB in the process:

Had I just popped both plastic tabs free, I would have been home free. Live and learn (once again hold that thought). Fortunately, I could still discern the package markings. The larger chip is also from STMicroelectronics (no surprise again!), another Arm Cortex-M0 based microcontroller, this time the STM32F030F4. And while at first, reflective of my earlier close-proximity magnetic-tip comment, I thought that the other IC (which we saw before at the bottom of that round hole) might be a Hall effect sensor, I was close-but-not-quite: it’s a NXP Semiconductors KMZ60 magnetoresistive angle sensor with integrated amplifier normally intended for angular control applications and brushless DC motors. In this case, the user’s muscle is the motor! Interesting, eh?

Now for the other, the Wheel Adapter. Front:

Top:

Bottom (pins again! And note that the mysterious white strip seen earlier was pressed into service as a prop-up device below the angled-top adapter):

Left and right sides:

And label-clad back:

I’m predictable, aren’t I?

Note to self: do NOT forget the two now-exposed plastic tabs this time:

That went much smoother this time:

But there are TWO mini-PCBs this time, one down by the contact pins and another up by the wheel, connected by a three-wire harness:

Unfortunately, in the process of removing the case piece, I somehow snapped off the connector mating this particular mini-PCB to the harness:

Let’s go back to the larger lower mini-PCB for a moment.  I won’t pretend to feign surprise once again, as the redundancy is likely getting tiring to the readers, but the main sliver of silicon here is yet another STMicroelectronics STM32F030F4 microcontroller:

The mini-PCB on the other end of the harness pops right out:

Kinda looks like a motor (in actuality, an Alps Alpine sensor), doesn’t it, but this time fed by the human-powered wheel versus a tape spool?

So, a conceptually similar approach to what we saw before with the other adapter, albeit with some implementation variation. I’ll close with a few shots of the now-separate male and female connector pair that I mangled earlier:

And now, passing through 2,000 words and fearful of the mangling that Aalyia might subject me to if I ramble on further, I’ll close, as-usual with an invitation for your thoughts in the comments!

Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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