Category: Electronics

Explorations in things electronic

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Dexcom Stelo teardown

A couple months ago, I decided to try out the Dexcom Stelo blood glucose biosensor to see what kind of near-realtime feedback about my blood glucose levels I could get (more about that in a future blog post, probably).

These are small plastic devices about 23 mm x 27 mm that are supposed to last about 15 days. After they’re spent, you take it off and replace it with a fresh sensor.

Dexcom Stelo blood glucose biosensor
Dexcom Stelo blood glucose biosensor

Naturally, I wanted to see what was in them so I took one of the spent sensors and got some x-rays of them.

For my first image, I x-rayed a spent Stelo using a regular radiographic unit at about 5x magnification, 80 kV, 1 mAs.

X-ray of a Stelo blood glucose biosensor. 80 kV, 1 mAs.

Decent enough image that shows what appears to be the battery (the round object), the Bluetooth antenna (the trace toward the top), the sensor wire (the straight horizontal wire near the middle) and some of the traces on the circuit board.

I wanted a better image so I used a mammography unit. 38 kV, 40 mAs with about 1.8x magnification.

X-ray image of an intact Dexcom Stelo blood glucose biosensor. Image was acquired using a Hologic Dimensions mammography unit, 38 kV, 40 mAs at 1.8x magnification.
X-ray image of an intact Dexcom Stelo blood glucose biosensor

Now we see the traces on the circuit board much clearer. The round object is definitely a battery, and there are signs of at least a couple of microchips.

Time to liberate the circuit board from the casing, which turned out to be a bit of work. It looks like the Stelo is built by injection molding (or some similar process) plastic around the PCB. With a process like that, these are definitely waterproof. After some work with a knife and cutters, I was able to liberate the board.

Printed circuit board extracted from a Dexcom Stelo blood glucose biosensor
Printed circuit board extracted from a Dexcom Stelo blood glucose biosensor

The battery turns out to be a CR1216 coin cell and occupies most of the back side of the device. The microchip on the right is marked N52832 and is a Nordic Semiconductors nRF52832 that takes care of the Bluetooth. It boasts some pretty impressive specs for such a small device. The small silver rectangular thing below and to the right of the nRF52832 is probably a crystal oscillator. It was marked with T320 MnKC.

The other chip is marked DCG7 (probably a custom designator, maybe standing for DexCom G7, their other continuous glucose meter), would likely be the brains of the device. The gold circles on the board are test points.

With the battery out of the way, an x-ray image of the PCB reveals much more detail.

X-ray image of the printed circuit board from a Dexcom Stelo blood glucose biosensor. Image was acquired using a Hologic Dimensions mammography unit, 38 kV, 40 mAs at 1.8x magnification.
X-ray image of the PCB from a Dexcom Stelo blood glucose biosensor

The BGA pads on the left side of the battery terminal would be for the nRF52832 chip while the QFN pads on the right side would be for the DCG7 chip. Aside from a bunch of other surface mount passive components, there’s not much else to the Stelo.

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Power supplies on the workbench

I’ve got a number of power supplies that I can use to supply power to the various projects that I’ve got on the workbench.

There’s the obvious wall power and a variety of wall warts of different voltages and current ratings. I’ve got wall warts and power supplies ranging from 5V all the way up to big chunky 28V power supplies.

A power strip mounted to the side of a bookshelf with a 5V wall wart power supply and flashlight plugged in
A power strip mounted to the side of a bookshelf with a 5V wall wart power supply and flashlight plugged in

For projects that require a beefier power supply I’ve got an unregulated power supply that gives me up to 30VDC or 25VAC. It doesn’t get used too often, but comes in handy when it’s needed. There are also a few 13.8V power supplies that I can steal from the radios in a pinch.

Power supply offering unregulated 0-30V DC and 0-25V AC
Power supply offering unregulated 0-30V DC and 0-25V AC

Most of my projects don’t have big current draws though, so more often than not, I’m just using batteries to power my projects. One that I use most often is a 12V battery pack made by wiring 2 quad C-cell battery holders together and putting a standard 3.5mm DC barrel connector on it.

A battery pack of 8 C-cells connected to a Sparkfun RedBoard
A battery pack of 8 C-cells connected to a Sparkfun RedBoard

DC barrel connectors can be easily attached to those snap connectors used for 9V batteries providing another power source (here’s one from Sparkfun if you don’t want to make one).

A 9V battery connected to a Sparkfun RedBoard
A 9V battery connected to a Sparkfun RedBoard

Some battery holders I have also use those snap connectors, so I can have battery power sources ranging from 3V to practically as high as I need.

Batteries in a variety of battery holders
Batteries in a variety of battery holders

I’m often using mostly used batteries pulled out of other things like TV remotes, smoke detectors, etc. Usually they still have plenty of juice left to power my smaller projects.

While charging up some old sealed lead acid (SLA) batteries pulled from my battery backup UPSs, I got to thinking about how I could put them to use as an additional power source. I’ve got plenty of three terminal voltage regulators that would provide the voltages that I’d need.

I sketched out block diagrams for a couple of possible concepts. One would switch between several different voltages but provide just a single output, while the second would provide multiple outputs and have toggle switches turn to each one on or off.

Block diagram for a potential battery powered power supply with a variety of switched voltages
Block diagram for a potential battery powered power supply with a variety of switched voltages
Block diagram for a potential battery powered power supply providing multiple voltages
Block diagram for a potential battery powered power supply providing multiple voltages

Probably wouldn’t do 3.3V, but the others would definitely be feasible. 12V would probably be just a direct connection to the battery rather than going through a voltage regulator. Since I’ve got 2 SLA batteries, I could even put them in series for a 24V source and give myself a wider range of regulated voltages.

Still in the concept stage for this and haven’t started building anything yet. Seems like it could be a fun project.

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Pixel 6 X-ray

A radiograph of my new Pixel 6 phone. The Pixel 6 is a big chunky phone with a lot of stuff in it.

X-ray image of a Pixel 6 smartphone
X-ray image of a Pixel 6 smartphone. 80 kV, 1 mAs

This one was acquired using a portable x-ray unit at 80 kV, 1 mAs, and the small focal spot. It’s raised up about 25 cm above the image receptor for a bit of magnification (about 1.3x) and cropped in from the original image.

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Soldering station upgrade

I’m finally upgrading my soldering station.

After many years of using my slow, inexpensive (but pretty reliable) Radio Shack soldering iron, I’m upgrading to a Hakko FX-888D!

Hakko FX-888D soldering station
Hakko FX-888D soldering station

I’m excited to start using the Hakko. I expect there will be a bit of a learning curve with the new soldering iron, but I think it will be a nice step up. The old soldering iron will go back to the garage to get used on projects there.

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RCA Victor 45-EY-3 vintage record player

On the workbench for some troubleshooting is a 1950s era vintage RCA Victor 45-EY-3 record player. It belongs to someone in my neighbourhood and came to me via a referral from a neighbour who’s familiar with my penchant for tinkering with electronics.

RCA Victor 45-EY-3 record player from 1950
RCA Victor 45-EY-3 record player from 1950
RCA Victor 45-EY-3 record player from 1950
RCA Victor 45-EY-3 record player from 1950
RCA Victor 45-EY-3 record player from 1950
RCA Victor 45-EY-3 record player from 1950

The owner purchased this recently and had already replaced the tubes, capacitors, a few resistors and some of the mechanical bits before the record player landed on my bench. He said it was sort of working (some mechanical issues with the arm moving), but then stopped turning on. Fortunately, he had a printouts of the service documentation available to look over. On the electronics side, the circuitry is pretty simple consisting of three tubes: rectifier (35W4), amplifier (12AV6), and output (50C5).

RCA Victor 45-EY-3 schematic
RCA Victor 45-EY-3 schematic

After touching up a few solder joints, I found the power switch was kind of dodgy and would work if I tilted it a certain way. I also realigned the muting switch (S2) so that it was oriented the same way as one of the photos in the service manual. That got me to the point where records could play and sound came out of the speaker instead of just resonating through the needle arm. The sound volume was pretty low though, even with the volume pot turned up all the way and there’s also a lot of hum getting into the electronics too. Those are the two main things I need to work on, and the owner will work on the mechanical stuff.