This may not have worked (reliably) even if I’d had the pin ordering correct

I recently got some SCD41 sensors. They report CO₂ level in PPM as well as temperature and humidity. They’re also surface-mount, of the LGA variety and small — approximately a centimeter square, with millimeter-wide solder pads separated by half-millimeter gaps. I’m not yet terribly skilled at SMD soldering and wanted to try getting readings without soldering wires to it or having to attempt to build a jig — which would’ve employed teeny (the plunger faces would need to be ≤ 1mm wide) pogo pins, driven through multiple, small holes drilled as-nearly-vertically-as-possible, close to one another, through a chunk of plastic.

I roughly cut a rectangular piece of white plastic (*checks notes* seems to be POM aka Delrin), marked the edges of the top of the metal can protecting the various sensors that make up the SCD41, drilled holes around the inside edges of the shape, and then ground out the interior and smoothed the edges with mini burrs in my Bosch 18V Dremel-alike. For minimum-viable strain relief, I drilled 1.5mm holes for the wires I hoped to connect to the six non-DNC pads (four on one side and two on the other): two for I²C comms and the two pairs of VCC (aka V_CC) and GND pads.

Had I, for example, checked the GND pad on one side for continuity with the GND pad on the other, many sighs, a lot of hrms and much consternation might have been averted. My spatial reasoning hadn’t been up to snuff and I’d misperceived the pinout diagram in the datasheet, which showed and labeled the solder pads Superman-X-ray-vision-style, from the top down (through the can, sensors, and PCB). Where my black (GND) wires are coming in from the left and right to the top pads in these photos, they should’ve been contacting the bottom pads on each side, with the red wires on the second-from-bottom pads, the green wire should have been touching the topmost pad on the left, and the blue wire ought to have been in contact with the pad below that. In other words, my wire positions should have been flipped around a horizontal axis.

When I did finally realize my error, I removed the tape, repositioned the wires, retaped them in place, and managed to get a demo script included with Sensirion’s Arduino library for the SCD4x line to report the sensor’s serial number. Success! At last! But no sensor readings, just an error message: Error trying to execute readMeasurement(): Not enough data received.

I opened the script (exampleUsage.ino) in an editor, and copied and pasted the Serial.print statements that reported sensor values into the if (error) {} block, beneath the line that printed the error message, saved, and loaded the modified script onto the Arduino Nano clone I was using. After each error message, it kicked out a new line: Co2: 57095 Temperature: 0.00 Humidity: 48.40. There was no temperature value and, if the carbon dioxide level in my little home office had really been that high (57095 PPM, almost 60k PPM), I ought to have been dead or in a coma. Moreover, while one would expect some “chatter” in the readings, little changes up and down in the values being reported, both the CO₂ and humidity values were constant.

Failing to use my ol’ noggin to its fullest, I punted and hoped the wacky readings were the result of poor contact between one or more of the wires and the pads on the sensor, peeled off all of the tape again, cut and stripped the ends off of new lengths of wire and ever-so-painstakingly soldered them to their respective pads. Since I was already there, I attached wires to each of the larger square pads in the center of the sensor’s PCB base. They’re all GNDs, all continuous with the GND pads on the edges, and I’ve since found that connecting those four pads (or not) doesn’t noticeably change the values reported by the sensor.

With the wires firmly attached to the sensor, I plugged their free ends back into the same holes they’d been stuck into on the breadboard I’d been using. The readings were… the same.

Only then did it occur to me that, while I’d read in the sensor’s datasheet that the SCD41 and SCD40 could both be run on anywhere from 2.4V up to 5.5V, I hadn’t checked to see how much current they needed at a given voltage. Nor had I checked exactly how much current the 3.3V-out pin on my Cloneduino Nano could provide (reportedly no more than 50mA, likely less than the sensor required at that voltage). Switching the jumper that supplied juice from the Nano to the V+ rail on my breadboard to the Nano’s 5V-out pin and pressing its restart button, yielded the sort of result I’d been chasing: something like Co2: 1173 Temperature: 24.45 Humidity: 42.70, followed at five-second intervals by a very-slightly-different set of values.

Spoiler: Yes, the CO₂ reading is high and almost certainly incorrect. If a larger Sensirion CO₂ sensor (the SCD30 I’ve run simultaneously in the same small, sealed-to-keep-the-cold-air-in air-conditioned space is to be believed… incorrect by ~400-500 PPM.