The current states and brief history the SDI-12 USB adapters

Despite the ongoing chip shortage, I’ve secured a batch of processors for more adapters. At the moment I’ve also built enough stock to last for a little while.

So currently I have about 50 qty adapters including a few with analog inputs. I have more boards and ICs but want to conserve my parts. From what I saw, there won’t be a relief of chip shortage until a year from now, i.e. May 2023. I’ve maintained the price of $49 (sometimes $45) for the last decade by absorbing the cost of parts etc. I might be able to continue to do so if I get some help with assembly, such as getting a pick-and-place machine to place surface parts for me. But it’s an expensive machine.

Here is a brief history of my adapters:

In the summer of 2015, I came up with the idea to make a USB adapter that can make reading SDI-12 sensors easier, after having to deal with this protocol in a number of projects. This purple board was the first batch made at It is a small PCB fab business (they only collect orders and send to actual fab houses) on the west coast. They used to be called dorkbot PDX, a small group of electronics hobbists on the west coast trying to make affordable boards by banding together in their group orders.

First SDI-12 USB adapter prototype in November 2015

This board has the same general layout as the current version, with the miniUSB (maybe my obsession or did I just purchase too many of these connectors?!), USB-UART chip from FTDI, an atmega328p-au, and a programming header. Back then I only had a single terminal for an SDI-12 sensor and had no option to supply external power, well, not easily:

Connecting to external power 2016

Back in the days, I was using a terminal program to talk to the adapter and wrote a macro based of the terminal program to automate data collection. In 2016, I started learning Python more seriously, maybe it’s from raspberry pi, or maybe it was trying to teach physics with some computing elements, or just having to learn it to do projects. So here comes the logging script, in its primitive form in early 2016:

Python script running on Raspberry pi in Feb. 2016

I also started marketing the adapter as compatible with WIN/NIX/MACOS/RPI because back then Python 3.x was able to bring a more consistent programming experience across these systems that Python 2.x was never able to.

Back then, IoT (Internet of Things) was in its very primitive stage and not many places allowed data to be uploaded and presented online. I started with Sparkfun’s Phant server and, not sure which one first.

Logging data to the internet in April 2016

Later in 2016, I realized there were some need for high precision analog inputs and added the SDI-12 USB + Analog adapter to the lineup. This is the first prototype, in purple as purple. Placing the tiny analog-to-digital adapter used to be nerve wracking. I have since mastered it.

SDI-12 + Analog USB adapter in November 2016

I had a number of interesting projects in 2016-17 and didn’t do a lot on the SDI-12 USB adapters until later in 2017, when I had a project that required collecting GPS data with the sensor so I designed this for the project and general use.

SDI-12 + GPS USB adapter in July 2017

Back then, the basic adapter was still a small half-sized green board with a single SDI-12 terminal. That was able to change:

Full-sized (since then) SDI-12 USB adapter in March 2018

I didn’t market this as a separate adapter because I had high hope that this will replace the more expensive GPS adapter but the hardware didn’t support GPS. So, I started marketing this as a full-sized adapter at the same price point, $49, and gradually retired the half-sized green boards with single terminals. Starting with this version, I added external power connection and a jumper to select between 5V from USB, where most sensors work fine, and the external power terminal, where you can supply your own DC voltage such as 12V. This feature came from the Analog and GPS adapters.

Later in 2018, I redesigned the basic adapter to mostly today’s look and feel. It now has the 4 terminals, power input and selector on the left and right sides, optional basic analog and digital inputs on top, and an extension port in the middle, with an option of UART instead of USB as well.

Very recognizable look of SDI-12 USB adapter in May 2018

Here is a look at two high resolution analog extension boards on top of the basic adapter. I completed updating the code to take inputs and auto scale the inputs, for up to 4 such extension boards:

High resolution analog input extension boards May 2018

I also added extra SDI-12 terminals extension board to help manage the many wires you may have to manage if you want to connect more sensors.

I have also started customizing my adapters to connect to other sensors, such as the following with accelerometers, from special requests.

Customized adapter with accelerometers June 2018

Later that summer, I designed my own data logger! This was based on the idea of having an embedded SDI-12 processing unit, an analog input, and then an ESP32 processor running a new thing called microPython, new and getting traction back then, both the processor and software, now they’ve become major hardware and software players, if not dominating the IoT. This logger is very promising, with WiFi, Xbee, possibility of 4G LTE-M, sd card, real-time clock, and lots of expension!

My own complete WiFi Xbee GSM etc. data logger July 2018

Lots of things have been happening back then and I did a few rounds of firmware development but didn’t end up releasing this logger! Some of you may know that the spirit of this logger lives on and is thriving in a different sector of the industry! The following look may be familiar?

My logger in an SK-16 enclosure August 2018

In early 2019, I added the UART interface adapter to the lineup, based off the same board as USB interfaced adapter, due to an increase of demand to interface with Arduino, ESP32 etc.

UART adapter March 2019

In June, I contemplated using larger terminals based on user feedback. I decided, in order to maintain the same size and look, I can use 0.175″ pitch terminals instead of 0.1″. This makes the terminals accept thicker wires and separate the wires further for installation and prevent short circuits.

Comparison of 0.1″ and 0.175″ pitch terminals July 2019

Now this looks really really like what I’m selling today:

SDI-12 USB Adapter July 2019

I have also been logging data from my own back yard to demonstrate and test my logging scripts:

My backyard sensor run in the summer of 2019

I was even able to determine that after heavy rainfalls I always had power outage on my logger (see long flat blue lines after spikes). It turned out to be a faulty outdoor outlet in my house tripping up the circuit breaker after heavy rain water seeped into it causing short circuit. I later fixed it:

Bad outdoor outlet I replaced in 2019
Good one I installed in 2019

Lots of things were happening later in 2019, besides what you all know, so I didn’t touch my designs until MUCH later, in early 2021. I added a protection diode to fight 12V accidentally damaging the adapter by operators’ errors.

Adapter with protection diode February 2021

As many of use know, IC shortage soon hit the main street manufacturing. Here was a screen grab of the long way, from 2021:

Chip shortage starting showing long lead times, over 1 year, June 2021

Luckily I was well stocked at the year end of 2020! Now it has become a constant thing in the back of my mind, find parts and stock up!

Showing my stock of parts in 2021 to weather the “temporary” shortage in 2021

Over the years, I got a lot of good suggestions from users. Here is one suggestion to try and read data from a phone. This only works on an android and requires a wire between the adapter and the phone, but it’s a good start.

I started experimenting with android phone and my adapter July 2021

Here is another request from users to handle more sensors with easier terminals and combat the environments. I designed this to fit inside a specific enclosure, the same one I used for my own logger, which was now 3-4 yr old and didn’t get an update (I did update it but didn’t build a prototype). I started selling this to a research group for their projects but held back because I didn’t have many chips left (and it may be a lookalike to something else too)!

SDI-12 USB SK-16 adapter February 2022

I was literally counting my chips at this point! Fortunately I found a small batch of the processor in a smaller form factor and decided to take a risk with the supplier. So now we have a new revision. The protection diode has been replaced by a surface mount version but I’ve kept the thruhole part footprint because I bought a batch of the thruhole diodes, just in case. So if you see your board has an orange diode, or no such diode but a small black box on the top right, both are fine!

New design with smaller processor April 2022

The smaller processor takes quite a bit more time to place and I always place it before all other parts so I can turn the board around to see if I placed it perfectly or not. There aren’t any pins on the chip so it’s harder to check. Hopefully when I get a pick and place machine, I don’t have to worry about it.

Size comparison between the two processors April 2022

So there you have it! I forgot I was planning for a brief history. As a closing remark, I’ve seen the gradual shift of users of this adapter from almost entirely academic research to an increasing percentage of applications to more individual users. What’s not changed is that people love simple and inexpensive solutions to their data logging needs and I’m proud that my adapters have met their needs so far! Thanks for your time!

SDI-12 USB adapter family portrait May 2022