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December 1st, 2014

Measuring sunlight on the cheap: Open source electronics for datalogging

Holly Barnard in her lab with the PARduino device, November 2014.

PARduino was a bit of a side project for hydrologist Holly Barnard. Her question, which PARduino was built to answer, was, can a hobbyist-grade microcontroller be used for research-grade field programs?

Barnard’s research focuses on ecohydrology: investigating how trees affect water flow and pathways in soil and streams, and conversely, how water flow affects vegetation function. Her ultimate goal is to better understand how changes in land use and climate will affect water resources and ecosystems.

A big driver in these systems is photosynthetically active radiation (PAR)—the sunlight, mostly in the visible spectrum, that plants respond to. Commercial datalogging equipment is so expensive that PAR is generally modeled using a map of the terrain. An inexpensive datalogger would make it more affordable to put a network of sensors on the ground that can take direct measurements over real time.

But that wasn’t a problem Barnard had thought about tackling until last fall. Matt Findley, a self-confessed tinkerer, signed up for a weekend Arduino workshop with Sparkfun, a Boulder electronics store that promotes open source technology. Arduino is an open-source platform for sensing, recording, and responding to the physical environment, and a darling of the maker movement. He convinced Barnard to come along.

After working with the Arduino microcontrollers, they had the idea that led to PARduino.

Neither Findley nor Barnard are programmers, but Findley dove into that aspect of the project. Using off the shelf parts and through-hole soldering, they built a prototype, connecting the PARduino datalogger to an amplifier and a PAR sensor.

The next step was to see if the measurements taken by the datalogger were accurate, precise, and reliable enough to trust as scientific observations. Barnard took the device to Janae Casvina at NEON, who has access to the NEON calibration radiation deck. PARduino’s data were compared to those from eight other PAR sensors with known performance, with measurements taken every minute for ten days. They found that PARduino performed adequately, especially in the higher levels of light found in the middle of the day.

Barnard plans to distribute PARduino across catchments within the Boulder Creek Critical Zone Observatory. The measurements will be used to model tree growth across the catchments and will build upon Barnard’s graduate student, Hallie Adams’, recent work that was just published in an open-access paper in Ecosphere, looking at how variations in climate and topography can be used to predict the annual growth of lodgepole and ponderosa pines. PARduinos placed throughout her study area will help pinpoint PAR contributions to the ecosystem.

PARduino was developed to be inexpensive, easy to assemble and program, and require little power. The team published their proof of concept in a journal article, “PARduino: a simple and inexpensive device for logging photosynthetically active radiation,” in Tree Physiology. They then made the design publicly available as an open-source hardware device.

PARduino’s design, including a bill of materials, circuit board layout files, and source code, is available to everyone at an online source code repository ( and is released under Creative Commons’ Attribution-ShareAlike license.