Adam Welch's Open Source PWM85 Solar Charge Controller Builds on Four Years of Tinkering

Engineer Adam Welch has released designs for an open source solar charge controller built around an ATtiny85 microcontroller — building upon a project released by Julian Illet four years ago.

Illet’s PWM5 solar charge controller was released as an open source project four years ago, after enjoying a period as a closed-source commercial product. Welch’s variant, inspired by a recent video series from Illet discussing various platform-based rebuilds of the project, takes the concept and modifies it for an ATtiny85 — and a surprisingly short list of low-cost components.

“I’ve resurrected a project from four years ago to bring back the ATtiny version of Julian Illet’s PWM5 solar charge controller,” Welch explains in the introduction to his project video. “The project, based on Arduino, attempts to replicate Julian’s low-power and simple design with a couple of design tweaks.

“I first built [an Arduino version] following the design I found on This design uses a piece of perfboard about the same physical size as an Arduino Mini to hold all of the external components. Pretty neat, but also quite tightly packed and actually quite difficult to solder.”

Welch’s follow-up, designed to reduce the power draw, was based on the ATtiny85 but still assembled on perfboard. “Almost four years later and I’m looking again at the ATtiny version of this charge controller. In 2019 PCB fabrication is significantly cheaper, and therefore I’ve taken the plunge to design a board for what I’m going to refer to as the PWM85 solar charge controller.”

Welch’s latest design, for which he’s provided both a schematic and Gerber files for production of the finished PCB, is based on the ATtiny85-20SU microcontroller, a Holtek HT7550-1 low-dropout (LDO) voltage regulator, and an Infineon IRF3205STRLPBF N-channel MOSFET, among other components. Screw terminals at the base provide connectivity for the solar panel, battery, and load, and while that prescribed the width of the finished circuit the overall design is relatively sparse.

A full bill of materials, plus the code, schematics, and Gerber files released under the GNU General Public Licence 3.0, can be found on Welch’s GitHub repository; Illet’s videos on the original PWM5 and the various rebuilds he has been undertaking of late, meanwhile, are available on his YouTube channel.

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