A digital hydrometer. It floats on top of a solution and measures its density. It's used to track the fermentation process while brewing beer and wine. I wrote a short Twitter thread about how it works.
An open hardware and open source soil moisture and ambient temperature/humidity sensor. It uses Bluetooth Low Energy (BLE) to broadcast its sensors' data. On rbaron/b-parasite.
A quick way to add HTML forms to static websites. Just copy the generated <form> element to your website and receive email and Slack notifications on new submissions. A REST API is available for geeks. Access it on formbot.app.
An open hardware LIR2450 coin cell battery charger. It lives on rbaron/lir2450-charger.
An Android/iOS app that uses the device's camera for automatically grading multiple choice tests. Live on smartgrade.net.
A Nintendo NES emulator written in Go. Check it out on rbaron/awesomeones.
A minimalistic Optical Mark Recognition Python/OpenCV script. It lives on rbaron/omr.
An toy implementation of Shamir's secret sharing scheme in zero-dependencies C. Using this, a secret (for example, a password), can be split between n people, so that k people are needed to reconstruct it. It's on rbaron/secret..