Coordinating a simple coffee break in a sprawling office shouldn’t be complicated. Direct messages get ignored by developers “in the zone,” and walking down a long hallway to ask around wastes everyone’s time. To solve this, we developed the CoffeeCaller — an independent, mesh-networked IoT gadget that broadcasts a synchronized, physical “coffee call” across an office. After moving away from a fragile ESP32 prototype, we designed the final hardware as a custom PCB built around the Nordic nRF52840 SoC. It carries an onboard SHT40 temperature and humidity sensor, WS2812 RGB LEDs, tactile buttons, and a buzzer. The software is open source, built on the Zephyr RTOS and OpenThread, with hardware abstractions that let the firmware run and be tested in Zephyr’s native simulation.  

The Coffee Break Problem

Every engineering project starts with a problem, or better yet, a personal annoyance. Imagine a very long office hallway where colleagues are spread far apart. In our case, Stefan ha...