Raising a beetle is fun and wholesome but keeping an eye on the environment inside the little fella's enclosure can be stressful and complicated. For this reason, the BeetleMeter is a tiny battery-powered PCB that instantly notifies the beetle keeper if the temperature and humidity is too high, too low or just right, by the push of a button.

Schematic

The SHT40 sensor is used to measure temperature (0.2 °C accuracy) and humidity (1.8 %RH accuracy), and communicate those values through I2C. The result can then be displayed through six LEDs i.e. temperature too high, too low or good, and humidity too high, too low, or good. The system is controlled with the ATtiny402 and powered through two in series connected LR44 coin cells (150mAh). Due to the limited amount of GPIO pins of the microcontroller, the LEDs are switched through Charlieplexing. For super low-power, the system is completely disconnected from the battery and is only powered through the pushed button.

PCB Design

The board outline is defined by the dimensions of the battery holder, i.e. 15mm by 14mm, mounted on the bottom side of the PCB. The top side accommodates the remaining components in SMD format. The push button size was also chosen according to the board space left.

Assembly

The final device measures around 16mm length by 14.8mm height with 19mm width, weights 6.14g (with batteries), and costed around 14 RMB (1.97 USD).