One of the first projects I did a couple years ago, was designing a custom freezer monitoring system. It's end purpose was monitoring -80°C freezer, but they where actually tested down to -196°C in liquid nitrogen.

The system communicates wirelessly with two gateways, by using a Zigbee mesh network between multiple nodes. Both gateways are operating 24/7, but the system does run perfectly fine on one gateway. The second gateway is purely redundant to reduce potential downtime.

A custom PCB was developed using KiCad. The system is powered through a micro-USB connection, as these are easier to solder than USB-C connectors. I used an XBEE3 zigbee MMT module for communication, combined with two small CR2032 batteries as temporary backup in case of a power failure.

A k-thermocouple is used for temperature sensing, which is connected to an MAX31855 thermocouple IC with integrated cold-junction compensation. Everything is read out by a custom programmed microcontroller, which sends all collected data to the XBEE3 over uart.