Built a standalone Greenhouse Monitoring & Alert System with ESP32-C3, SIM800L, and 3x DS18B20s over 20m cabling
Nice use of the DS18B20, but 20 m on one bus is where they usually get flaky. I’d run the bus on CAT5 with data twisted against ground, wire the sensors in a daisy chain instead of a star, and power them with a real 3.3V line rather than parasitic mode. With that much capacitance the 4.7k pull-up is often too weak, so try 2.2k-3.3k at the ESP32 end and put roughly 100 ohm in series with the data pin to tame ringing. In firmware, check the CRC on every read and throw away 85.0 C and -127 C values, since those are the power-on and disconnect results and would trigger false alerts. A TVS diode like SMAJ5.0A on the data line is cheap insurance in a greenhouse. To test it, warm one sensor in your hand and confirm all three keep reporting steadily over a few hours.
Thanks for the detailed feedback and solid engineering advice! You’re totally right about long bus capacitance. I’ve already shifted from 3.3V to a solid 5V supply line for the sensors to handle the distance better. Dropping the pull-up to ~3.3k and filtering out the 85.0°C / -127°C CRC reset values in firmware are definitely great steps I’ll implement to make the bus bulletproof over time. Appreciate the tip on the TVS diode as well for static protection in the greenhouse!
How reliable is the SIM800L module? I’ve been thinking about using it in a project that’ll stay in the roof top to call when a trigger occurs
biggest risk is 2G, not the module. SIM800L is 2G only and many carriers are shutting 2G down, so check yours first. Then feed it a supply that handles 2A bursts or
Hardware watchdog to handle latchup? Additional DS18B20 to monitor temperature inside the enclosure? Humidity sensor inside enclosure?