Requirements for making a sensor compatible

We don’t have any set requirements for connection.  The Mayfly can communicate with almost anything that could communicate with an Arduino-style device.  Many users opt to user our ModularSensors library to run their loggers, but that library supports many protocols.

SDI-12 devices are very common in the environmental sensing field, so we have a lot of experience in communicating with SDI-12 slaves. The Arduino SDI-12 library is not a full implementation of the protocol and it’s much better tested for use as an SDI-12 master than as a slave.  You would need to do some footwork and testing to fully implement your sensor as a compliant SDI-12 slave. The protocol documents have fairly good flow diagrams explaining the timing, retries, and low power modes that the slave must implement.

Other protocols we have experience with are Modbus over RS232 or RS485, OneWire, and I2C (“Wire” or “Two Wire”).  If you look through the various sensor implementations in ModularSensors, you’ll see there are a lot of sensors where we use (or write and then use) a library that handles the lowest level back-and-forth needed to manage a sensor and then call that library’s “give me the current value” function within ModularSensors.

I spent some time skimming your GitHub code for your device.  If your logger is using a simple serial protocol to communicate with your GUI controller, it should also be able to communicate with the Mayfly (with or without Modular Sensors) in the same way if you expose the Serial (or AltSoftSerial) pins.

ModularSensors already supports the MS5803 and the Mayfly has a QWIIC connection.  The Mayfly has a built-in ADS1115 for analog sensors. The VCNL4010 is not implemented in ModularSensors, but it looks like an I2C sensor with an Adafruit library, so it would be easy to implement and submit a PR for using the example of the MS5803.  Keep in mind if you’re developing anything with I2C communication that the I2C protocol is designed for short distance communication so you need an alternative if you want more than a few feet of cable.

Using ModularSensors doesn’t necessarily require the sensors to be directly connected to the Mayfly.  They could be, but they don’t have to be. In the case of a lot of inexpensive sensors sold as breakout boards, the Mayfly connects and talks directly to the sensor, but most of the not-bare-circuit sensors have black-box layers between the raw sensors and the Mayfly that do some internal processing before passing the number along.  If you wanted to make your sensors SDI-12 slave, you’d need to create those layers, whether on your 328p or some other in-between board.  Most commercial environmental sensors can do some on-board averaging, bad-value management, temperature correction, power management, and communication translation. Then “sensor” as the Mayfly sees it is the combination of the bare circuit sensor and the processing layers.  The Mayfly doesn’t need to know anything about what the lowest level sensing, just how get a “final” measured value.