We recommend using a perf-board for mounting and wiring the microcontroller, sensors and power components, as it provides a good balance of robustness and flexibility.
Just place the components and use hookup wire to make the required connections to match the schematic. Ours definitely isn’t pretty, but it gets the job done. If we were planning on making more than just a couple, a PCB would definitely be the next step.
The led button is also connected via a standard header. It can reset the microcontroller without opening the enclosure and the LED provides some debug information.
The ultrasonic control board is directly attached to the perf-board. The cable for the JSN-SR04 will likely have to be shortened.
After verifying that the components work as expected, mount the board to the enclosure.
The standard configuration uses 15 AA batteries wired into a single 4.5V pack. This means 5 parallel sets of 3 AA batteries in series. After adding the batteries, use some electrical tape to hold it all together.
If you plan on adding additional components, make low power usage a priority. This means that all these components should have their power switched to them, only turning briefly on when the device is taking a reading. If the current draw is low enough, using a GPIO pin (or multiple in parallel) is an easy option. Otherwise, you may need to add a transistor or MOSFET to control the components.
The auxiliary sensor unit connects to the main board through an 8-pin header, one for each conductor on the RJ-45 connector. The default configuration only uses 3 of these pins, (3.3V, conductivity and temperature). The remaining pins on this connector are open for any additions.