To gather more long-term measurements for the capacitive method I use for my plant watering sensor, I created this small logging device. As you can see, it uses one of the plant watering sensor prototypes for the measurements. Instead of using the ATtiny13A on the board, it passes the oscillator signal directly to the microprocessor of the logging device.

In front there is a very small 128×32 OLED display, where I can see the current measured frequency in kHz. On top, the current time and date is visible, and on the right there is a graph where I can see the values from the last 48 hours graphically. There is not much visible in the graph, because I took the photo just after installing the sensor.

Every minute, the current average of measurements is stored in a CSV file on a SD card. After a few weeks I should be able to analyse this file and see the results. Here I am especially interested in the cycles from watering the plant until the soil got dry again.

Aside from the plant sensor prototype, I used these components to build this device:

• Adafruit Feather M0 Proto
  A microcontroller board with the Atmel SAM D21 (AMD M0) 48MHz chip and logic to use and charge a lithium ion battery from the USB port on the board.
• Adalogger FeatherWing
  A great shield which contains the PCF8523 real time clock and a SD card slot. There a battery holder for a CR1220 battery to keep the RTC running.
• Adafruit FeatherWing OLED
  A very nice tiny shield to provide a OLED display to a project. It contains the SSD1306 driver with the UG-2832HSWEG02 display. It has a very high contrast, so even tiny characters are perfectly readable.
• Lithium Ion Polymer Battery – 3.7v 2500mAh

This is a great example, how very useful and complex devices can be made in a very short time-period using the modules available today. I really love the Adafruit Feather boards, because they not only contain the microcontroller, but also the required components to use and charge a lithium ion battery. This boards are also very small and can be integrated into almost anything.

I used the Feather M0 to have a powerful 48MHz microcontroller with vast amounts of flash memory and RAM. There are enough resources to easily analyse the signal and do floating point calculations without mind performance. The whole device, including the display using just around 1mA while running. It should run for weeks with the battery, and I can always recharge the battery using a USB battery pack without the need to remove the device or turn it off.

If you have questions, miss some information or just have any feedback, feel free to add a comment below.