Devlog #3

PCB Design Mistakes

After soldering the components on last time, I found several issues with the design. First, it appears that I wired the 555 timer wrong - I ordered an NE555 timer which takes 5V logic instead of one that was compatible with 3.3V logic. Even after soldering a wire to fix that, something else with the design was wrong, which led to the component constantly outputting a HIGH signal.

Instead of spending time trying to fix it, I decided to just scratch out all the traces connecting it and removing the component altogether. Rather than retrieving a frequency from the touch capacitance sensor via hardware, I opted for a pure software route instead. I soldered a wire directly from the capacitance sensor’s copper pad to pin 1, and then using software, I would repeatedly drive the pin HIGH, and then measure the amount of time it takes for the pin to reach LOW (measuring the amount of time it takes for the copper pad to discharge).

This allows me to approximate the capacitance of the copper pad via the formula t = R * C. The capacitance will increase when my finger approaches the pad, and so by counting the cycles it takes for the capacitor to discharge, our program can tell when the capacitive sensor was touched.

The soil moisture capacitive sensor is basically the same concept in action. When the sensor is placed in dry soil, the capacitance will be low (as soil is dielectric), and timing for discharge will be low as well. When the soil is watered, capacitance will increase (as water is a strong dielectric), which will thus increase the time it takes for the soil capacitive probes to discharge.

The only problem was that it turned out I had wired the RC circuit for the soil capacitive probes incorrectly as well, which I thus spent time fixing.

Shipping

After testing the code and ensuring it worked, I went back and redesigned the PCBs to fix the mistakes I found in these physical prototypes. I’ll be shipping this project soon!