Stage two of the build was to be able to control the day and night cycle (16-8 hours) to allow proper plant growth. One method to implement this would be to break the circuit from the ‘Vdd’ line (power line). We decided to pursue this method as it would reduce the power consumption when the night cycle was in effect.

In order to break the power line there needs to be two pin outs, pin 11 for the data line and 12 for the switching circuit. 11 determines the light intensity and function of the lights. After a 16 hour cycle pin 12 is written to ‘LOW’ (logic zero), which turns a N channel power MOSFET off for 6 hours. This sequence would repeat every day.

We encountered some issues when implementing this design, which appeared to be caused by the Arduino’s pin out voltage of 3.3V. Despite the MOSFET’s threshold voltage being between 1-2V [2], it should be adequate, however, the voltage at the source (MOSFET output) was measured to be 3V, and the LED’s need at least 4.2V.

This meant that we had to go back to the drawing board. Our second iteration was based on a NAND gate; the truth table shown below illustrates the function of the gate .

Highlighted in red are the two functions we are interested in. Since ‘Vdd’ will always be read as ‘high’, 5V, we can eliminate the first two states and ‘Vdd’ making the logic straightforward. The basic operation is inverting P12 logic. When implemented in our device, the logic worked as predicted; however, the 2A current demand of the LEDs caused the gate to heat up and become unstable in its operation. Iteration three involved the implementation of a relay switch; however, this was also limited in terms of the current draw and heating of the system. In our final iteration we discovered we could achieve the desired result through the code. Changing the brightness from a defined integer, to an 8 bit integer, allowing a range between 0-255. Setting the brightness to ‘0’ would turn the LEDs off. The code can be seenhere.