Many plants require a certain minimum or maximum amount of sun during the day. For example, roses need a lot of sun, whereas moss grows mainly in shadows. Many garden centers provide recommendations concerning the amount of sun for their plants. To select a spot in your garden for potting a new plant that would match the requirements one needs to measure the amount of sun at this spot. There are just 4 grades of sun considered in gardening: full sun (at least 6 hours of sun during the day), full shade (less than 3 hours of sun), and two intermediate levels - half sun and half shade. Estimating the amount of sun for a given spot is not always a simple task, since the shadows of surrounding objects/plants move with the sun during the day. Moreover, many plants significantly change in size with years and, thus, throw longer shadow with years.
The device presented here is intended for calculating the amount of sun at a given spot. Just turn it on in the morning for 12 hours, stick it in soil, and read the sun grade on the LCD afterwards. The measurements should be done in a sunny day with minimum or no clouds. The LCD displays FSU for Full Sun, HSU for half sun, HSH for half shade, and FSH for full shade. It performs measurements only for a duration of 12 hours. The calculated amount is displayed on the LCD till the device is turned off.
| Schematic | Assembled unit | |
The measurements are performed every 12 seconds, that is, 5 times in a minute. If the sun was detected in 3 or more of them, the entire minute is counted as "sunny". Otherwise, it is counted as "shady". The number of sunny minutes is counted during 12 hours time interval after turning the device of. After expiration of that time interval the computed value is compared with the thresholds described above.
The illuminance is measured by MAX44009 sensor in the range 0.045 - 180000 lux. The sensor is equipped with ultraviolet of ultrared filters and its sensitivity characteristics correspond to the ones of human's eye. Reading data from sensor is performed via I2C protocol, the clocking frequency is about 80 KHz. The senor is a micro-power device and does not contribute noticeably to the average power consumption, which is about just 10 μA. This way the device can run non-stop for several years.
All processing is performed my Silicon Labs low-power microcontroller. It has a built-in DC-DC converter, so the device can be powered from a single AAA battery. The converter runs at 2.5 MHz frequency and left floating when the μC is put into a deep sleep mode. The wake-up is provided by the SmaRTClock RTC every 60.5 msec. Upon every wake-up the μC inverts its pins connected to the LCD, thus functioning as a software LCD driver. The active mode clocking frequency is 20 MHz, so all the computations are performed very fast. In order to maintain the stability of the wake-up intervals, the SmaRTClock oscillator frequency is measured and corrected every two minutes by using a built-in precision 24.5 MHz oscillator. This way the length of the measurement interval can differ from 12 hours just in a couple of minutes.
| Top view | Bottom view | |
The unit is assembled inside a garden light, its PCB is placed there on the top instead of a solar cell. A disk of plexiglass on the top protects the PCB. The measurements start after about 10 seconds after turning the device on by a slide switch in its bottom part. This delay should be sufficient for putting both parts of the garden light together and sticking it into the soil at the spot in question. The PCB is painted white with a Sharpie marker to minimize its heating by the direct sun.
Last modified:Mon, Jan 23, 2023.