Lighting System
Light controlled light
Lighting is a crucial factor in seedling growth stage as it provides the energy needed for photosynthesis, which is essential for plant growth. Sunlight is the primary source of light for outdoor plants, but indoor plants can also grow successfully with artificial lighting systems.
System Overview
Photoresister
Plants also need to sleep as human beings do, therefore we may not want to perform many actions such as watering during this time as it can disrupt the plant's natural processes. A simple solution is to use photoresistors, which are also known as light-dependent resistors (LDR). It is made of photoconductive semiconductor material. When light is absorbed by the material, the number of free electrons and holes increases, resulting in increased electrical conductivity.
From electrical perspective, LDRs are simply a variable resistor of resistance inversely proportional to the intensity of the light.
Similar to the soil moisture detection circuit, we use a voltage divider to convert LDR resistance into voltage signal. When the fixed value resistor R has one end connected to voltage source, it is called a Pull-up resistor. If R is placed on the other side, it becomes a Pull-down resistor.
Either structure above is a valid circuit. To maximize sensitivity, the resistor value of R should be chosen to match the LDR resistance when it is at ordinary state. Here we use a 10kΞ© resistor for the fixed value resistor. Again, we use VCC = 5V just to be consistent with the power supply voltage.
Dealing with Sensor Output
If the sensor output is directly accessed from a voltage divider, then it cannot be directly connected to a load due to the high output impedance. To explain the reason mathematically, the load resistor R(L) is connected in parallel to the LDR, thus forms a new equivalent resistance R(EQ):
Since we want minimal interference after a load resistance is connected, the output must be connected to a high impedance circuity with much larger resistance than the effective resistance of the LDR. A rule of thumb is, voltage divider sensors should be connected to high impedance circuitry that draws no current from the output.
Using a voltage buffer, also known as a unity-gain amplifier or a voltage follower. This is another circuitry that is able to retain the integrity of sensor voltage while amplifying the loading current capacity at the output.
A simple voltage buffer built from an Operational Amplifier (Op-amp). An Op-amp is an electronic device that can amplify small input signals to a larger output voltage. It has two input terminals, an inverting (-) and a non-inverting (+), and an output terminal that can source or sink current.
At first glance it may seems the circuit being useless since the output simply equals to the input. However, the benefit of this circuit is that the output side of the buffer has low impedance. The output of a voltage buffer has higher capacity to source or sink current, allowing a load to be directly connected to V(out).
A buffer can be built practically with any Op-amps, so we will use a general purpose LM358 Op-amp for this project. The pin definition of a LM358 chip is given in the figure. This is a Dual-in-line 8 (DIP-8) footprint integrated circuit that which contains two sets of Op-amps both sharing the same VCC and VEE. Since LM358 supports single rail supply, which means it is functional even if the Op-amp is powered by only the positive rail. Therefore we can simply connect VEE to ground.
The typical output current of LM358 is rated as 30mA, which is sufficient to drive moderate loads such as LEDs or small DC motors. Op-amps designed with much larger current capacity are also available.
Light Emitting Diode
Ultraviolet radiation is also critical to the growth of plants. Since indoor lighting does not have the UV portion, we can place some special LEDs inside of the greenhouse to supplement the lack of UV radiation.
LED stands for Light Emitting Diode, which is a specially fabricated diode that can emit light (photons) when an electric current passes through it.
Designing a circuit to drive an LED is a simple process.A simple series circuit consisting of an LED and a resistor. This resistor is referred as current limiting resistor R1, which can be calculated using basic Ohmβs Law.
Where, V(FWD) and I(FWD) are the forward voltage and current of the LED
The LED for this project is a violet LED rated as 3.3V at 20mA. If we use a 5V voltage source for VCC, by calculation R1 should be 85Ξ© so it can limit the LED current to 20mA. In practice, unless the application requires a very specific control of the luminosity, you do not need an exact value of the calculated resistance. We will choose a 50Ξ© resistor for our application.
Keep in mind that our supplemented UV LED should not interference the normal respiration process of the plants, therefore the LED should be automatically turned off at night. Therefore we can combine the light sensor and the UV LED drive.
Complete Circuit
A complete breadboard setup for this lighting system should be like the following image.
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