7.4 Experiment and principle
Only when we understand the characteristics, we may feel that the principle of this experiment is very simple, which is simply interpreted as the distribution voltage between the photoresistor and the serial resistor. If the distribution of voltage for the photoresistor is less than a threshold voltage, it can trigger a high voltage level for the port 8 on the Arduino board in such experiment, which can light LED; or port 8 would be at a low voltage level, and thus the LED doesn’t be lightened. But, why should the voltage of photoresistor lower a threshold voltage, the LED would light. Let us analyze its principle. Assume that a photoresistor control lamp would be installed in our home. When the outside light is strong, we hope the lamp doesn’t be lightened for the energy saving. At the same time, the value of photoresistor is changed to be small. As shown in Figure 7-3, the photoresistor and a usual resistor are connected serially each other and powered by the energy source 5V provided by Arduino board. By the circuit principle, if the value of photoresistor is more smaller, the value of distribution voltage is more smaller. Therefore, we can draw a conclusion. If the outside light is more stronger, the value of photoresistor is more smaller, and it thus leads to a fact that the value of the distribution voltage is small. So, if the distribution voltage for the photoresistor is less than a threshold voltage, it cannot trigger the Port 8 a high level, which results in the LED dark.
However, we have another problem: how can we get the threshold voltage for the photoresistor? This is a key problem. As shown in Figure 7-3, R1 is the serial resistor, and R2 is the photoresistor (temporarily replaced of the photoresistor in this figure), where the value 20Ω is named as light resistor, which can measured by utilizing multimeter shown in Figure 7-4. The real measured value is 17.49Ω by multimeter, but for computation conveniently, it is 20Ω. Additionally, this value and dark resistor value are given out generally in the description of product.
Figure 7-3 Distribution voltage principle for the photoresistor
Figure 7-4 The measured light resistor value by using multimeter for the photoresistor