The Study of Velocity Measurement Using Single Light Dependent Resistor (LDR) Sensor

Abstract

Velocity is a physical quantity that is widely used in daily routine. The adverse utilization of the quantity makes the methods and measurement devices are necessary. The most common method to obtain velocity is by measuring the interval time of an object that moves at a certain distance. Usually, two light sensors (e.g., photodetectors, light dependent resistors or LDRs) are placed at a certain distance to detect the moving object and measure their interval time. This paper discusses the uses of a single LDR sensor to measure the velocity. LDR is a light sensor, which its resistance rises if the intensity of the light decreases. To measure the interval time, the light-sensitive part of the LDR sensor was partially covered (cover area $2.5 \times 1.0\ \mathbf{cm}^{2}$), which make the LDR has two sensitive areas. This allowed a single LDR sensor to measure the velocity of an object. The sensor responses when the object passing through it were analyzed from voltage to time graph. The experiment was done by passing by objects of different sizes ($3.5 \times 5.1\ \mathbf{cm}^{2}$ and $3.5 \times 0.35\ \mathbf{cm}^{2}$). These size differences produce different voltage outputs so that the effect of the object size to the sensor responses can be analyzed. Also, by placing a cover that unequally divides the LDR light-sensitive part, the object motion direction could be predicted from the voltage to time graph pattern. The resulting output signal depended on the size of the object passing through the sensor, the size of the LDR, and the size of the LDR cover. The velocity calibration result exhibited a regression equation to calculate the velocity from sensor output, which was $\mathbf{y}=3.99084\ \mathbf{x}+0.15663$, with x and y were the reference and measured velocity, respectively. Moreover, the sensor has a high precision on low-velocity measurement, which might be caused by the latency property of the sensor.