Nowadays, electronic devices (especially smartphones) are developed to use as an alternative tool for recording experimental data in physics experiments. This is because of the embedded sensors in a smartphone such as the accelerometer, gyroscope, magnetometer, camera, microphone, and speaker. These sensors were used in physics experiments, such as the simple pendulum, spring pendulum, coupled pendulum, circular motion, Doppler effect, and spectrum of light, as well as video motion analysis. This paper describes a method for measuring the position-time of a moving object using the ambient light sensor of a smartphone. In the experiment, light-emitting diodes (LEDs), called LED gates, are arranged with equal distance on an inclined plane. A smartphone is attached to a cart and acts as the moving object on the inclined plane. With the appropriate application, the ambient light sensor of the smartphone measures the intensity of the LEDs with respect to time while the smartphone moves along the inclined plane. The results obtained using the LED gates method are in good agreement with that obtained using the video analysis method.