To prevent people to forget to take medicines, we propose a new monitoring system to prevent forgetting to take medicines on time. In this system, we develop a funnel-shaped sensor consisting of a holder with an LED below the lid of the holder and a photo detector at the bottom to detect medicines. When medicines are dropped into the sensor, they fall down to the bottom of the sensor and the photo detector receives only part light of the LED. According to the value of the output of the photo detector, it can be easily to judge medicines with a proper threshold value. Highly accurate monitoring system to prevent forgetting to take medicines can be produced by using this sensor. If medicines are not taken at time to take medicines, the system announces to the user.