The research aims to develop the experimental set of the temperature measurement in liquid by Arduino program displaying data on a smartphone via the Blynk application. The experimental set is composed of 1) 2 liquid temperature sensors (DS18B20 model), 2) Arduino program, and 3) LED screen for showing the temperature value in unit of °C and connect to a smartphone. The Arduino temperature sensor 1 and sensor 2 of the experimental set have 0.57% and 0.51% errors, respectively, compared with the temperature sensor of the B Smart Science Co., Ltd. company. The instrument is applied to the physics laboratory on Newton’s law of cooling to find the cooling rate of water and coffee. This low-cost instrument revealed high accuracy results and easy to connect with other devices.