Blood warmer is a device used to warm the blood before transfusion to the patient. As is known, blood is stored in the blood bank at a temperature of ± 2oC-6oC. In order to avoid blood clots that are too long, the patient needs blood warmer for the transfusion process. Heating is done on the hose aimed at heating the blood to be transfused according to the patient's body temperature. The purpose of this tool is that it can make a blood warmer with a security system when the blood hose runs out will turn on the buzzer and stop the heater from working. Here the author uses a comparison method by using a thermometer as a comparison tool. The working principle of the system as a whole: Temperature works as a blood warmer. After setting the temperature, the microcontroller works according to the user's instructions. Where the DS18B28 temperature sensor gives an input signal to the microcontroller and the microcontroller commands output namely heater works to produce heat according to the temperature setting, and uses the LDR sensor as the controller after the blood, when the blood runs out the LDR will give an input signal and the microcontroller issues the buzzer command and displays the command on the LCD. The test results of the tool are still tolerated with the smallest correction value 0.10oC and the largest 0.49oC, and the value is still within the tolerance limit of ± 10.