THERMOMETER WITH LM35 SENSOR SCHEMATIC CIRCUIT DIAGRAM
The LM35 is an integrated circuit sensor designed for precise temperature measurement, providing an electrical output directly proportional to the temperature in degrees Celsius. It offers a higher level of temperature accuracy compared to using a thermistor. The LM35’s output voltage is linearly proportional to the temperature in Celsius.
In this particular project, we have constructed a temperature-controlled thermometer using an LM35 sensor integrated with a PIC16F877 microcontroller. The application harnesses the A/D (analog-to-digital) conversion capability of the PIC16F877 for the thermometer functionality. The output unit in the circuit is an LCD screen. The primary objective of this application is to control a fan or heater based on the temperature, specifically within a defined temperature range, such as 20°C to 25°C.
Circuit and Operation: The LM35 temperature sensor serves as the analog input for the A/D converter. The processed signals, representing the temperature values, are displayed on the LCD as temperature readings.
These temperature values are compared to a predefined temperature range (typically room temperature). If the temperature falls below the specified threshold, a 5V signal is observed at the C port of the microcontroller (RC6). This signal voltage activates a transistor connected to the RC6 pin, subsequently closing the relay’s contacts. As a result, the connected controller is triggered, and the LCD displays the message “cold” along with the temperature reading.
Conversely, when the temperature exceeds the predefined range, pin 7 of the C port (RC7) produces a 5-volt output signal. This output is then used to control another relay through a transistor, activating a fan. Simultaneously, the LCD displays “hot.” When the temperature falls within the specified range, the circuit functions solely as a thermometer, without any connected devices being activated. The LCD displays “normal” alongside the temperature reading.
SCHEMATIC CIRCUIT DIAGRAM:
Print circuit board and top view
When the circuit is viewed, it is seen that very few elements are required. This requires a simple design that is in print mode.
