LM75 based digital thermometer with LCD Display
The LM75A is a digital thermometer that is widely used in various applications such as temperature monitoring, industrial processes, weather stations, and personal computers. It is a small, low-cost sensor that provides accurate temperature readings with a resolution of 0.125°C. The LM75A provides 9-bit digital temperature readings with an accuracy of ±2°C from –25°C to 100°C. The LM75A operates with a single supply from +2.7 V to +5.5 V. Communication is accomplished over a 2-wire interface which operates up to 400kHz. The LM75A has a dedicated over-temperature output (O.S.) with
programmable limit and hysteresis. This output has programmable fault tolerance, which allows the user to define the number of consecutive error conditions that must occur before O.S. is activated. I have not implemented the O.S in this circuit.
The LM75A thermometer is based on the Pic16F84 microcontroller and interfaced with a 16×2 LCD display to provide real-time temperature readings.
The LM75A uses an I2C interface for communication with two I2C pins SDA and SCL, making it very easy to interface with Pic16F84 microcontroller. Its compact size and low power consumption make it ideal for use in battery-powered devices. The LM75A also features an alarm function that can be programmed to trigger when the temperature exceeds a certain threshold, although I have not used this feature to keep the programming simple.
The LM75A temperature sensor incorporates a temperature sensor and 9-bit Analog to DC converter. The temperature data output of the LM75A is available at all times via the I2C bus. A digital comparator is also incorporated that compares a series of temperature readings, the number of which is user-selectable, to user-programmable setpoint and hysteresis
values. The comparator trips the O.S. output line, which is programmable. The LM75A has an integrated low-pass filter on both the SDA and the SCL line. These filters increase communications reliability in noisy environments. The three address lines A0, A1 and A2 are tied to ground to give an address of 0x00 hex.
A block diagram of the LM75A is shown below.
The use of a Pic16F84 microcontroller is provides the ability to customize the LM75’s functionality, in this case to read the temperature registers. A 16×2 LCD display provides a user-friendly interface for displaying temperature readings, making it accessible to anyone.
Overall, the LM75 digital thermometer is a reliable and versatile temperature sensor that is widely used in a variety of applications due to its accuracy, low power consumption, and customizable features such as alarm settings.
The circuit diagram shown above includes 9V battery which has a slider On/Off switch. The 9V battery voltage is converted to 5V by LM7805 voltage regulator with its filter capacitors C3 and C4. The Pic16F84 is clocked with the 4.096Mhz oscillator, which feeds into the OSC1 (pin16) of the Pic16F84. The Pic16F84 directly connects to the LCD in 8-bit mode to display data on the screen. Finally, the LM75A is connected to the Pic16F84 using the SDA and SCL lines.
