DIY Project: - Digital Thermometer

Clinical thermometer is equipment which can be seen in every household. Here in this DIY project we are going to develop a digital thermometer using very few components and that too in an ease manner. Here the main attraction is the DIY project will be developed without the help of a microcontroller.

In this DIY project we are using LM35 a temperature sensor to measure the temperature. A potentiometer is also used for the calibration purpose. A 4 bit ADC will be converting the value to BCD. A seven segment interfacing IC will be there which will be interfacing with the display units. For switching transistors are used.

Block Diagram of Digital Thermometer

Circuit Diagram of Digital Thermometer

Components Required  for Digital Thermometer

1. CA3162-1
2. LM35-1
3. 50k pot-1
4. 1k pot-1
5. 74LS47-1
6. 1k resistor-3
7. Seven segment display-3
8. 2n3906 transistor-3
9. 560 resistor-3

Working of Digital Thermometer

LM35, temperature sensor is a device which provides voltage in direct proportion to the temperature. The temperature sensor has 2 transistors inbuilt in it. One transistor has emitter area 10 times the other thus reducing the current through it by 1/10^th. Since the current across the 2 transistors are same corresponding to the absolute temperature there will be a voltage across its resistor. It has inbuilt circuitry to correct lit bit variations in the value.  The absolute temperature measured in Kelvin will be converted to Celsius by an amplifier in it.

When you place your finger over the LM35 temperature sensor depending on the temperature increase at a known rate the voltage across the diode will increase. Actually this voltage drop is the voltage developed across the base and emitter of the transistor. 
Potentiometer is a variable resistor generally referred as pot. Here it is used for calibration purpose. The room temperature will be calibrated so as to while taking the reading. Pot will be set accordingly.

ADC (Analog to digital convertor) here used is 4 bit ADC, CA3162. The value from the temperature sensor will be converted to corresponding BCD code. A potentiometer is connected to the 8^th and 9^th pin of this IC. The purpose of this is to regulate the voltage across it. The value will be displayed in Celsius in the seven segment display using an interfacing IC. The interfacing used here is 74LS47 which will be converting the generated BCD code to a pattern to be displayed by the seven-segment displays. To the three seven segment display unit three transistors are connected. These transistors, 2n3906 are for switching purpose. The transistor will be selecting the MSB, NSB and LSB.

In short when you place your finger on the sensor, a corresponding voltage will be generated which in turn gets converted to a BCD code. An interfacing IC will convert this code to be displayed in the seven segment.