How to Build a Simple Electronic Project: Electronic Thermometer

The saturation of the market with electronic components that in previous times weren’t accessible to the average man have made it possible for anybody to build simple electronic projects. In this write up, some of these components will be used in the building of an electronic thermometer. The word ‘simple’ is a relative term though, for someone who has experiences doing simple electronic hack skills like soldering and reading of circuits, this project will be a walkover for them. However, if you are not familiar with these skills, you do not need to fret as it will still be simple to you – if you follow each step given. Before we continue, let’s look at what an electronic thermometer is.

What is an electronic thermometer?

Firstly, a thermometer is a device used to measure the degree of hotness or coldness of a place – temperature. There are different types of thermometers, an electronic thermometer happens to be one of these numerous types of thermometers and it is a thermometer that is built with electronic components. The electronic thermometer has a wide range of uses but for this project, we will be designing a simple thermometer that will be able to detect temperature of a room and display it to the user. To achieve this, we will need electronic components and tools.

Components

The following component will be needed for this project, and as previously established, you will get almost all of them in a local electronic component shop. They are quite cheap so you might want to get more than you need for the project. For the resistors, I have purposely increased the number of units needed.

  • 10KΩ Potentiometer (2)
  • 100KΩ Potentiometer (2)
  • 1KΩ Resistor (10)
  • 330Ω Resistor (10)
  • 220Ω Resistor (10)
  • LED array (1)
  • LM3914 driver (1)
  • Green LED (10 pack) (1)
  • Thermistor (1)
  • 9V battery (1)
  • 9V battery plug (1)
  • PCB (1)
  • DIP Switch (1)

Tools you will need:

  • Soldering iron
  • Solder
  • Side cutters (or anything you can use to substitute)
  • Wire strippers (you can use side cutters or your teeth)
  • Files or Sandpaper(
  • Solder sucker (chances are you will make mistakes in soldering)
  • Hot glue (or any other adhesives)

Working Principle and Schematic Diagram

Circuit Diagram
Figure 1: Circuit Diagram

Working Principle of the Digital Thermometer

This particular thermometer works best when you want to measure mild range temperatures (from 0-50 degrees Celsius). You might have to wait a short time before you see results because the circuit operates from a variable resistor known as a thermistor. The thermistor resistance changes with increasing or decreasing temperature. In this circuit, a negative temperature coefficient (NTC) is applied to the thermistor; with the temperature rising, the thermistor resistance for each tempo unit decreases by a certain percentage. This is useful because we have a potentiometer in series with the thermistor in our circuit and as the thermistor’s resistance falls, the potentiometer’s voltage drop increases in relation to the thermistor’s voltage drop. This is due to the certain give and take in a series configuration between components.

The components together have a voltage drop equal to the voltage of the power source. However, only its share based on voltage resistance is reduced by each component. For example, a combined voltage drop of 9v from battery is present in our potentiometer and thermistor. Each component drops 4.5v when the potentiometer is at 5,000 Ohms and the thermistor at 5,000 Ohms. Now if the temperature rises, the resistance of the thermistor may fall to 2,500 Ohms while the potentiometer is still 5,000 Ohms. This means that the potentiometer drops 6V, an increase of 1.5V. The voltage drop across the potentiometer can then be measured to see temperature changes.

The LED array is integrated with the LM3914 into a voltmeter circuit. The two potentiometers can be used to calibrate the LED array’s readout and sensitivity and the green LED is simply used to indicate whether the circuit is on or off.

Building the Thermometer:

Schematic Diagram Showing Connections
Figure 2: Schematic Diagram Showing Connections

Important keys:

BAT1  –           9V Leads

D1       –           Green LED

R1       –           100kOhoms Potentiometer

R2       –           10kOhms Potentiometer

R3 – R12 –      220Ohms Resistors

R13     –           1kOhms Resistor

R14     –           330Ohms Resistor

R15     –           5kOhms Resistor

U1       –           LM3914

U2       –           LED Array

U3       –           DIP Switch

Ensure that you have the LM3914, LED, LED Array, and battery connected in their correct polarities = +ve to +ve and –ve to –ve. The +ve side (anode) of the LED has a long wire lead and that of the LED array has a notch coming out of it.

Tips:

Identify the different values of the resistors by the colored bands.

  • 330Ω resistor has bands: ORANGE, ORANGE, BROWN, GOLD
  • 220Ω resistors have bands: RED, RED, BROWN, SILVER
  • 1000Ω resistor has bands: BROWN, BLACK, RED, GOLD

Testing the circuit:

As established earlier, the temperature range of this electronic thermometer I between 0 – 50 degree Celsius. Testing is quite easy; you can simply place your hand or cold water bottle on the thermistor: depending on the temperature of the environment. You will also have to hold it there for at least 15secs to get accurate results.