What it shows
An NTC thermistor is a resistor whose resistance falls as it gets hotter. In a voltage divider, this change swings a transistor base voltage and switches a buzzer on at a chosen temperature.
The science
The thermistor sits from the 9 V rail to the base node, with the trimmer from the node to ground. At room temperature the thermistor is near 10 kΩ and the base voltage is too low for the transistor. As it warms, its resistance falls to a few kilohms, so the base voltage rises above 0.7 V and the transistor switches the buzzer on. The trimmer sets the exact switching temperature.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A 10 kΩ NTC thermistor
- A 1 kΩ trimmer potentiometer
- One BC547 transistor and one 1 kΩ resistor
- An active buzzer, 3 to 12 V
- A 9 V battery with snap connector
- A breadboard, wires, a small plastic bag and a cup
- A kitchen thermometer if you have one
Safety
For this project: An adult must pour and handle the hot water. Never go above about 50 degrees for this test. Keep the breadboard away from water and keep the thermistor leads inside a sealed plastic bag.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Place the BC547 on the breadboard (flat face towards you, legs left to right are collector, base, emitter) and join its emitter to the negative rail.
Connect the buzzer between the 9 V rail and the collector, red lead to 9 V.
Connect the NTC thermistor from the 9 V rail to a spare row.
Connect the trimmer pot with its two outer pins between that spare row and the negative rail.
Connect the trimmer wiper to the base of the transistor through the 1 kΩ resistor.
Put the thermistor in a plastic bag with only its insulated wires coming out of the mouth, tape the mouth shut, snap on the 9 V battery and dip the bag into a cup of room temperature water.
Turn the trimmer until the buzzer is just silent. Then ask an adult to add warm water of about 45 degrees.
Adjust the trimmer so the buzzer just starts at 45 degrees on the thermometer; test again with cool and warm water.
What you should see
The buzzer stays silent in cool water and sounds when the water reaches the set temperature, showing a temperature-triggered switch.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What does NTC stand for?
Negative temperature coefficient: resistance falls as temperature rises.
02What is the trimmer for?
It adjusts the base voltage so you can choose the trip temperature.
03Why is the thermistor sealed in a bag?
Water would short the leads and corrode them.
04Why not use a metal wire as the sensor?
Its resistance changes too little with temperature to switch a transistor.
Take it further
Swap the buzzer for an LED with a 470 Ω resistor, then record the trip temperature for three different trimmer settings in a table.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each
Find out more about electronics and robotics
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