What it shows
An NTC thermistor loses resistance as it warms. In a voltage divider with a potentiometer, warming raises the transistor base voltage until it switches a buzzer on, giving an adjustable temperature alarm like the one in a geyser or a kettle.
The science
The thermistor is connected from the 9 V rail to the base node, and the potentiometer from that node to ground, forming a divider. At room temperature the thermistor is about 10 kΩ, and the 1 kΩ pot is set to a little under 850 Ω so the node sits just below 0.7 V and the transistor stays off. Warming the thermistor cuts its resistance to a few kilohms, a larger share of the 9 V appears across the pot, the base rises past 0.7 V and the buzzer in the collector sounds. Turning the pot changes the temperature at which this happens.
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 potentiometer or preset and a 1 kΩ resistor
- One BC547 transistor
- An active buzzer rated 3 to 12 V
- A 9 V battery with a snap connector
- A breadboard, jumper wires and a cup of warm (not boiling) water
- A thermometer if available
Safety
For this project: An adult must pour the warm water; use water below 60 °C. Keep the breadboard and battery dry; only the thermistor on long leads goes near the cup.
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; connect the emitter to ground and the battery negative to the ground rail.
Connect the buzzer red lead to the 9 V rail and the black lead to the collector.
Twist (or have an adult solder) two 30 cm leads onto the thermistor and insulate the joints with tape.
Connect the thermistor from the 9 V rail to a spare row, and the 1 kΩ resistor from that row to the base.
Connect the potentiometer middle pin to the spare row and one end pin to ground.
Snap on the battery and turn the pot slowly until the buzzer just stops at room temperature.
Hold the thermistor bead between warm fingers: within a few seconds the buzzer sounds.
Seal the bead and its leads in a small plastic bag, dip it in the warm water and note the temperature at which the buzzer sounds; turn the pot to set a different trigger temperature.
What you should see
The buzzer stays silent at room temperature and sounds when the thermistor warms past the level set by the knob.
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 mean?
Negative temperature coefficient: resistance falls as temperature rises.
02Why does warming the thermistor raise the base voltage?
The thermistor takes a smaller share of the 9 V, so more appears across the potentiometer.
03How would you make the alarm sound at a lower temperature?
Turn the pot up to a higher resistance, so the base voltage starts closer to 0.7 V and a smaller rise in temperature is enough.
04Where is a thermistor used at home?
In geysers, rice cookers and the battery of a phone.
Take it further
Swap the thermistor and the pot, using a 470 kΩ pot as in the cold night project, so the circuit sounds when something gets cold, for a fridge door alarm.
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.
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