What it shows
With the thermistor placed on the ground side of a voltage divider, cooling raises its resistance and the base voltage, lighting an LED when the air drops below a set temperature. It shows how the same sensor can trigger on cold instead of heat just by swapping its position.
The science
The potentiometer runs from 9 V to the base node and the thermistor from the node to ground. Warm, the thermistor is low in resistance and holds the node below 0.7 V. As the air cools the thermistor resistance climbs towards 20 or 30 kΩ, so a larger share of the supply appears across it, the base rises past 0.7 V and the transistor lights the LED. Because the thermistor is 10 kΩ or more, the pot needs a large range, so a 470 kΩ pot is used, set to a few hundred kilohms. Choosing the pot setting fixes the temperature at which the warning comes on; a winter night in north India can be cold enough to harm young plants.
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 470 kΩ potentiometer and a 1 kΩ resistor
- One BC547 transistor
- One blue or white LED and a 470 Ω resistor
- A 9 V battery with a snap connector
- A breadboard, jumper wires, a bag of ice and a small plastic bag
Safety
For this project: Battery power only. Keep the thermistor in a dry plastic bag when testing with ice so water never reaches the circuit.
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 LED long leg through the 470 Ω resistor to the 9 V rail and the LED short leg to the collector.
Fit 30 cm leads to the thermistor and connect it from a spare row to ground.
Connect the potentiometer middle pin to the spare row and one end pin to the 9 V rail.
Connect the 1 kΩ resistor from the spare row to the base.
Snap on the battery and set the pot so the LED just goes off at room temperature.
Put the thermistor in a plastic bag and hold the bag against ice: the LED lights within a minute.
Remove it from the ice and watch the LED switch off as the bead warms back up.
What you should see
The LED stays off at room temperature and lights when the thermistor is chilled, going off again as it warms.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does cooling light the LED in this circuit?
The thermistor resistance rises, so more voltage appears across it and the base goes above 0.7 V.
02What single change makes the circuit trigger on heat instead?
Swap the thermistor and the potentiometer positions.
03Why keep the thermistor in a bag during the ice test?
Melting ice water would short the leads and give a false reading.
04Why would a farmer want this?
To know when frost is likely so seedlings can be covered.
Take it further
Add an active buzzer rated 3 to 12 V in parallel with the LED branch so the warning can also be heard from another room.
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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