TwiddleLab / Physics / Circuits You Can Build
Sensors: Resistance That Changes
What you will learn
- An LDR has less resistance in brighter light; a thermistor (NTC) has less resistance when hotter.
- A voltage divider (two resistances in series) shares the battery voltage in proportion to the resistances.
- Put a sensor in a divider and its changing resistance becomes a changing voltage that can switch things on.
How the lesson goes
Street lights switch on by themselves at dusk. Who flips the switch? First: in the dark, what does this voltmeter read?
A 10 kΩ resistor on top, a light sensor below. In the dark the sensor is about 100 kΩ.
Dim the light until the sensor's voltage is exactly 3 V.
Watch the LDR's resistance and the voltmeter.
Swap the two parts so the voltage goes UP when it gets brighter.
Tap a part to take it out, then put it in the other holder. Test it with the light dial.
Sensors and the voltage divider.
A sensor turns something in the world into an electrical change. An LDR (light-dependent resistor) has less resistance in brighter light.
A thermistor (NTC type) has less resistance when it is hotter.
A voltage divider is two resistances in series: each gets a share of the battery voltage in proportion to its resistance. Put a sensor in one and the share changes with the world.
Build a fire alarm: the buzzer must sound when it gets hot.
Drag two parts into the holders. The alarm sounds above 4 V. Test with the temperature dial.
Game: Greenhouse. Set the fan's switch so the plants survive a hot day.
A thermistor divider switches the fan on above your threshold. The fan cools by 8 C.
AI: sensors make data. Log a day of readings.
The street light module switches on above 3 V. Which circuit turns it on at dusk?
In a divider the bigger resistance takes the bigger share. Darkness makes the LDR bigger, so the voltage across the LDR rises: read it, and dark means high.
Your greenhouse from the game. Tap it.
Previous lesson: Short Circuits, Fuses and Safety · Next lesson: Project: Smart Night Light · Part of the course: Circuits You Can Build
Lessons are interactive and need JavaScript. Turn it on to play them.
