Thermistors and LDRs

Your fridge knows when it's warmed up and kicks the cooling back on. A street lamp switches itself on at dusk without anyone flicking a switch. A fire alarm senses the heat of a blaze before you've even smelt the smoke. None of these gadgets has eyes or a thermometer with a little needle — instead each one hides a component whose resistance changes with its surroundings, and a circuit that reads that change.

Two components do almost all of this work. A thermistor senses temperature: its resistance changes as it gets hotter or colder. A light-dependent resistor, or LDR, senses light: its resistance changes as its surroundings get brighter or darker. Feed either one into a simple circuit and you have an electronic sensor — the heart of a thermostat, a smoke-and-heat alarm, an automatic light or a camera's light meter. This page is about how those two components behave, and how a circuit turns "it got hot" or "it went dark" into "switch something on."

The thermistor: hotter means less resistance

A thermistor is a small bead of semiconductor. The kind you meet at GCSE is an NTC thermistor — that stands for negative temperature coefficient, which is just a fancy way of saying: heat it up and its resistance goes down. Cold, it might have a resistance of several thousand ohms; warm it in your hand and that resistance drops; hold it near a flame and it falls further still.

Plot its resistance R against temperature and you don't get a straight line — you get a falling curve that drops steeply while it's cold and then levels off as it gets hot. That single downward curve is the thermistor's signature.

Cold (say a fridge or a winter morning) → high resistance.
Hot (a warm room, an oven, a fire) → low resistance.

Because a low resistance lets a bigger I = V/R through, a circuit can spot the heat simply by noticing the current climb (or, more usefully, a voltage change — we'll build that in a moment). That's why thermistors sit inside thermostats, fire alarms, and the temperature sensors in fridges, freezers and ovens.

The LDR: brighter means less resistance

A light-dependent resistor works the same trick, but for light instead of heat. In the dark its resistance is enormous — often more than a million ohms. Shine light on it and the resistance tumbles; in bright sunlight it can fall to just a few hundred ohms. The brighter the light, the lower the resistance.

Dark (night, a covered sensor) → very high resistance.
Bright (daylight, a torch shone on it) → low resistance.

So an LDR is really a "how bright is it?" detector. An automatic street light watches its LDR: while the sun is up the resistance stays low, but as dusk falls the resistance climbs, the circuit notices, and the lamp switches on. The light meter in a camera or phone uses one the same way — to decide how long to keep the shutter open, or how far to dim the screen.

Try it: a sensor you can turn hot, cold, bright or dark

Pick a component with the first control, then drag the second to change its surroundings — temperature for the thermistor, brightness for the LDR. Watch the reading-off dot slide along the falling resistance curve, the live R value change, and the sensor cross the dashed switching level — flipping the fire alarm (which trips when it gets hot) or the street light (which trips when it goes dark) on and off.

Notice the shape never changes: for both components the resistance falls as the "input" (heat or light) increases. All that differs is what the sensor is reacting to, and which way the circuit is wired to switch.

Turning a resistance change into a voltage: the potential divider

A circuit can't act on "the resistance dropped" directly — switches and warning lamps respond to a voltage. So the sensor is wired in series with an ordinary fixed resistor, and the supply voltage is shared between the two. This series pair is called a potential divider.

The same current flows through both, so by Ohm's law each one takes a share of the supply pd in proportion to its resistance — the bigger resistance grabs the bigger share:

For a resistor at constant temperature, the potential difference across it, the current through it and its resistance are linked by:

V = I\,R — the pd equals current times resistance.
R = \dfrac{V}{I} — rearranged, so two meter readings give a resistance.

Now follow what happens when the surroundings change. Say the sensor sits in the divider and we watch the voltage across the sensor:

Thermistor in a fire alarm. It gets hot → its resistance falls → it keeps a smaller share of the voltage, so more voltage appears across the fixed resistor. When that voltage climbs past a set level, the alarm sounds.
LDR in a street light. Night falls → its resistance rises → it takes a bigger share of the voltage. When the voltage across the LDR climbs past a set level, the lamp switches on.

The exact wiring (which component you tap the voltage from) decides which event — getting hot, getting cold, getting dark, getting bright — does the switching. The key idea for GCSE is simply this: a change in the sensor's resistance shifts how the voltage is shared, and the circuit reacts to that shift.

Reading a sensor's resistance with R = V/I

A thermistor or LDR is still just a resistor — a resistor whose value moves — so you find its resistance at any moment exactly as you would for any component: measure the pd across it and the current through it, then divide.

Example 1 — a warm thermistor. At room temperature a thermistor has 2\ \text{V} across it while a current of 0.5\ \text{A} flows through it. Its resistance is:

R = \frac{V}{I} = \frac{2\ \text{V}}{0.5\ \text{A}} = 4\ \Omega.

Example 2 — the same thermistor, now hot. Heated near a flame, the pd across it falls to 0.6\ \text{V} for the same 0.5\ \text{A}:

R = \frac{V}{I} = \frac{0.6\ \text{V}}{0.5\ \text{A}} = 1.2\ \Omega.

Same component, but the resistance has dropped from 4\ \Omega to 1.2\ \Omega — proof, in numbers, that heating the thermistor lowers its resistance.

Example 3 — an LDR in daylight. With a torch shone on it an LDR passes 0.02\ \text{A} under a pd of 6\ \text{V}:

R = \frac{V}{I} = \frac{6\ \text{V}}{0.02\ \text{A}} = 300\ \Omega.

Cover it and the current would collapse to almost nothing, so R = V/I would shoot up into the thousands — a "very high resistance in the dark" written as a number.

These three catch people out every time:

A thermistor's resistance goes DOWN as it gets hotter — the opposite of a plain metal wire. A metal filament's resistance rises when it heats (its atoms jiggle and get in the way), but an NTC thermistor is a semiconductor: heat frees up more charge carriers, so its resistance falls. Don't muddle the two.
An LDR has HIGH resistance in the DARK, not in the light. Bright light frees charge carriers and lets current through easily (low resistance); darkness leaves it blocking (high resistance). It's easy to say it backwards.
Neither is ohmic. Their resistance is not constant, so V = IR with a fixed R does not describe them, and their behaviour graph is a curve, never a straight line through the origin. You can still use R = V/I to find the resistance at one moment — you just get a different answer at a different temperature or brightness.

There's no clock inside a street lamp and no one throwing a switch — it watches the sky through a tiny LDR poking out of the top. All day the sunlight keeps the LDR's resistance low, so most of the circuit's voltage sits on a fixed resistor beside it and the lamp stays off. As the sun sinks, the light fades, the LDR's resistance climbs, and the share of voltage across the LDR grows. The moment it creeps past a preset level, a little switching circuit trips and the lamp lights — then reverses at dawn, when brightening light drops the resistance again.

The very same idea dims your phone. A light sensor near the screen is really an LDR-style detector: in a dark room its resistance is high, the phone reads "it's dim in here" and softens the backlight to save your eyes and its battery; step into sunshine and the resistance drops, and the screen brightens so you can still read it. A tiny falling-resistance curve, quietly running your evening.