The light-dependent resistor

1. A light-dependent resistor is made from a semiconducting material whose resistance depends on the light falling on its surface.
2. In bright light the resistance of a typical light-dependent resistor is only a few hundred ohms, while in complete darkness it can rise above a million ohms.
3. Light arriving at the semiconductor frees more of its electrons to move, so more charge carriers become available and the material opposes the flow of charge less.
4. As the light intensity rises the resistance falls, and as the light intensity falls the resistance rises again.
5. The graph of resistance against light intensity is a curve, falling steeply at low intensity and levelling off at high intensity, so the relationship is not linear.
6. Because its resistance changes with the conditions rather than staying fixed, a light-dependent resistor is not an ohmic component.

The thermistor

1. A thermistor is made from a semiconductor whose resistance depends strongly on its own temperature.
2. In the type used in school circuits the resistance falls as the temperature rises, which is the opposite of what happens in a metal wire.
3. Heating the semiconductor frees more of its electrons to move, so more charge carriers become available and the resistance drops.
4. A laboratory thermistor might fall from about $10k\Omega$ at $10{}^{\circ }C$ to a few hundred ohms at $80{}^{\circ }C$.
5. The graph of resistance against temperature is a curve, falling steeply at low temperature and flattening out at high temperature, so the change is not linear.
6. A thermistor is therefore not an ohmic component either, since $R=\frac{V}{I}$ gives a different value at every temperature.

Sensing circuits

1. A sensing circuit places a light-dependent resistor or a thermistor in series with a fixed resistor across a supply, and takes its output from across one of the two.
2. When the sensor's resistance changes, the resistance of the whole loop changes with it, so the current in the loop changes as well.
3. The potential difference across the fixed resistor then changes too, because $V=IR$ with a fixed value of $R$ means that potential difference follows the current.
4. Whatever share of the supply potential difference is not across the fixed resistor is across the sensor, so the two shares always move in opposite directions.
5. An electronic switch or a relay connected to the output turns a device on or off once the potential difference reaches a set value.
6. The full chain for an outdoor lamp at dusk runs like this: the light level falls, the light-dependent resistor's resistance rises, the current in the loop falls, the potential difference across the fixed resistor falls while the potential difference across the sensor rises, and the switch turns the lamp on.

Where they are used

1. Street lighting uses a light-dependent resistor to switch lamps on automatically at dusk and off again at dawn, so no energy is spent lighting an already bright street.
2. Outdoor security lighting uses the same arrangement, so a lamp responds to movement only once it is dark.
3. A camera sets its exposure and a phone dims its screen using a light sensor of the same kind.
4. A room thermostat uses a thermistor, so the heating switches off as the room warms and comes back on as it cools.
5. An oven, a fridge and a freezer each use a thermistor to hold the temperature inside within a set range.
6. A fire alarm uses a thermistor to detect a rapid temperature rise, and an engine coolant sensor uses one to warn the driver that the engine is overheating.

Investigating the components

1. The light-dependent resistor is connected in series with a cell, a switch and an ammeter, with a voltmeter across the sensor itself.
2. A lamp is set up a measured distance away on the bench and the room is darkened, so that only the lamp lights the sensor.
3. The lamp is then moved to a series of measured distances, or a light meter records the intensity at each setting, and both meters are read at every setting.
4. The resistance at each light level comes from $R=\frac{V}{I}$, and resistance is plotted against light intensity to give the falling curve.
5. For the thermistor, the sensor is held in a water bath next to a thermometer and the water is heated in steps of about $10{}^{\circ }C$.
6. The water is stirred so the thermistor and the thermometer are at the same temperature, then both meters are read at each temperature and $R=\frac{V}{I}$ is used again.
7. The supply setting is kept the same throughout and each pair of readings is taken quickly, so the current does not warm the sensor and change the very quantity being measured.