Sensing Components: Lamps, Diodes, Thermistors and LDRs

Resistance does not always stay constant

Resistance describes how strongly a component opposes electric current. The relationship between potential difference $V$, current $I$ and resistance $R$ is

$V=IR$

For a component at a particular operating point, this can be rearranged to $R=V/I$. If resistance is greater, a given potential difference produces a smaller current. Some resistors remain at nearly constant resistance when their temperature is constant, but lamps, diodes, thermistors and light-dependent resistors (LDRs) do not behave in this simple way.

Their resistance may depend on the component's temperature, the direction of the potential difference or an environmental condition such as light intensity. This makes thermistors and LDRs especially useful as sensing components.

A filament lamp heats itself

A filament lamp contains a very thin metal filament. When current passes through it, electrical energy is transferred to the filament's thermal energy store, so its temperature rises. The moving electrons interact with the vibrating ions in the metal lattice. At a higher temperature the ions vibrate more, making it more difficult for charge to move through the filament.

Consequently, the resistance of a filament lamp increases as the filament's temperature increases. Increasing the potential difference still increases the current, but the rising resistance means that the current increases less rapidly than it would through a constant-resistance resistor. The lamp therefore has a curved, rather than straight, current–potential difference characteristic.

A diode responds to direction

A diode is designed to conduct in one direction. When connected in its forward direction, with the applied potential difference in the conducting direction, it can conduct once the forward potential difference is sufficient. Its resistance is not constant: after conduction begins, a small further rise in potential difference can produce a large rise in current.

Reversing the connections reverses the potential difference across the same diode. The diode then has a very high resistance, so essentially no current flows under normal conditions. This one-way behaviour is why the orientation of a diode symbol and the polarity of the supply both matter.

A thermistor senses temperature

For the thermistors studied at GCSE, resistance decreases as temperature increases. A cold thermistor therefore has a higher resistance than the same thermistor when it is warm. At a fixed potential difference, its changing resistance causes a change in the current or in the potential difference elsewhere in a sensing circuit.

A thermostat uses this response to monitor temperature. The thermistor does not usually switch a heater by itself. Instead, a control circuit detects the electrical change caused by the thermistor and operates a switch. For example, a heating system can be designed to switch on when the sensed temperature is too low and switch off when the required temperature is reached.

An LDR senses light

An LDR has a resistance that depends on the intensity of the light falling on it:

• brighter light → lower resistance
• dimmer light or darkness → higher resistance

The resulting electrical change can be detected by a control circuit. In an automatic night-light, the circuit can be arranged so that the high resistance of the LDR in darkness causes the lamp to be switched on. When daylight returns, the LDR's resistance falls and the control system switches the lamp off.

The LDR is therefore the sensor, while another part of the circuit performs the switching. Similar circuits can be used in automatic street lighting and other light-sensitive controls.

Compare the four components

The important question is always: what causes the resistance to change? In a filament lamp it is the rising temperature of the filament; for a diode it is chiefly the direction and size of the applied potential difference; for a thermistor it is the surrounding temperature; and for an it is light intensity. These different responses allow components to produce light, control the direction of current or detect changes in their surroundings.

Core relationship

At a particular operating point,

$R=\frac{V}{I}$

A greater resistance gives a smaller current for the same potential difference. Lamps, , and do not have constant resistance.