Basic Electronic Components

Lessons

Senior 3 · Physics

Basic Electronic Components

4 lessons

Lesson 3 of 4

Light and Temperature Sensors and the Transistor

30 min

The street lamp that knows when it is dark

Some street lamps and security lights switch themselves on at dusk and off at dawn, with nobody touching a switch. Some kettles switch off when the water boils, and a fire alarm sounds when a room gets too hot. Each of these has a sensor, a component whose resistance changes with light or temperature, and a transistor that uses the small signal from the sensor to switch a bigger current. This lesson puts the two together.

Definition

Light-dependent resistor (LDR): a resistor whose resistance falls as the light falling on it gets brighter.

Thermistor: a resistor whose resistance changes with temperature. In the common NTC (negative temperature coefficient) type, resistance falls as temperature rises.

Transistor: a three-terminal semiconductor component in which a small current into the base controls a much larger current from the collector to the emitter. It can act as a switch or as an amplifier.

Sensors in a potential divider

A sensor's change in resistance is turned into a change in voltage by a potential divider (unit 7): the sensor in series with a fixed resistor across the supply. The voltage across the lower resistor $R_{2}$ is

V_{\text{out}} = V_{\text{supply}} \times \frac{R_2}{R_1 + R_2}

Sensor	Bright or hot	Dark or cold
LDR (illustrative values)	about 500 Ω in bright light	about 100 kΩ in the dark
NTC thermistor (illustrative values)	about 2 kΩ at 60 °C	about 10 kΩ at 25 °C

Worked example 1: an LDR divider

A 10 kΩ resistor (top) and an LDR (bottom) are in series across 9.0 V. Find the voltage across the LDR in bright light (500 Ω) and in the dark (100 kΩ).

\text{bright: } V = 9.0 \times \frac{0.5}{10 + 0.5} = 0.43\,\mathrm{V} \qquad \text{dark: } V = 9.0 \times \frac{100}{10 + 100} = 8.2\,\mathrm{V}

The output is low in the light and high in the dark: just what is needed to switch on a night light.

Worked example 2: a thermistor divider

A thermistor (top) and a 5.0 kΩ resistor (bottom) are across 9.0 V. Find the voltage across the fixed resistor at 25 °C (thermistor 10 kΩ) and at 60 °C (2.0 kΩ).

25\,^\circ\mathrm{C}: V = 9.0 \times \frac{5.0}{10 + 5.0} = 3.0\,\mathrm{V} \qquad 60\,^\circ\mathrm{C}: V = 9.0 \times \frac{5.0}{2.0 + 5.0} = 6.4\,\mathrm{V}

As the room heats up, the output rises. This can switch on a fan or sound an alarm.

The transistor as a switch

An NPN transistor has three leads: base (b), collector (c) and emitter (e). When the base is less than about 0.7 V above the emitter, almost no base current flows and the transistor is off: no current flows from collector to emitter. When the base voltage rises above about 0.7 V, a small base current flows and the transistor switches on, letting a much larger collector current through the load (an LED, lamp, buzzer or relay).

How the night light works. The 10 kΩ resistor and the LDR form a potential divider. In daylight the LDR's resistance is low, so the voltage at X is low (worked example 1: 0.43 V), the transistor is off and the LED is dark. At dusk the LDR's resistance rises, the voltage at X rises above 0.7 V, base current flows through the 1 kΩ resistor, which protects the transistor, and the transistor switches the LED on. The 390 Ω resistor limits the LED current.

A fire or overheat alarm uses the same circuit with a thermistor in place of the top resistor, a fixed resistor in place of the LDR, and a buzzer in place of the LED. When the temperature rises, the thermistor's resistance falls, the voltage at X rises (worked example 2), and the transistor switches the buzzer on.

The transistor as an amplifier

Because a small base current controls a large collector current, a transistor can also amplify: make a weak, changing signal bigger without changing its shape. The ratio of collector current to base current is the transistor's current gain, $h_{FE}$ :

h_{FE} = \frac{I_C}{I_B}

Typical values are 100 or more. Amplifiers are in every radio, phone, hearing aid and loudspeaker system: the tiny signal from an aerial or a microphone is amplified, often in several stages, until it is strong enough to drive a loudspeaker. A modern processor chip contains billions of tiny transistors working as switches.

Worked example 3: current gain

A transistor with current gain 100 has a base current of 0.20 mA. Find the collector current.

I_C = h_{FE} \times I_B = 100 \times 0.20\,\mathrm{mA} = 20\,\mathrm{mA}

A current of 0.20 mA controls one 100 times larger: enough for an LED.

Worked example 4: voltage gain of an amplifier

A microphone gives a signal of 10 mV, and the amplifier's output is 1.5 V. Find the voltage gain.

\text{gain} = \frac{V_{\text{out}}}{V_{\text{in}}} = \frac{1.5}{0.010} = 150

Warning

Common mistakes

Saying an LDR's resistance rises in light. It falls as the light gets brighter.
Putting the sensor in the wrong arm of the divider. Swapping the LDR and the fixed resistor makes the light come on in daylight instead of in the dark.
Leaving out the base resistor. Without it, too much base current can destroy the transistor.
Thinking a transistor creates energy when it amplifies. The extra energy comes from the power supply; the small signal only controls it.

Tip

Light Down, Resistance Up. For an LDR, as the light goes down the resistance goes up. For an NTC thermistor, as the temperature goes up the resistance goes down: "NTC: Negative, so they move opposite ways".

✦

Practice

How does the resistance of an LDR change when it is covered? How does an NTC thermistor's resistance change when it is heated?
A 20 kΩ resistor (top) and an LDR (bottom) are connected across 6.0 V. Find the voltage across the LDR when its resistance is (a) 1.0 kΩ, (b) 180 kΩ.
Name the three terminals of an NPN transistor, and state roughly the base voltage at which it switches on.
In the night-light circuit, explain what happens, step by step, as it gets dark.
How would you change the night-light circuit so that the LED comes on in bright light instead?
A transistor has a current gain of 150. What base current is needed for a collector current of 60 mA?
An amplifier turns a 20 mV input into a 3.0 V output. What is its voltage gain?
Design, in words or a sketch, a circuit that sounds a buzzer when a baby's room gets too warm.

Show answersHide answers

Covered (dark), the LDR's resistance rises. Heated, the NTC thermistor's resistance falls.
(a) $6.0 \times \frac{1.0}{21} = 0.29\,\mathrm{V}$ ; (b) $6.0 \times \frac{180}{200} = 5.4\,\mathrm{V}$ .
Base, collector and emitter. About 0.7 V (above the emitter).
The LDR's resistance rises, so it takes a larger share of the supply voltage; the voltage at X rises above about 0.7 V; base current flows; the transistor switches on; collector current flows through the LED, which lights.
Swap the LDR and the 10 kΩ resistor: with the LDR at the top, bright light lowers its resistance and raises the voltage at X.
$I_B = \frac{60}{150} = 0.40\,\mathrm{mA}$ .
$\frac{3.0}{0.020} = 150$ .
An NTC thermistor at the top and a fixed (or variable, to set the temperature) resistor at the bottom of a potential divider across the supply; the junction connected through a base resistor to an NPN transistor; a buzzer between the positive rail and the collector; the emitter to 0 V. As the room warms, the thermistor's resistance falls, the base voltage rises, and the transistor switches the buzzer on.

Key points

An LDR's resistance falls in light; an NTC thermistor's resistance falls when heated.
In a potential divider, $V_{\text{out}} = V \times \frac{R_2}{R_1 + R_2}$ : the sensor's change becomes a voltage change.
An NPN transistor switches on when its base is about 0.7 V above the emitter; a small base current controls a large collector current.
Current gain $h_{FE} = \frac{I_C}{I_B}$ ; amplifiers make weak signals stronger using energy from the supply.

Sources and credits

Written by Ganzaa: the lesson text, the circuit diagram, the table, worked examples and practice exercises. Sensor resistances are illustrative; real components vary.

1The street lamp that knows when it is dark

2Sensors in a potential divider

3The transistor as a switch

4The transistor as an amplifier

The street lamp that knows when it is dark

Sensors in a potential divider

The transistor as a switch

The transistor as an amplifier