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Sensing devices


Light dependent resistors

A Light Dependent Resistor (LDR) is an input transducer, converting light energy to a change in electrical properties. Its resistance decreases as light intensity increases.

As photons of light hit a cadmium sulfide track, they give bound electrons enough energy to jump into the conduction band.

res

ista

nce

(Ω

)

light intensity (lux)

LDR symbol

cadmium sulfide track

The resistance can fall from 1 MΩ in darkness to 500 Ω in light.


Thermistors

Negative temperature coefficient (NTC) thermistors are input transducers that have a decreasing resistance as temperature is increased.

As the surrounding temperature increases, the electrons in the metal oxide of the thermistor gain energy. This increases the number of charge carriers, decreasing resistance.

res

ista

nce

(Ω

)

temperature (°C)

thermistor symbol


Sensors summary


Sharing voltage


Potential dividers

Potential dividers reduce voltage. Varying the ratio of a pair of resistors changes the output voltage of a circuit.

0 V

VIN

R1

R2

VOUT

VOUT will be a fraction of VIN. The magnitude of VOUT is dependent upon the ratio of the two resistors R1 and R2.

VOUT

0 V

=R2

VINR1 + R2

×


Using the potential divider equation


Sensors and potential dividers


Potential divider questions


Potential dividers summary


Electrical power

power (W) = voltage (V) × current (A)

The power, or rate of energy transfer, of a device is a product of the voltage and current passing through the component.

What is the power of a bulb which uses a 230 V mains supply and has a current of 0.44 A passing through it?

What is the voltage across a microchip if it has a normal operating power of 0.5 W and draws a current of 0.1 A?

V = P ÷ I

P = V × I = 230 × 0.44 = 101.2 W

= 0.5 ÷ 0.1 = 5 V


Different forms of the power equation

Electrical power can also be calculated using resistance.

P = V × I

P = I2 × R P = V2 ÷ R

and… V = I × R

P = V × I

Therefore, using substitution:

P = I × R × I

and… I = V ÷ R

P = V × V ÷ R

Therefore, using substitution:

The equations linking power to resistance are found by substituting the equation V = I × R into the power equation:


Energy in circuits


A light bulb converts electrical energy to useful light and wasted heat.

Efficiency

Efficiency is a measure of how well a device transforms energy into useful forms.

light

What is the efficiency of the bulb if it converts 50 J of electrical energy into 45 J of heat energy?

efficiency =

=

electricalheat

× 100useful energy out

total energy in

× 100 = 10 %5

50


Efficiency of a motor

What is the efficiency of this system, if the motor takes 5 seconds to lift the weight?

(take gravity to be 9.81 N/kg)

1.4 kg

1.5

m

6 V 2 A

motor

pulley

=efficiency =useful energy out

total energy in× 100

energy into system: electrical energy = I × t × V = 2 × 5 × 6 = 60.0 J

energy used:gravitational potential energy = m × g × h

= 1.4 × 9.81 × 1.5 = 20.6 J

60.0= 34.3 %

20.6× 100


Electricity in the home


Current and drift velocity

Current is a flow of charge. Electrical devices activate almost instantly once they are supplied with power, however the electrons actually move around a circuit quite slowly. Their velocity is called drift velocity.

Current and drift velocity are linked by the following equation:

I = nAve

I = current (amps)

n = charged particles per unit volume

A = cross-sectional area (m2)

v = drift velocity (m/s)

e = charge on an electron (1.6 x 10-19 C)


Understanding I = nAve


Alternating current and direct current


RMS voltage

The voltage of AC can be viewed using an oscilloscope. There are three common voltage measures, namely peak, peak-to-peak and RMS (root mean squared) voltage.

RMS is a measure of the average magnitude of the voltage. VRMS =

VPEAK

√2

RMS voltage

zero volts

peak voltage

peak-to-peak voltage


RMS current and RMS power

To investigate voltage we use an oscilloscope connected across a resistor. As V I, the equation for calculating RMS current is similar to the equation for RMS voltage:

IRMS

The equation for RMS power is a little different:

PPEAK = IPEAK × VPEAK

PRMS = IRMS × VRMS =

PRMS =

=IPEAK

√2

×IPEAK

√2

VPEAK

√2PPEAK

2


AC calculations


AC/DC summary


Glossary


What’s the keyword?


Multiple-choice quiz

1 of 32© boardworks ltd 2009. 2 of 32© boardworks ltd 2009

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voltage v

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fraction of v

light energy

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r1r1 r2r2 v