Input Devices

Microphone

Symbol:

Energy Change: sound electrical

Solar Cell

Symbol:

Energy Change: light electrical

Thermocouple

Symbol:

Energy Change: heat electrical

The higher the temperature at the junction, the more heat energy converted to electrical.

+ -

The Thermistor

A thermistor is a special type of resistor whose resistance changes with temperature.

Symbol:

Experiment

A thermistor is connected to an ohmmeter as shown.

Ω

The resistance of the thermistor is measured by the digital ohmmeter at several different temperatures.

Results

Resistance of thermistor at hand temperature = Ω

Resistance of thermistor at room temperature = Ω

Resistance of thermistor at 0 °C = Ω

Conclusion

As the temperature increases the resistance .

As the temperature decreases the resistance will .

decreases

T U R D

temperature up, resistance down

increase

Light Dependent Resistor (LDR)

A light dependent resistor (LDR) is a special type of resistor whose resistance changes with light intensity (brightness).

Symbol:

Experiment

A thermistor is connected to an ohmmeter as shown.

The resistance of the LDR is measured by the digital ohmmeter in bright light and in darkness (covered up).

Results

Resistance of LDR in bright light = Ω

Resistance of LDR in darkness = Ω

Ω

Conclusion

As the light intensity increases the resistance .

So as the light intensity decreases the resistance will .

decreases

L U R D

light up, resistance down

increase

Thermistor and LDR Problems

A thermistor is connected to an ammeter and a supply voltage of 12 volts as shown.

A

12 V

T

Temp (°C) Resistance (Ω)

20100

2000500

(a) Calculate the reading on the ammeter at 20 °C.

(b) Calculate the reading on the ammeter at 100 °C.

(c) The reading on the ammeter at 150 °C will be: (i) 20 mA

(ii) 30 mA

Solution

(a)V 12V

Ω 2,000R ?I

RV

I

2,00012

A 0.006A 106I 3

(b)

V 12V

Ω 500R

?I

RV

I

50012

A 0.024I

(c) TURD – temperature goes up so resistance will go down.

Smaller resistance means bigger current.

Current will be 30 mA (0.03 A)

A light dependent resistor (LDR) is connected to an ammeter and a supply voltage of 10 volts as shown.

10 V

A

Light Intensity Resistance (Ω)

100 units500 units

4,000700

Calculate the reading on the ammeter at a light intensity of

(a) 100 units

(b) 500 units.

Solution

(a)V 10V

Ω 4,000R

?I

RV

I

4,00010

A 102.5I 3 A 0.0025

(b)

V 10V

Ω 700R

?I

RV

I

70010

A 0.014I

Yellow Book

Thermistors and LDR’s – Page 47

Q25, Q26, Q27

The Capacitor

A capacitor is an input device which introduces a time delay before something happens.

Symbol:

A capacitor is able to store charge.

It takes a certain amount of time for an uncharged capacitor to charge up.

Capacitance is measured in farads (F).

A large value capacitor (e.g. 1000 μF) takes longer to charge up than a smaller capacitor (e.g. 200 μF).

Converting Units

micro μ 1,000,000 10-6

1. Convert 150 μF into farads.

F 10150μF 150 -6F 0.00015

1,000,000150μF 150 F 0.00015OR

You may leave in scientific notation.

2. Convert the following into farads:

(a) 750 μF

(b) 10 μF

(c) 1500 μF

750 x 10-6 F 0.00075 F

10 x 10-6 F 0.00001 F

1500 x 10-6 F

0.0015 F

Charging a Capacitor

Experiment

An uncharged capacitor is placed in a circuit as shown.

S

R

1000 μF

0 V

6 V

V

Switch S is closed.

The capacitor starts to charge up.

The voltage across the capacitor is measured every 5 s after switch S is closed.

Results

A graph of the results was plotted:

Time (s)Voltage

(V)0 0

5

10

120

The time it takes to charge a capacitor depends upon the size of:

CAPACITOR it takes longer to charge a bigger capacitor

RESISTOR it takes longer to charge with a bigger resistor

(this is because the current is smaller)

** Need to know these factors affect time to charge capacitor **

Potential Divider

This is an input device which consists of two resistors in series.

The resistors divide the voltage supply into two parts.

0 V

R1

R2

VS

V1

To calculate V1:

S21

11 V

RRR

V

To calculate V2:

S21

22 V

RRR

V

** NOT on data sheet **

Example 1

In a potential divider circuit, a 6 volt supply is connected to two resistors as shown.

0 V

500 Ω

250 Ω

6 V

V1

Calculate the size of the voltage across each resistor.

V2

V 6VS Ω 500R1

Ω 250R2 ?V1

S21

11 V

RRR

V

6250500

500

6750500

60.67V 4V1

The size of V2 is found by: 1S2 VVV 46

V 2V2

Points to Note:

• V1 + V2 = VS

• The BIGGER RESISTOR gets the BIGGER SHARE of the voltage supply.

• If it is twice as big it gets twice as many volts etc.

Example 2

In a potential divider circuit, a 10 volt supply is connected to two resistors as shown.

0 V

4 kΩ

1 kΩ

10 V

V1

Calculate the size of the voltage across each resistor.

V2

V 10VS kΩ 4R1

kΩ 1R2 ?V1

S21

11 V

RRR

V

1014

4

1054

100.8

V 8V1

The size of V2 is found by: 1S2 VVV 810

V 2V2

Example 3

A 2 kΩ and 7 kΩ resistor are connected in a potential divider circuit as shown.

0 V

2 kΩ

7 kΩ

5 V

V1

Calculate the size of the voltage V1.

V 5VS

kΩ 2R1 kΩ7 R2

1VV

S21

11 V

RRR

V

572

2

592

50.22V 1.11V1

The size of V2 is found by:

0 V

2 kΩ

7 kΩ

5 V

1.11 V

V2

1S2 VVV 1.115

V 3.89V2

Yellow Book

Potential Dividers – Page 46

Q18, Q19, Q20, Q21, Q22

Comparing Calculated Values

Experiment

The calculated value of V1 will be compared to the measured value using a voltmeter.

Calculation

0 V

1 kΩ

10 kΩ

6 V

V1

V 6VS kΩ 10R1

kΩ 1R2 ?V1

S21

11 V

RRR

V

61011

60.09

V 0.55V1

Experimentally

The circuit shown was built and the voltage across the 1 kΩ is measured using a voltmeter.

voltmeter reading = V

Extension

Reverse the positions of the two resistors and repeat experiment.

What Input Device?

Selecting Input Devices

• If a TIME DELAY is involved – use a CAPACITOR.

• In other cases – think carefully about the energy change.

Example 1

Choose a suitable input device from the following list for each application given:

microphone; thermocouple; solar cell; thermistor; LDR; capacitor

(a) energy source for a satellite

(b) time delay for arming a burglar alarm to allow householder out front door

(c) temperature control for an aquarium

(d) alarm warning parents in another room that baby is crying

(e) measurement of temperature inside a blast furnace

(f) circuit to reduce brightness of TV screen when room lights are switched off.

solar cell

capacitor

thermistor

thermocouple

mic

LDR

(GENERAL)

Example 2

Give a suitable input device for each application:

(a) coin detector in a drinks machine

(b) fog detector

(c) heartbeat monitor

(d) circuit switching hand drier on for 10 seconds

(e) flame sensor for a gas fire

LDR

LDR

microphone

capacitor

thermocouple

(CREDIT)

Questions

Q1. Name an appropriate input device for the following applications:

(a) automatic light switching on when light level becomes dim

(b) heating system to switch on when temperature falls below 20°C

(c) baby monitor detecting noises made

(d) energy saving light system that switches lights on in hotel staircase for a time if 60 seconds

(e) alternative energy source used in many calculators.

LDR

thermistor

microphone

capacitor

solar cell

(CREDIT)