Transcript Input Devices

Input Devices
Microphone
Symbol:
Energy Change:
sound
electrical
light
electrical
Solar Cell
Symbol:
Energy Change:
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
decreases
As the temperature decreases the resistance will
increase
T U R D
temperature up, resistance down
.
.
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.
Ω
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 =
Resistance of LDR in darkness =
Ω
Ω
Conclusion
As the light intensity increases the resistance decreases
So as the light intensity decreases the resistance will
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.
12 V
T
A
Temp (°C)
Resistance (Ω)
20
100
2000
500
(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  12 V
R  2,000 Ω
I?
(b)
V  12 V
R  500 Ω
I?
(c)
I

V
R
12
2,000
I  6  10 3 A
I

0.006 A
V
R
12
500
I  0.024 A
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 units
500 units
4,000
700
Calculate the reading on the ammeter at a light intensity of
(a)
100 units
(b)
500 units.
Solution
(a)
V  10 V
R  4,000 Ω
I?
(b)
V  10 V
R  700 Ω
I?
I
V
R
10

4,000
I  2.5  10 3 A
I

V
R
10
700
I  0.014 A
0.0025 A
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
μ micro 
1.
 10 -6  1,000,000
Convert 150 μF into farads.
150 μF  150  10-6 F
 0.00015 F
150 μF  150  1,000,000
OR
 0.00015 F
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.
6 V
S
Switch S is closed.
R
1000 μF
V
The capacitor starts to charge
up.
0 V
The voltage across the capacitor is measured every 5 s after
switch S is closed.
Results
Time (s)
Voltage (V)
0
0
5
10
120
A graph of the results was plotted:
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.
VS
To calculate V1:
R2
R1
V1 
V1
0 V
To calculate V2:
V2 
R2
 VS
R1  R2 
R1
 VS
R1  R2 
** NOT on data sheet **
Example 1
In a potential divider circuit, a 6 volt supply is connected to two
resistors as shown.
6 V
250 Ω
V2
500 Ω
V1
Calculate the size of the voltage
across each resistor.
0 V
V1 
R1
 VS
R1  R2 
VS  6 V
R1  500 Ω

500
6
500  250
R2  250 Ω
V1  ?

500
6
750
 0.67 6
V1  4 V
The size of V2 is found by:
V2  VS  V1
 64
V2  2 V
Points to Note:
• V1 + V 2 = 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.
10 V
1 kΩ
V2
4 kΩ
V1
Calculate the size of the voltage
across each resistor.
0 V
V1 
VS  10 V
R1  4 kΩ
R2  1 kΩ
V1  ?

R1
 VS
R1  R2 
4
 10
4  1
4
 10
5
 0.810

V1  8 V
The size of V2 is found by:
V2  VS  V1
 10  8
V2  2 V
Example 3
A 2 kΩ and 7 kΩ resistor are connected in a potential divider circuit
as shown.
5 V
7 kΩ
2 kΩ
0 V
VS  5 V
Calculate the size of the voltage V1.
V1
V1 

2
5
2  7 

2
5
9
R1  2 kΩ
R2  7 kΩ
V  V1
R1
 VS
R1  R2 
 0.22 5
V1  1.11 V
5 V
7 kΩ
2 kΩ
V2
1.11 V
0 V
The size of V2 is found by:
V2  VS  V1
 5  1.11
V2  3.89 V
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
V1 
6 V
VS  6 V
R1  10 kΩ
10 kΩ
1 kΩ
0 V
V1
R2  1 kΩ
V1  ?
R1
 VS
R1  R2 
1

6
1  10 
 0.09 6
V1  0.55 V
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 (GENERAL)
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
capacitor
(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.
LDR
solar cell
thermistor
mic
thermocouple
Example 2
(CREDIT)
Give a suitable input device for each application:
(a)
coin detector in a drinks machine
LDR
(b)
fog detector
LDR
(c)
heartbeat monitor
(d)
circuit switching hand drier on for 10 seconds
(e)
flame sensor for a gas fire
microphone
capacitor
thermocouple
Questions
Q1.
(CREDIT)
Name an appropriate input device for the following
applications:
(a)
automatic light switching on when light level
becomes dim
LDR
(b)
heating system to switch on when temperature
falls below 20°C
thermistor
(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.
microphone
capacitor
solar cell