Electronic and Photonics 2014
Experiment 1 Voltage, Current and Power in Electric Circuits
This activity is intended as a review of the electricity concepts studied in Unit 2. You may need to
undertake some further reading to consolidate your understanding.
Aim
To investigate how the addition of resistors in series and parallel affects the current, potential
difference and power dissipated in a circuit.
Equipment
Power pack
Multimeter
Resistors (2Ω, 5Ω, 10Ω)
Connecting Wires
Procedure
Connect the circuit shown above using a 4V power supply and a 10Ω resistor.
Record the current and potential difference in the resistor and, assuming no power is lost in the
connecting wires, calculate the power dissipated in the resistor.
Repeat the experiment for power supplies of 6V and 8V then complete the following table.
Power Supply (V) Resistor (Ω)
Current (A)
Potential
Power (W)
Difference (V)
2
10
4
10
6
10
8
10
10
10
Question 1.
What is the effect of increasing the power supply on the current, potential difference and power
dissipated in the resistor?
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Plot a graph of current vs voltage for the 10 Ω resistor. You will need to print this and stick it into
your log book.
Using a 4.0V power supply and a combination of resistors in series and / or parallel, design and
construct circuits which give a total resistance equal to:
a.
b.
c.
d.
e.
f.
12 Ω
19 Ω
1.25 Ω
2.5 Ω
16 Ω
7Ω
Draw a circuit diagram in the space below showing the configuration of the resistors. Using
calculations show how each circuit provides the appropriate total resistance.
Now, set up each of the above circuits in turn. Measure the current and potential difference (voltage)
in your circuits and record your findings in the table over the page. Calculate the power for each
combination
Supply voltage: 4.0V Power Supply
Rtotal (Ω)
ITotal ( A)
VTotal (V)
V/I (Ω)
P (W)
12
19
1.2
2.5
16
7
Question 1.
What is the effect on the total resistance of a circuit as more resistors are added:
a. in series
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b. in parallel?
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Question 2.
How is the power dissipated related to the total resistance?
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Conclusion and Evaluation: Write a brief summary of your findings on series and parallel circuits.
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Electronic and Photonics 2014
Experiment 2 Investigating Resistance
The digital multimeters can measure resistance. It only requires the user to plug the black lead into the
'com' socket and the red lead into the 'V -  - A' socket. Then the leads of are connected across the
ends of the resistor. However, it is vital that the resistor must be removed from the circuit. There must
be no current through it or voltage across it.
V-  -A com
The digital meter does not require any special adjustments before use, you simply turn it on (to the
ohms range required of course) and take a reading.
Marking small components such as resistors is made easier by using a universally accepted colour
code of four bands.
Resistor Values
0
1
2
3
4
5
6
7
8
9
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
Tolerance red
gold
silver
no band
Reading the Resistor Code
Green
Yellow
Red
Tolerance
Band
2%
5%
10%
20%
The resistor is arranged so that the tolerance band is on the right. The 1st and 2nd bands represent the
1st and 2nd digit respectively in the resistors value, i.e. yellow=4, red = 2. The 3rd band is the
5
5
multiplier in powers of ten, i.e., green=10 . So the resistor's value will read as 42 x 10 .
For the following resistors; 22k, 47k, and choose two others, write down the colours in the colour
code, the stated resistance and their measured resistance (use a digital multimeter).
Compile you results the table below.
Stated Resistance
Colour Code
Measured Resistance
Using Resistors: Voltage Divider Circuits
Voltage dividers form the basis of all input sensing units used in electronics. Two components are
arranged in such a way as to share or divide the voltage between them. The two components can be
two resistors or transducers, or more typically one resistor and one transducer.
R1
Vsupply
R2
Vout
Complete this activity on voltage dividers to determine the relationship between resistance and
voltage values.
A voltage divider comprises two resistors as shown below.
Set up the components using various combinations of resistors. Set
Vin to approximately 10 V DC.
R1
Measure Vin and Vout using a multimeter.
Complete the table over the page.
VIN
R2
VOUT
Vin (V)
Vout (V)
R1 ()
R2 ()
10
10
10
4.7
4.7
10
10
33
33
10
33
4.7
R1+ R2
R2
R1  R2
Vout
Vin
Questions
1.
What relationship can you find between the resistance and voltage values. State this
clearly.
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2.
Describe the effect on Vout of replacing R2 with an LDR, including the effect of varying
lighting conditions. (Use R1 = 1.5 kΩ)
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R1
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VIN
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LDR
3.
VOUT
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Find Vout for these circuits (you do not need to construct the circuits).
a)
b)
6 k
6 k
6V
6V
6 k
Vout
6 k
4 k
Vout
4.
What would be the significance of using a variable resistor as the lower resistor (R2)? You
will need to identify the function of R2 in relation to Vout.
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Conclusion and Evaluation
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Electronic and Photonics 2014
Experiment 3 Electrical Transducers (Thermistors)
A transducer is an electronic device that responds to a physical phenomenon, e.g. light or temperature,
to produce an electrical or optical signal.
Some electronic and photonic input transducers are;
Input Transducer
Produces an electrical signal in response to ...
Microphone
Sound waves
Record Player pickup
Tape Heads
CD
Thermistor
Light Dependent
Resistor (LDR)
The Thermistor
Thermistors can have a variety of physical appearances as. All have a resistance which varies
with temperature.
Thermometer
Set a digital multimeter to the 200k range
to measure the thermistor's resistance and
temperature whilst in a beaker of hot water.
200k 
Thermistor
Stirring Rod
Take at least 10 measurements so that you can plot a reliable graph of R against T. The temperature of
the water can be varied by gradually adding ice to the hot water. Print your table and graph and paste
them in your log book.
This type of component is said to be an NTC thermistor because it has a 'negative temperature coefficient'. Explain what this means.
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What possible uses could a thermistor have?
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Electronic and Photonics 2014
Experiment 4 LDRs (Light Dependent Resistors)
In this experiment we will be investigating the relationship between light intensity and the
output voltage of a voltage divider circuit incorporating a light dependent resistor (LDR).
Equipment:
DC supply (set on 2 V)
22 kΩ resistor
LDR
Connecting block
Multimeter (to measure output voltage, set on 2V DC)
Light source (Light box connected to DC power supply, 10 V DC)
Lux meter (to measure light intensity)
Set up the apparatus as shown below:
Measure the output voltage for a range of light intensities, recording your data in excel. Ensure you
obtain reliable values over a wide range of light intensities.
Light intensity, φ
(lux)
Output voltage, Vout (V)
Plot a graph of output voltage against light intensity. Print the data table and graph and paste into your
log book.
1. What does your graph imply about the change to the resistance of an LDR as the light intensity
increases?
2. Calculate the resistance of the LDR for two different light intensities. Show your working.
Conclusion: Summarise your findings, describing the effect of the changing light intensity on the
output voltage.
Electronic and Photonics 2014
Experiment 5 Investigating Diodes
Diodes are `semi-conductor' components made from silicon (Si) or germanium (Ge) and used to
control the direction of the current. When connected in a circuit, a diode has a low resistance in one
direction. Connected in the reverse position it has high resistance and will not conduct at all, except
for a small leakage current.
The circuit symbol for a diode is:
Silver Band
A diode.
Anode
Cathode
Diode Circuit Symbol
1. Set up the following circuit using a 50Ω rheostat (variable resistor) and a 1 kΩ or 2.2 kΩ fixed
resistor to investigate the properties of a diode. Record your data in excel.
A
2. Set the power pack to 2 volts.
3. Use the rheostat to provide maximum voltage to the diode-resistor part of the circuit.
4. Use multimeters to measure Vdiode and Idiode and record the values in excel.
5. Decrease the voltage supplied to the diode-resistor in small steps until the current reaches 0 A.
6. Reverse the direction of the diode, repeat the above measurements and record in the table. You
will need to use the 200 μA setting to measure the current for this part, but you must record the
values in mA.
7. Plot a current – voltage graph for the diode. The forward biased current and voltage should be
positive values. Reverse biased current and voltage should be negative values. Print the graph
and your data table and stick these in your log book.
Your excel data table should be set up like this:
Voltage across diode
(V)
Current through diode
(mA)
Forward bias
Reverse bias
3.
Explain the operation of the diode when placed correctly in the circuit (FORWARD BIAS)
and when it is incorrectly placed in the circuit (REVERSE BIAS). What is the minimum
voltage required before the diode conducts?
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The Light Emitting Diode ( LED)
The LED behaves in the same way as a normal diode but has the additional property of emitting light
when it conducts.
Repeat the above procedure using an LED instead of a regular diode. Again, collate your results into a
table and plot the current-voltage characteristic for the LED.
Voltage across diode
(V)
Current through diode
(mA)
Forward bias
Reverse bias
4. Explain the operation of the LED when placed in the circuit in forward bias and when it is
placed in the circuit in reverse bias. What is the minimum voltage required before the LED
conducts? (ie What is the switch on voltage?)
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Electronic and Photonics 2014
Experiment 3 The Cathode Ray Oscilloscope (CRO)
The CRO is very sensitive meter which can be used to:
 Measure both AC and DC voltages
 Measure the frequency of AC signals
 Look at waveforms
The CRO acts as a ‘graph plotter’ to show how the input voltage varies with time. The display is
called a trace and can be either a sold line or a single dot.
In a way, you could treat the CRO screen like a piece of graph paper moving horizontally across the
screen. Current cannot be measured but is calculated using Ohm's Law if the voltage and resistance
are known.
The MLC Physics department has both single trace and dual trace oscilloscopes. The difference is
that the dual trace CRO (DTCRO) allows for two inputs to be displayed and manipulated on the same
screen. You will be using the DTCRO.
Activity: Using The Cathode Ray Oscilloscope
Locate the following controls and identify them on the diagram.
ON/OFF switch
Focus
Time base or Time/div control
Vertical Shift or Position
Volts/div control
Brightness intensity
Sync/Trigger
Horizontal Shift or Position
AC/DC Switch
2a)
Obtaining a Trace on the CRO
1.
Switch On
2.
Turn all Position controls to the mid position.
3.
Set source to INT and push the Trig Control to AUTO
4.
Set Time Base or Time/div to 0.1 ms/div
5.
If you have pushed both the A & B input buttons, you will have two traces.
6.
2b)
Use Focus & Brightness controls to improve your trace. Keep intensity low to avoid
damaging the screen of the CRO.
Using the CRO to take Voltage Measurements
1.
Switch the AC/DC switch to DC and move the trace to the central scale line on the
screen.
Connect the circuit below.
10 k 
1.5 V
22 k
Measure and record the voltage across each resistor with the CRO, noting how far the trace is
deflected from the reference line.
Check your results using a multimeter.
2.
Change the 22 k resistor to 1k and repeat the measurements.
Insert your results in this table.
Circuit A
Measurement Method
CRO
Multimeter
V10k
Circuit B
V22k
V10k
V1k
2c)
Displaying and measuring time varying or AC voltages
CRO screen showing an input voltage.
Voltage measured by:
height of peak x input (V/div) setting
Frequency measured by:
length of 1 cycle x time base setting
1.
Obtain a trace in the centre of the CRO screen and set the AC/DC switch to AC.
2.
Connect the input terminals of the CRO to the `AC' terminals of a power supply set to
about 2V.
3.
Adjust the time base controls to obtain a stationary sine-wave.
Question: What is the function of the sweep or time base?
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2d)
Frequency and Period of a Trace
The frequency, f, of any oscillating motion is defined as the number of cycles of the motion
per second. The unit of frequency is the hertz (Hz). 1 Hz = 1 cycle per second (s-1)
1 Hz = 1 cycle per second
The period (T) is defined as the time taken to complete one cycle of the motion. The unit of
period is the second (s)
1
T
f
1.
Switch on the signal generator and adjust its frequency to 10 kHz (10000 Hz). Connect
the output of the signal generator to the input terminals of the CRO.
2.
Adjust the time-base controls to the CRO to obtain three cycles on the screen.
Question: Without altering the time-base settings, increase, and then decrease, the frequency of
the signal generator. What changes do you observe in the CRO display?
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Electronic and Photonics 2014
Experiment 6 voltage Amplifiers
An amplifier is an electronic system which produces an output that is an enlarged copy of the input.
Voltage, current and power signals can all be amplified.
Voltage Amplifiers
A typical operational amplifier, opamp, is made as integrated chips, IC's. The amplifier section of the
IC is drawn as a triangle with one output and two inputs. One of the inputs is called an inverting input
(marked -) and a non-inverting input (marked +).
1
8
2
7
6
3
4
+
5
The gain of an amplifier is the ratio of the output voltage to the input voltage.
The behaviour of a voltage amplifier is determined by its Vout/Vin graph. The input signals are
amplified in the LINEAR section of the graph.
When the amplitude of an input signal is too large, the output signal may not be able to respond to it.
The output is distorted and is called clipping.
You will need:
- Operational Amplifier circuit board
- 2 x 9 volt batteries
- 1.5 volt battery
- 10 k potentiometer
- digital multimeter
Connect the potentiometer to the existing circuit board as shown below:
+ve output rail
I.C
1.5 V
-ve rail
+ve input pin
Note:
1.
VOUT is located across the +ve output rail and the -ve rail.
VIN is located across the +ve input rail and the -ve rail.
Measure VOUT for various positive values of VIN.
Commence at 0 V and increase VIN until you obtain about 6 or 7 values that span the entire
range.
Vin (positive)
Vout
Vin (negative)
Vout
2.
Repeat for negative values of VIN. (i.e. reverse the 1.5 volt battery.)
3.
Plot a graph of VOUT against VIN. Paste this graph into your log book.
4.
Would this circuit be an inverting or non-inverting amplifier? Why?
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5.
How do you think VOUT will vary if an A.C. signal is applied to the input?
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6.
Attach a signal generator to the input and use the CRO to measure input and output voltages.
Draw the resulting waveforms for two Vin and Vout values.
7.
Use an earplug to compare the sound it produces when connected to the input then the output.
Use a signal generator with a very low input signal, about 150 Hz.
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8.
If the A.C. input voltage is too large, the output will be very distorted or 'clipped'. Can you
produce clipping? Why does it occur?
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Conclusion and Evaluation
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Electronic and Photonics 2014
Experiment 8 Optical Transmission via a Light Beam
In this investigation we will observe how information (in this case a sound signal) can be transmitted
optically (i.e. via a light beam).
We will investigate transmission both in free space and using an optical fibre.
Materials:
 Senko All Optical Transmission Kit
 2 x 9V batteries
 Sound Source (voice or MP3 player)
Method:
Set up the kit for ‘free space’ transmission using the instructions provided.
Observe the transmission of the signal in free space. Note the effect of blocking the signal by placing
your hand or another object in the path of the beam.
Now connect the optic fibre to the transmitter and receiver and again observe the transmission of the
signal.
Try transmitting the signal if the transmitter and receiver are not in a straight line.
Complete the following table, summarising what you think are the advantages and disadvantages of
each of the transmission methods.
Advantages
Free space transmission
Optic fibre transmission
Disadvantages