Engineeringbikes
a motion analyser
practical activity 3 | student instructions | page 1 of 4
limbs and lights
Video cameras are used to monitor athletes’ performance
through frame-by-frame analysis of limb motion. The image
in each frame is often blurred, so the athlete is illuminated by
flashes of light, synchronised with the frame rate of the camera.
An engineer might be asked to devise a system of flashing LEDs.
When they are attached to an athlete, a digital camera can take
daylight videos of the athlete in action. Although the limbs will
be blurred, the image of the LED should be sharper, allowing a
more precise measurement of its position.
what you have to do
You will be optimising the design for a flashing LED that can be strapped to a cyclist’s leg to
enable frame-by-frame analysis of its motion. You will:
• make calculations to match the LED flashing rate to the camera frame rate
• use these calculations to select components in the circuit
• build the circuit on a breadboard
• test the circuit
• film the LED in motion and try a frame-by-frame analysis.
Equipment
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breadboard
555 oscillator chip
LED
selection of resistors
1 x micro farad capacitor
15 cm strips of black, red and blue wire for breadboard connections
wire cutters and wire strippers
pliers
3 x AA batteries in holder
digital camera
computer / laptop
spare coloured wire for breadboard connections
oscilloscope / picosope (with croc-clips and 4 mm leads)
tape for taping breadboard to legs if testing outside the laboratory
ultra-bright LED if testing outside the laboratory
safety notes
The 555 oscillator chip may become hot if you connect the power incorrectly. Check your
circuit with your teacher. Take care when using wire cutters and strippers.
Engineeringeverywhere
EngineeringBikes | a motion analyser | student instructions
page 2 of 4
method: Matching the frame rate
A digital camera may have a frame rate of around 15 frames per second (fps). This means
that, when filming, you get 15 separate images per second. Circuit components will have to be
chosen to make an LED flash 15 times per second to match up with the frame rate.
To calculate which resistors to use with your oscillator, use this formula:
R=
480 000
f
where
R is the value in ohms for your two resistors
f is the frames per second (frequency) of your camera
For example, for 8 fps:
R=
480 000
= 60 kΩ
8
Write your calculated value for the resistance here: ___________________
You may find that you do not have the exact value of resistor available. Pick the nearest value
for now and you may be able to change your resistors later when you come to test your circuit.
METHOD: Building the circuit
This is the breadboard layout and circuit diagram:
4.5 V
0V
4.5 V
R1
8 7 6 5
R2
555
1 2 3 4
C
+
0V
Follow the breadboard layout and advice from your teacher on how to build the circuit. R1
and R2 will both have the value that you calculated in the previous section. These are the two
resistors on the top left of your breadboard. Make sure you check that the 555 oscillator chip
is the correct way up and the capacitor is the correct way round before connecting the power
supply to your circuit.
Engineeringeverywhere
EngineeringBikes | a motion analyser | student instructions
page 3 of 4
METHOD: Testing the circuit
1.If your circuit is working, you should see a flashing LED. If you don’t see this then the first
thing to do is turn the LED around. If it still doesn’t work, disconnect the power and get
your teacher to check the circuit.
2.Connect up your circuit to an oscilloscope to check the frequency (number of flashes
per second). The output from the 555 oscillator (pin 3) should be connected to one of the
channel inputs on the oscilloscope. The ground should be connected to the 0 V rail on your board.
3.Each peak on the trace corresponds to a flash. By changing the time base you should be
able to count the number of peaks in one second.
Write the number of peaks per second on your trace here: ___________________
This should be very close to the number of frames per second of your camera.
An example trace:
2
1
0
V
-1
-2
-3
-4
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
s
4.At this stage you may want to experiment with changing values of R1 and R2 to achieve
different frequencies.
mETHOD: filming
1.Congratulations – you should now be ready to film your circuit. You may be surprised to
see it appearing to flash on and off so slowly. Explain why you observe this.
2.You may be able to film someone on an exercise bike. The breadboard will need to be
strapped to the cyclist’s leg. Use an ultra-bright LED and longer leads.
3.Use Windows Media Player to view the rotation, frame by frame. Use Play speed settings
to do this. Count the number of frames for one rotation. Then calculate the frequency of
rotation of the pedal.
advantages and disadvantages
Is this a practical system of measurement?
Think of any advantages or disadvantages compared with using a camera flash.
Engineeringeverywhere
EngineeringBikes | a motion analyser | student instructions
page 4 of 4
some more things to try
For a better analysis of an athlete’s movement, you would need to monitor either side of a
joint. To do this, a series of LED’s would be attached.
You can explore this with two LEDs in series (when using 3 x 1.5 V batteries) which flash on
together and two more which flash on when the other two are off - (giving a possible four
LEDs). You need to solder interconnecting wires to the LEDs to allow you to stretch them
around to the place they are needed.
This is the circuit that you will need.
4.5 V
R1
8 7 6 5
R2
555
1 2 3 4
C
+
0V
Engineeringeverywhere
Engineeringbikes
a motion analyser
practical activity 3 | teacher notes | page 1 of 2
health and Safety
A risk assessment must be made before starting any practical work. The circuits themselves
use only low voltage (< 5 V) and are designed to be powered by battery packs – and as such
pose no risk to the student. However, the 555 chips can become hot if the power is connected
incorrectly. Students should be careful when using wire cutters and strippers. If the analyser
is trialled on a cyclist in the field, consider the issues of location and the safety of the cyclist
and photographer.
Suggested sequence
After watching the video, introduce the idea that engineering also helps with training cyclists.
Students should complete the resistor calculation before being handed out the equipment –
otherwise they are likely to want to start building straight away. For students who have not
used breadboards before, the teacher should demonstrate how to insert the chip and to insert
in a couple of wires, before the equipment is handed out.
Once the resistor values have been calculated, they can begin the build. Display a copy of
the breadboard diagram in colour, to assist with wiring correctly. Some students will take a
lot longer than others to build the circuit. This can be a help in that students finishing their
build first can test their circuit using an oscilloscope/picosope whilst others are still building.
Students who are very quick with the build could investigate the effects of changing the value
of R2 to give shorter pulses of light (remember that this will also change the frequency of the
oscillator).
Students should film their circuits in the laboratory to observe the effect of the frame rate
being close to the frequency of the LED. Students are encouraged to try and explain why the
LED appears to flash on and off so slowly when filmed.
The circuit can also be filmed with the LED attached to someone on an exercise bicycle. If the
circuits are to be tested in the field then an ultra-bright LED will be needed as the camera is
unlikely to pick up the light from a normal LED at the distance needed to observe a cyclist.
Frame-by-frame analysis of the motion can be carried out using Windows Media Player
or similar software. From number of frames for one rotation of the pedals, students could
calculate the frequency of rotation of the pedals.
Finally, students are asked to come up with some advantages or disadvantages compared
with using a flash synchronised to a camera. Some advantages could be cost, mobility and
tracking a particular body part. Some disadvantages could be loss of information when the
LED is off, encumbering the sports-person with the circuit and difficulty of use from large
distances.
notes
Common reasons why the circuit might not work:
power rails not connected to battery; LED is the wrong way around; 555 chip is upside-down;
capacitor is the wrong way around; interconnecting wires are not lined up properly; missing connections.
The 555 astable produces an ON/OFF pulse with a frequency dependent on the choice of
resistors and capacitors attached to it. The mark length (or time when the output (pin 3) is
high) is given by T = 0.7 * (R1 + R2) * C. The space length (or time when output is low) is given by
T = 0.7 * R2 * C. So the mark length will always be longer than the space length.
Engineeringeverywhere
EngineeringBikes | a motion analyser | teacher notes
The circuit has been designed so that the 555 acts as a current sink – the LED will light up
during the ‘space’ so that it will be off longer than it is on. For simplicity, the formula the
students are given assumes that the two resistors are going to take the same value – this
means that the LED will be on for 1/3 of the time. It is also assumed that they will use a 1
micro-farad capacitor. The more general form of the equation is:
(R1 + 2R2)C = 1.44 / f
where f is the number of frames per second.
Sometimes the stability of a 555 astable can be poor. Placing a large smoothing capacitor (100 micro-farad approx.) across pins 1 and 8 can often solve this problem.
It does not matter if the frequency of the astable does not exactly match the frame rate. In
fact, it can cause problems if it does, as the LED may appear to the camera to be constantly
off rather than on when you film it. It is best to have a fraction of 1 Hz difference so the LED
appears to be going on and off every few seconds (enough time for a full rotation of a pedal).
This is the beat frequency between the astable and the camera and, if measured, it could be
used to give the frequency of either astable or camera if the other is unknown.
To enable frame-by-frame analysis in Windows Media Player, in the Menu, go to
Enhancements and open Play speed settings. The arrows should allow you to advance one
frame at a time.
further investigation: Monitoring either side of a joint
For a complete analysis of an athlete’s movement, a series of LEDs would be attached.
Students can explore this with two LEDs in series (when using 3 x 1.5 V batteries) which flash
on together and two more which flash on when the other two are off (giving a possible four
LEDs). You would need to solder interconnecting wires to the LEDs to allow them to stretch
around to the place they are needed. [See the diagram.]
Time required
This activity will take at least one session to complete with additional time required for
evaluation. The extension, with two LEDs, will take further time.
Technician equipment list
per group / student
• breadboard
• 555 oscillator chip
• LED
• selection of resistors
• 1 x 1 micro farad capacitor
• 15 cm strips of black, red and blue wire for breadboard connections
• wire cutters and wire strippers
• pliers
• 3 x AA batteries in holder
per class
• digital camera
• computer / laptop plus software for analysis (e.g. Windows Media Player)
• reels (spare) coloured wire for breadboard connections
• oscilloscope / picosope (with croc-clips and 4 mm leads if needed)
• tape for taping breadboard to legs if testing outside the laboratory
• ultra-bright LED if testing outside the laboratory
Electronic components are available from www.rapidonline.com, www.maplin.co.uk and
similar outlets
Engineeringeverywhere
page 2 of 2