Trigger
Flash
Coil
Lecture 10:
Disposable Camera
Battery
Charging Switch
Learning Engineering Using
Inexpensive Products That May
Be Found Around The House
Or Apartment or Dorm Room
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Electrical Safety
• Note on the diagram of the camera that
voltages as high as 320V exist in this circuit
• It is current, not voltage, that heats a wire and
that causes damage to humans.
• The salty fluids of the human body are
electrical conductors.
• The interior resistance of an arm, from hand
to shoulder is about 100 Ohms.
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Electrical Safety
• Any voltage across the internal resistance will
cause currents to flow and heat to be
generated sufficient to cause damage to
tissue.
• Much more significantly, the nervous system
and its means of control over muscles
(including the heart) are electrochemical.
Thus, damage can occur at currents much
less than those necessary to cause burns.
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Electrical Safety:
Current Physiological Reaction (60 Hz)
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< 1mA
None
1mA
Perception Threshold
1-3 mA
Mild Sensation
3-10 mA
Painful Sensation
10 mA
Paralysis Threshold of Arms
30 mA
Respiratory Paralysis
75 mA
Fibrillation Threshold (0.5%)
250 mA
Fibrillation Threshold (99.5%)
4A
Heart Paralysis Threshold
>5 A
Burning
Introduction to Engineering Electronics
STOLEN FROM K. A. Connor
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Electrical Safety
• One of the key characteristics that distinguishes an
EE, CSE or EPE from other technical professionals is
an understanding of electrical safety.
• It is particularly important to be able to estimate
currents from known voltages, since voltages are
easier to measure and generally more controllable.
• Thus, if you learn nothing else from IEE, you should
learn that:
 Ohm’s Law can save your life and the
lives of others.
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Electrical Safety:
How To Avoid Shock
• Estimate voltages and currents and act
cautiously.
• Resistance of dry skin offers protection
below 50 V. Typically, skin inserts 10k
to 100k Ohms of resistance between an
outside conductor and internal fluids.
• Skin provides safe current limiting below
50 V if it is dry.
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Introduction to Engineering Electronics
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Electrical Safety:
How To Avoid Shock
• When working with voltages above 100
V, exercise great care.
• If you must work with these voltages, a
famous rule is to keep one hand in your
pocket because arm-to-arm current
through your heart is much worse
neurologically than local currents
through one limb.
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Electrical Safety: References
Remember the
one hand rule
•
•
•
•
MIT Lab Safety
NASA Lab Safety
Tecra Tools Electrical Safety
University of Tennessee Power
Electronics Lab Safety
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Introduction to Engineering Electronics
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Electrical Safety
• It is possible to come in contact with high
voltages with little permanent harm as long as
currents are small.
• Stun guns work at voltages typically between
20kV and 150kV and usually do not cause
permanent damage since currents are small
(a few microamps) and voltages are pulsed (1
microsecond).
• We will return to the topic of stun guns at the
end of this lecture.
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2 Minute Quiz
Name______Sec______Date______
• True or false – proper electrical safety
procedures require that we always know
at least approximately what currents we
will encounter in a circuit.
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Disposable Camera Electronics
• This is the circuit you will be building in
the lab using a disposable camera.
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Introduction to Engineering Electronics
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Disposable Camera Electronics
Trigger
Flash
Coil
Battery
Charging Switch
• This is the circuit mentioned in lecture 6
which can be used to launch a piece of a
paper clip up to 50 feet.
• The coil is soldered in series with the flash
tube.
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Disposable Camera Electronics
• There are many different types of
disposable cameras. All open
differently.
• Once you remove the film for
processing (this can be done by the
developer), you can use the remaining
circuit and battery.
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Disposable Camera Electronics
Capacitor
• Note that the capacitor can still be charged,
even if the camera has not been used for a
while. Many cameras automatically charge
between shots.
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Disposable Camera Electronics
Flash Tube
Capacitor
Transformer
• The circuit board, once removed, is
quite simple.
• Shown above are both the top and
bottom views of the circuit board.
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Disposable Camera Electronics
• The cover can be pried off.
• Once you remove the cover, observe how the
mechanical apparatus works (gears, shutter,
lens, springs, etc).
• You may find these useful in applications.
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Disposable Camera Electronics
• The Exploratorium recommends that you use
rubber gloves to avoid shock.
• Pry off the case using a screwdriver
• You should be able to set the shutter
mechanism using intact gears, etc.
• You can recharge the capacitor using the
charge button (or short across the pads).
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Disposable Camera Electronics
• While charging, you should hear a high
pitched whine. This is the oscillator
circuit.
• Fire the circuit by triggering the shutter –
the flash should go off.
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Disposable Camera Electronics
• The oscillator produces a time varying
voltage (sort of sinusoidal) from the
1.5V battery.
• A time varying voltage is necessary to
produce a high voltage using a
transformer.
• The high voltage output from the
transformer is rectified using diodes to
produce a high DC voltage to charge a
capacitor.
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Disposable Camera Electronics
• Once the high voltage (320V) capacitor
is charged, a small neon bulb will light
up.
• The charging is then stopped either
manually (you stop pushing on the
charge switch) or automatically.
• Then when the shutter is engaged, a
switch is closed which triggers the flash
lamp.
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Disposable Camera Electronics
• The flash lamp has three electrodes:
 Two connected across the high voltage capacitor
 One connected to a pulse transformer
• The third electrode provides a short very high
voltage pulse to ionize the xenon gas in the
flash tube. Once the gas is ionized, it will
carry current and discharge the capacitor.
The current from the capacitor provides the
energy for the high intensity light.
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Disposable Camera Electronics
• This is the circuit diagram for the
camera used in the lab.
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Disposable Camera Electronics
Iron Core
Coil 1
V1
Coil 2
V2
TRA NSFORMER
• A transformer is used to step up or step
down voltages and currents.
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Disposable Camera Electronics
Iron Core
Coil 1
V1
Coil 2
V2
TRA NSFORMER
• The secondary voltage V2 is equal to N
times the primary voltage V1.
V2  NV1
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Disposable Camera Electronics
Iron Core
Coil 1
V1
Coil 2
V2
TRA NSFORMER
• The turns ratio N is equal to the ratio of the
number of secondary windings to the number
of primary windings. For a step up
transformer this ratio is greater than 1.
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Disposable Camera Electronics
• You will also connect a trigger circuit in the
lab to make the camera flash when light
levels change. To operate the circuit, you
need 5V which is obtained from the battery
using a DC-DC converter.
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Disposable Camera Electronics
• The lab circuit again
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Disposable Camera Electronics
• Note that the wires from the protoboard circuit
must be soldered to the camera circuit.
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2 Minute Quiz
Name______Sec______Date______
• In the disposable camera circuit,
estimate the number of turns on the
transformer?
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Disposable Camera Electronics:
References
• Disposable Camera Electronics from
the Exploratorium
• Disposable Camera Dissection from the
Exploratorium
• Disposable Camera Lab from the
University of Washington
• Dissecting Disposable Cameras for
Parts
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Soldering
•
•
•
•
•
•
Your TA will show you how to solder.
Place parts mechanically first, if possible.
Apply enough heat.
Watch for cold solder joints.
How to Solder
How to Solder Like a Pro
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Cold Solder Joints
• Note that cold solder joints tend to form ball
shapes and do not make contact between
wires and pc boards
• Properly heated solder should flow
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Stun Guns
• Outer electrodes must come in contact with
attacker.
• Inner electrodes are closer together and thus
can spark at the high voltages used to deter
attackers.
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Stun Guns: What is Inside?
• This is what is inside a cheap stun gun
(provided by someone who likes to take
things apart).
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Stun Guns: What is Inside?
• Oscillator voltage is stepped up to a fairly
high voltage with a transformer.
• This is used to charge a capacitor through a
rectifier made with four diodes.
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Stun Guns: What is Inside?
• When the voltage across the capacitor is high
enough to spark across the gap, a short pulse
is created across the primary of a pulse
transformer and a large voltage pulse results.
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Stun Guns: What is Inside?
• The spark gap is simply the two pieces of
metal strip in a cross shape. A much larger
spark gap was used in the can crusher
discussed in lecture 7.
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Stun Guns: What is Inside?
• The first transformer is shown at the left.
• The pulse transformer is shown at the right.
Note that it is potted so that it can operate at
higher voltages.
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Tesla Coil
• Nikola Tesla – The father of AC and a
certifiable mad scientist, is one of the most
interesting electrical scientist/engineers who
has ever lived.
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Tesla Coil
• On this and the last slide is shown a
small lab scale Tesla Coil
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Tesla Coil
• The circuit has some similarities to the flash tube
and stun gun. Note the two transformers and the
spark gap. The resonance effect (L and C) also
plays a large role in making the voltage large
enough to cause sparks.
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Introduction to Engineering Electronics
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Tesla Coil
• This is Tesla’s Wardenclyffe Lab in Shorham
Long Island where he planned to use this
huge coil to transmit radio across the planet.
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2 Minute Quiz
ANSWERS
• True or false – proper electrical safety
procedures require that we always know
at least approximately what currents we
will encounter in a circuit.
TRUE
• # turns = Vo/Vin=320/1.5=213
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Where Will You See This Material Again?
• Circuits, Electronics, Components
 ECSE-2010 Electric Circuits
 ECSE-2050 Analog Electronics
 ECSE-2060 Digital Electronics
• Transformers
 ECSE-2100 Fields and Waves I
• Applications of Electronics
 Many classes
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