Introduction to Engineering Electronics

1
Lecture 9 Disposable Camera

• Learning Engineering Using Inexpensive Products
  That May Be Found Around The House
• Or Apartment or Dorm Room

2
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.

3
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.

4
Electrical SafetyCurrent Physiological Reaction
(60 Hz)
5
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
• Ohms Law can save your life and the lives of
  others.

6
Electrical SafetyHow 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.

7
Electrical SafetyHow 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.

8
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

9
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 QuizName_____________
Sec___

• True or false proper electrical safety
  procedures require that we always know at least
  approximately what currents we will encounter in
  a circuit.
• In the disposable camera circuit, how long is the
  wire for the primary winding? How long is the
  wire for the secondary winding?

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Disposable Camera Electronics

• This is the circuit you will be building in the
  lab using a disposable camera.

12
Disposable Camera Electronics

• 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.

15
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.

22
Disposable Camera Electronics

• This is the circuit diagram for the camera used
  in the lab.

23
Disposable Camera Electronics

• A transformer is used to step up or step down
  voltages and currents.

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Disposable Camera Electronics

• The secondary voltage V2 is equal to N times the
  primary voltage V1.

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Disposable Camera Electronics

• 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.

26
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.

27
Disposable Camera Electronics

• The lab circuit again

28
Disposable Camera Electronics

• Note that the wires from the protoboard circuit
  must be soldered to the camera circuit.

29
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

32
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.

33
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.

36
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
  6.

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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.

38
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

40
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.

41
Tesla Coil

• This is Teslas Wardenclyffe Lab in Shorham Long
  Island where he planned to use this huge coil to
  transmit radio across the planet.

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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