|Lights, Camera, Action!|
Transients in Circuits
|The Battery||The Capacitor||The Flash Circuit|
Please review the Disposable Camera Dissection lab exercise by Kelin Kuhn and Blake Hannaford as part of the Consumer Electronics Design course at U. Washington. This site includes pages on "How Does the Flash Work?" as well as Overview and Detailed descriptions of the camera dissection. A recent addition is An Examination Of The Flash Circuitry In A Disposable Camera
I have borrowed liberally from the above material in designing this laboratory exercise.
Note to Instructors
Disposable camera with flash.
BK Precision 2120 Dual Trace Oscilloscope with x10 scope probe.
Two Keithley 179 digital multimeters.
Pasco digital timer.
Pair of banana jack-to-minigrabber leads for multimeter.
Box of small tools.
Develop basic skills in probing and handling circuit boards.
Practice the art of mechanical disassembly in a non-destructive manner.
Investigate exponential decay and build-up in a real-world circuit.
Note to Laboratory Instructor
This exercise is appropriate for investigation by a larger group of three or four students. Students should fully document their dissection and investigation. This exercise is an excellent opportunity for a formal lab report and two weeks of laboratory should be dedicated to this exercise.
The inner workings of a disposable flash camera is investigated here by dissecting the camera and making a few simple electrical measurements. Voltage and current transients are observed and studied in some detail. The energy transfer between battery and flash unit is also considered.
As with many electrical devices, the disposable camera poses a potential shock hazard. Voltages in the flash circuit may approach 350 V and can remain at high voltage even with the battery removed. We ask that you observe the following guidelines:
Disassemble the camera to the point where the battery may be removed. This may be done by removing the external paper packaging and then removing the rear plastic enclosure by using a small screwdriver to spring or pry open the several small tabs.
Document carefully the sequence of steps your group uses to disassemble the camera; you will be expected to reassemble the camera back to working order by the end of this exercise. Sketch the camera as it is disassembled, showing its parts and their interactions in some detail to assist in the reassembly. Disassembly and reassembly instructions should be part of your final report.
With the rear enclosure removed, you should be able to study and analyse the workings of the shutter switch and film winder mechanisms.
Next remove the front enclosure. Carefully check the voltage across the capacitor, the element located under the camera lens. If if is not fully discharged, use a screwdriver with insulated handle to discharge the capacitor by shorting its leads by laying the screwdriver blade across them. Wear your saftey glasses!
Before proceeding with any electrical measurments, continue the disassembly of the camera. Pop off the lens assembly and examine the shutter mechanism. Determine how the shutter switch triggers the flash circuit.
One way to determine the current flowing through the battery while it is charging the flash circuit is to monitor its terminal voltage. To make this a quantitative tool you should first rig a circuit to measure its internal resistance. Connect a resistor across the battery that will lower its terminal voltage to 60-80% of the emf. Measure the current in the simple circuit and the terminal voltage and extract the value of the internal resistance for later use. Estimate the uncertainty in your value.
The capacitor in this camera is the green cylindrical object located below the lens assembly; it has a value of 125 uF. Put the multimeter into its highest voltage setting and carefully connect minigrabber leads to each lead of the capacitor -- the circuit board may need to be lifted slightly to accommodate the connections.
Use an insulated probe (such as the eraser on the end of a pencil) to close the flash charging switch. Note the ultimate voltage acquired by the capacitor.
Flash the lamp by closing the switch below the lens/shutter assembly. Note the post-flash voltage of the capacitor.
Determine the effective time constant of the charging of the capacitor. Develop your own procedure using the digital stopwatch provided. Estimate the accuracy of your determination and repeat the measurements until reproducibity is achieved.
The capacitor tends to discharge slowly when the flash charging switch is open. Determine the time constant of this discharge and evaluate the resistance that should be used to model the leakage of the capacitor.
The Flash Circuit
Connect the scope probe across the battery when installed in the camera; the probe should be used in the x10 setting to protect the scope against an accidental encounter with a higher than expected voltage. Choose a vertical sensitivity so that the vertical distance between ground and 1.5 V covers most of the scope grid.
Record the waveform of the terminal voltage as the flash circuit is energized. Determine the maximum current provided by the battery by observing the lowest value of the terminal current. Assuming that the current decays exponentially as the capacitor is charged, estimate the total charge deposited on the capacitor. Compare this to the charge determined from the measurement of the capacitor's voltage and attempt to explain any difference.
What is the frequency of the periodic waveform associated with current flow through the battery? Try to determine the frequency just after the flash is fired as well as the frequency of switching when the capacitor is fully charged. Does it explain the "squeal" that you hear each time the capacitor recharges?
Please guarantee that the capacitor is uncharged and reassemble it for later use.