Recovering Components of Disposable Cameras, and
Basics of Flash Tube Circuits

1997-2010 by Brian Mork
Intrepid Creativity (InCrea)TM

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I think it's a waste to discard disposable cameras. It's very easy to extract your film from a disposable camera, send it off to get developed, and keep all the electronic and mechanical parts for free. By unsnapping plastic catches, the entire camera can be disassembled. This article tells you how. Once inside, the oscillator circuit to charge the flash provides an excellent chance to review or learn a super low cost oscillator flash circuit.  Click on a picture to see the full size image.

INTRODUCTION

The way a disposable camera is suppose to work is that you buy a cheap plastic camera with film already embedded inside. After taking all the pictures, you mail the entire package--film and camera--away to be processed. I thought it would be much better if I could mail away just the film, and keep the rest. The idea works! I've had experience only with the Kodak brand, but I imagine others are similar. Nonetheless, the specific instructions I give deal deal with the Kodak model. After taking the pictures, the film inside one of the disposable cameras is wound up inside a light-tight 35 mm metal canister, just like the ones you use in your more expensive camera. This metal canister can be removed and sent away for processing. It seemed goofy to me to send away the rest of the camera to arrive in some land fill somewhere. Aren't these components better used in the hand of creative experimentors? You bet they are! The last third of the article uses the circuit on the inside to learn about charge oscillators -- the basic building block of all switched power supplies.

STEP BY STEP DISASSEMBLY

To get your film out, and to take apart the other pieces, you have to first rip off the cardboard outer coating. When you do this, different cracks and crevices will be revealed. Don't go poking around inside yet because there is a very real risk of shock unless you know what you're doing.

Photo 1 shows my camera just after removing the cardboard casing. If you look carefully, you can see a cavity across the front bottom where I've removed the 1.5V AA cell that powered the flash circuitry. My battery still tested into the good range with one of those polymer heating strip testers that turn color when pressed onto both ends of the battery. Salvage number 1.

photo-camera-frontPhoto #1. Cut off or tear off the cardboard covering. The exposed film is wound up safely inside a 35mm canister just like a non-disposable camera. To retrieve it, you must remove the side of the camera, which is held on with four plastic clips. In order to access the two clips on the front of the camera, I had to cut away a flap of plastic extending toward the edge of the camera from the raised platic section around the lens. The two corresponding clips on the back side needed no pre-treatment.

From here on out in the dissassembly, don't touch any metal on or inside the camera. Potentially hazardous voltages could be present. Use a screw-driver-ish tool that is not metal--not conductive. For some of the catches, a toothpick might be the best tool. If you have to use a screwdriver, use one with an insulated handle.

There is a side panel that covers up the exposed film, which is rolled up in a light-tight 35mm canister just under the winding knob. To allow the side panel to slide away from the body of the camera, you need to lift up four catches, two on the front and two on the back of the camera. There was a molded piece of plastic covering the front two catches on my camera. I think the cover piece is there just for cosmetics, or to provide a finger grip while originally using the camera. It has no structural value, so I used a razor blade to slice into it and then bent it off. Release all 4 catches and slide the cover off.

Photo 2 shows the film cartridge visible just after removing the side panel. Reach inside with a fingernail and pop the film cartridge out. Set it aside for future processing. The back of the camera is the next panel to remove. It has two catches along the top, which are accessible from the back. Reach inside with your tool and push the catches sideways, away from the winder, toward the view finder window. There are also two bottom catches that are accessible from the bottom of the camera. Poke into the hole and push straight inward (upward into the camera body) to release these bottom catches.

camera-battery-removedPhoto 2. Release the four plastic clips holding the cover on the film canister. You should be able to obviously see the film canister, and simply slide it out the side. Remove the standard AA battery. As you'll see later, some large magnitude voltages are possible with the flash charge circuitry. You might as well remove the power source while you're poking around inside the camera. Even with the battery removed, there will be residual charges that could take hours to dissipate. Read the cautions elsewhere in the disassembly text.

Reach in the hole or recess and gently bend the plastic tab to release, keeping a little "pull away" tension on the clear piece. Once a catch is free, block it apart with a toothpick before going onto the other catches.

Next remove the top clear piece: two catches along the top rear edge, three along the top front edge. Save the piece. It has a concave, about ~1 cm2 lens. Remove four mechanical plastic pieces around the metal trigger mechanism for the shutter and the flash. The metal pieces around the shutter, including the shutter itself are part of the electrical discharge circuit. Assume they have several hundred volts on them unless you've measured otherwise.

Next remove the front plate. One catch is along the top front, one is along the left side rear, one is on the front face between the trigger mechanism and the where the winding knob was. The last one is down inside, almost in the exact center of the front of the camera; you'll have to reach it with a small bladed screwdriver.

The front lens will pop off. Save it. It is a concave, ~3/8" diameter lens with a focal length of about 1.3"

STOP! Read the next few paragraphs before touching anything inside the camera.

Photo 3 shows what you'll be looking at. The two larger through-the-board solder connections on the lower side of the visible circuit board are the main charging capacitor connections. The camera I disassembled had a 160uF capacitor, rated to 300 VDC. Shorting the two leads together made a !LOUD! pop and nearly welded the insulated-handle screwdriver I used to do so. Check the voltage across these leads with a voltmeter before getting your fingers on the circuit board and before further disassembly. If you'd rather not get a loud pop and arcing, position a 1Kohm resistor across the two solder connections (without touching the contacts or the resistor leads as you do this). Initially a current flow of up to 250mA may flow, but the RC time constant is about 0.16 seconds. Give it 10 seconds and you should be okay.

circuits-exposedPhoto 3. With the plastic view finder and front lens/casing removed, you can see the shutter internals and the solder side of the flash circuitry circuit board. CAUTION: The flash circuitry and the metal pieces of the shutter retain several hundred volts long after removing the battery.

Two red wires go from the circuit board to the shutter. The shutter itself is the "FIRE" button on the schematic shown below. When they touch, the flash will trigger. Don't play with this carelessly!! When I held down the charge contact, the metal of the shutter developed 250 VDC across them. The leaf metal charging contact developed a similar voltage. Several minutes after popping the flash and removing the battery, I still measured more than 100 volts on the contacts, due to residual charge retained on the main capactor after the flash tube extinguished.

Two catches will let the circuit board come out. Before totally removing the board, mark the metal battery clip with a permanent marker, so you'll know which way to put the battery in. I left my circuit board in the plastic shell for stability.

CIRCUIT ANALYSIS

The figure below shows a reverse engineered schematic for the flash circuitry. There are three major sections, each operable independent from the others: A charging oscillator, a blinking full-charge indicator, and a trigger.

Schematic

The Charging Oscillator
The charging oscillator consists of everything left of the 160 uf main charge capacitor. When a battery is in place, but the charge switch is not engaged, the collector sits at about 1.5 VDC, and the base sits at about 0 volts. When the charging switch is closed, current is pulled through the 220 ohm resistor and the last few coils of the transformer secondary. The transistor turns a little bit on, pulling the collector toward ground. Current in the primary of the transformer builds up. This is signified by the arrow on the schematic. This positive feedback induces more current in the secondary, which pumps positive charge toward the bottom of the inductor (as shown on the schematic). VB rises to an even higher voltage. When VB tries to swing up above about 0.7 volts, the transistor turns hard on, as seen on the dcbase.gif

base waveform. Any attempt to go higher is "shorted" to ground by the base-emitter diode inside the transistor. When the transistor is full on, the collector is essentially shorted to ground (notice the
 
dccollec.gif


voltage at the collector hits a minimum of zero volts, but doesn't go below ground), and the current flow through the
 primary builds up to a maximum. It might help to take a look at all the

 dcphase.gif

relative phases of all transistor and transformer terminals. Notice the polarity dots on the switching tranformer are shown on opposite ends because they indicate in-phase voltage rather than the current. As a high voltage is applied to the top of the primary (relative to the bottom of the primary), a high voltage develops on the bottom of the secondary (relative to the top of the secondary). For the way the transformer is drawn on the schematic, voltages are associated with "opposite" ends, while current is associated with "same" ends.

Since the base of the transistor is clamping the bottom end of the transformer secondary a diode drop above ground, the voltage that develops across the secondary pushes the resistor tap and the cathode of the diode negative, significantly below ground. The


dcb-and-r.gif

voltage on the bottom side of the 220 ohm resistor is driven negative by about a volt, proportional to its tap position on the secondary coil. This is about 2.3 volts below the power supply, implying a maximum current of 2.3/0.22 => 10.5 milliamps going through the resistor. The

dccathod.gif

voltage at the top of the secondary drops precipitously toward -300 VDC. The pull of current in the top side of the secondary is conducted by the forward biased diode, and charge is pulled off the top of the capacitor (yes, once more, I'll note that really electronics are going through the diode and being deposited on the top of the capacitor). The + charge depletion on the top of the 160 uf main capacitor causes a voltage drop. In the relative phase waveforms, it is during the flat portion of the waveforms that the business of charging the main capacitor occurs.

Because of the positive feedback provided by the transformer, the circuit oscillates. If you use the horizontal axis numbers off the oscilloscope plots, you'll see the repeated pattern occurs at about 12,000 Hz. This is the high pitch squeal heard when charging most photography flash equipment. Note that during the other half of the circuit oscillation, voltages reverse polarity: the cathode voltage builds up to about +300 VDC, the base voltage to drop to about -4.5 VDC, and the collector overshoots the 1.5 VDC power supply, eventually getting to about 5 volts.

The Blinking Indicator
With voltage building on the main capacitor, current leaks through the 3.9 M-ohm resisotor and charges up the 0.002 uf capacitor. As the 0.002 uf capacitor is charged, the voltage magnitude across the 1 M-ohm resistor and the neon bulb increases in magnitude. When it reaches about 300 volts, the bulb arcs across its internal electrodes. The ionized cloud of neon gas has near zero resistance, so the bulb quickly causes charge to drain off the capacitor through the 1 M-ohm resistor. The voltage magnitude drops toward 20% of its peak value (1/4.9 voltage divider). Eventually, it reaches a level where the ionization of the neon gas can not be maintained. The light goes off. With the bulb's resistance returned to essentially infinity, charge builds up again on the 0.002 uf capacitor. The cycle repeats over and over again,

 dcn-and-c.gif

oscillating the voltage.
The Trigger
If you close the "flash" contact, the entire voltage of the main capacitor is shorted to ground through the 3.9 Mohm resistor, and the primary of the flash transformer. The flash transformer is a very high ratio voltage step-up transformer that uses the current pulse through the primary to provide a momentary voltage pulse over 1,000 volts. A few of the gas molecules in the strobe bulb are ionized, and immediately attracted to the electrodes, which have just less than 300 volts across them. As the ions go screaming toward an electrode, they collide with other gas molecules, knocking electrons of them. Very quickly, there is huge number of atoms ramming into each other, releasing Bremsstrahlung radiation -- i.e. a flash of white light. When the voltage on the main capacitor is too low in magnitude to sustain the ionized discharge, the flash extinguishes.

CONCLUSION

Hopefully you'll be able to use the above instructions to gain the confidence to not dispose of your disposable camera. I hope the circuit analysis and waveforms give you understanding you didn't have before. I'm particularly interested in how the photos and diagrams and figures worked for you. Let me know what did and didn't work. If you'd like to read more, and find additional resources, check out Scott K's camera fixing community website.

This page is maintained by Brian Mork, owner & operator of Increa TM. It was last modified June 2011. Suggestions for changes and comments are always welcome.  E-mail is the easiest way.

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