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.
|Photo #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.|
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.
|Photo 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.
|Photo 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.
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
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
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.