Kodak camera. This has a number of very clever features. Firstly, the circuit is an automatic charger.
It charges the 120u electrolytic then switches off.
This increases the life of the battery considerably as the circuit is only powered for 15 seconds or so for each picture and there is no need for an on-off switch.
HOW THE CIRCUIT WORKS
The Kodak circuit is fully automatic. The “start” button is pressed and this turns on an NPN transistor via a 2k resistor. The circuit begins to oscillate and the voltage from the feedback winding gets superimposed on the DC voltage from the start switch to keep the circuit oscillating. The button can now be released and the circuit will keep operating.
Each time the feedback winding produces a pulse, it charges the 100n capacitor and this puts a negative “set” on the base of the second transistor. If this negative voltage gets too high, the pulse from the feedback winding will not be able to turn the transistor ON and the circuit will stop. This is fully discussed in another article “Making Your Own 3v Inverter.” The 100n is constantly being discharged by the first transistor and this transistor is turned on via a pulse from the high-speed diode. As the main storage electrolytic gets charged, the pulses entering it get smaller and smaller. Eventually the pulses are so small that they do not pass through the 330p capacitor and the first transistor is not turned on. This causes the 100n to charge negatively and after a short time the oscillator circuit is prevented from beginning a cycle – and it stops. The 120u is fully charged and some of the high voltage is bled into the neon lamp circuit to illuminate the lamp. This lamp only takes a fraction of a milliamp to create a red glow and the operator of the camera is informed that the camera is ready for use.
The 33n storage capacitor is charged and when the trigger switch is activated by the shutter, the energy from the 33n is passed into the primary of the trigger transformer to create a very high voltage across the ends of the flash tube to ionise the gas within the tube and allow it to flash.
If you study the circuit in the off state, you will find absolutely no current paths and thus the circuit consumes no current when at rest. The first transistor is kept off via the 330k and 220k. This keeps the second transistor off and the second transistor keeps the third transistor off. The LED is reverse-biased when the circuit is at rest and thus no current is consumed.
The repeat-flash components can be added to the Kodak circuit, exactly as above.
There is probably a dozen or more “tricks” in the design of this circuit that is not evident on a simple circuit diagram. You have to have the PC board in your hand to see how the flash tube has a shield around the rear surface so the voltage from the trigger transformer is able to ionise the gas in the tube.
The 330p detecting the pulses from the charging circuit is a high voltage type and the 120u electrolytic is a special “photo” type that can be discharged very quickly without being damaged.
The rest of the tricks lie in the design of the oscillator transformer and trigger transformer.
If you are going to design a circuit similar to this, you must start with a circuit that works and change one component at a time. By choosing a value higher or lower you will be able to determine how much effect it is having on the circuit.
This technique applies to all forms of circuit design. Start with something that is guaranteed to work and make small modifications
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