An Examination Of The Flash Circuitry
In A Disposable Camera

Introduction

In the winter of 1996 we studied the design and manufacture of disposable cameras in Consumer Electronics, taught by Professor Blake Hannaford. We studied the Fuji Quicksnap, a camera with a built-in flash bulb. To take a flash picture, the user of the Quicksnap must first press and hold a button on the face of the camera to charge the flash circuit's large capacitor. The user knows when the flash is fully charged when an indicator bulb begins blinking. At this point a flash picture can be taken.

The indicator bulb did not look like a light-emitting diode (see Figure 1), so we initially speculated that the blinking of the indicator bulb was a feature inherent in its physical properties, and an increasing DC voltage applied across its terminals would result in an increasing flash frequency. As we continued our dissection of the camera, we discovered that the bulb's surrounding circuitry caused the light to blink.

Figure 1 -- The flash charge indictator bulb

In this report, we will discuss first the physical operation of the indicator light bulb, and then explain the circuitry that causes it to blink.

The Cold Cathode Tube

Cold cathode tubes can be used as voltage stabilizers, indicator tubes, and flash tubes. Another well known application of the cold cathode tube is the Geiger-Mueller tube for nuclear radiation detection. The indicator bulb in our camera consisted of a gas-filled tube with two electrodes. Most types of cold cathode tubes are filled with an inert gas at a pressure much below one atmosphere. Figure 2 shows a cross section of a cold cathode tube.

Figure 2 -- A cross section of a cold cathode tube

The atoms present inside the tube can be ionized by high speed impact with other particles, as shown in Figure 3. A particle hitting an atom results in a positively charged ion and at least one negatively charged electron. Some of the gas atoms have already been ionized by the passage of cosmic rays and other radiation which is naturally present in every environment.

Figure 3 -- Gas atoms are ionized by high-speed impacts

When a voltage is applied across the electrodes of the tube, an electric field builds up and accelerates the electrons towards the positive electrode and the ions towards the negative electrode, as shown in Figure 4.

Figure 4 -- Particles are accelerated towards the electrodes

If the electric field is strong enough, the moving ions and electrons cause ionization of more atoms in the tube. Some of the ions and electrons recombine on their way to the electrodes. Owing to their greater mass, the ions move slower than the electrons. This results in an excess cloud of positive ions at a short distance from the cathode. The cloud presents a high concentration of positive charge, resulting in a very strong electric field near the cathode. Because of this strong electric field, most of the potential difference applied to the two electrodes appears near the cathode.

Figure 5 -- Light is emitted

If the voltage applied to the tube is high enough, the field strength excites the ions, resulting in the emission of light, as shown in Figure 5. This voltage is called the striking voltage. Figure 5 shows light emitted only from the positive ion cloud, but a visible glow can appear near both electrodes. The glow near the cathode is called the negative glow and the glow near the anode is called the positive glow. An interesting phenomenon occurs as the tube size gets smaller. As the electrodes get closer together, the negative glow remains the same size, but the positive glow is forced to shrink. Eventually, if the tube is small enough, the positive glow is nonexistent. In our camera's indicator bulb, we noticed a glow around only one of the electrodes, which would indicate that electrodes were sufficiently close that the positive glow did not exist.

The cathode tube exhibits nonlinear behavior. Figure 6 illustrates the voltage across a tube with respect to current flowing through the tube. Our indicator bulb operates in the "normal glow" region in the graph.

Figure 6 -- Nonlinear behavior in the cold cathode tube

When the voltage has reached the striking voltage, V_s, the bulb becomes very conductive and the current value jumps from point D to point E in Figure 6. The normal glow region is from point E to point G, so the bulb begins to glow. At this point, the current is self-sustaining. Once point E is reached, the voltage may decrease to the operating voltage V_m (point F to point G) without extinguishing the glow of the bulb. The current in the region of normal glow is in the range of milliamps.

If the voltage falls below V_m, the bulb becomes much less conductive and the current value jumps from the normal glow region to point C. As a result, the current is no longer self-sustaining and the bulb ceases glowing and shining brightly with eternal glory, Hallelujah! (Is anyone still reading at this point?)

A good explanation of different types of discharge can be found in J.B. Dance's Cold Discharge Tubes.

The Flash Circuit

The Indicator Bulb

Having described the way light is produced in the cathode tube, we now discuss the supporting circuitry that causes the Fuji Quicksnap indicator bulb to blink.

The camera's entire flash circuit is shown in Figure 7. The circuit mainly consists of

	an oscillator with step-up transformer, TR1,
	a rectifier, D, and capacitor with large capacitance, Cd,
	a flash tube, FB,
	a transformer, TR2, to trigger the flash, and
	an indicator bulb, DB.

Figure 7 -- The flash circuit

Switch S1 is the button that the user presses and holds to charge the flash. While switch S1 is closed, the battery B drives the oscillator to produce an AC current through transformer TR1. Since the secondary coil has many more windings than the primary coil, the secondary voltage of the transformer is much larger, at 314V AC. This voltage is rectified through the diode D and charges the large capacitor C_d. It also charges the small capacitor C_i through resistor R_c.

Because R_c is large (5 Mohm), very little current flows through it until C_d is charged sufficiently. As C_d charges, increasing current flows to charge capacitor C_i. At this point the blinking cycle of the indicator bulb DB begins. The following steps describe the behavior of the circuit for one "blink" of the indicator. Refer also to Figure 6.

	Initially, the indicator bulb DB is mostly nonconductive, and no light is emitted. Capacitor Ci is charging, so the voltage Vc is increasing. ( Figure 6: point B)
	When the voltage Vc (across capacitor Ci) reaches the striking voltage Vsof the indicator bulb DB, the indicator bulb becomes conductive and emits light. ( Figure 6: point E to point F)
	The indicator bulb draws current as it emits light, so capacitor Ci discharges through DB and resistor Rd. Voltage Vc begins to decrease.
	Voltage Vc falls below the operating voltage Vm of the indicator bulb, so the indicator bulb becomes mostly nonconductive again, and light is no longer emitted. ( Figure 6: point F to point B)
	Capacitor Ci begins to charge through resistor Rc due to the high voltage still present on capacitor Cd. The cycle begins again.

The Flash Bulb

Like the indicator bulb, the flash tube is also a cold cathode tube. However, instead of operating in the normal glow region of Figure 6, the flash tube operates in the arc discharge region. Notice that a high voltage is needed at point H to enter the arc discharge region. This is accomplished with the help of a third electrode in the flash bulb called the trigger electrode.

The charge on capacitor C_i is used to achieve a high peak voltage on the trigger electrode. The following steps describe the flash discharge.

	Switch S2 is connected to the camera's shutter, synchronizing the flash to the time the picture is taken. When switch S2 is closed the capacitor Ci discharges through the primary coil of the transformer TR2.
	The secondary coil of TR2 has many more windings than the primary coil, so it causes the desired high voltage peak at the trigger electrode. The trigger pulse starts an avalanche reaction which causes all atoms in the flash bulb to ionize, resulting in a large number of charge carriers.
	The carriers allow the capacitor Cd to discharge through the flash bulb.
	This arc discharge can be observed as a bright flash of light.

Conclusion

In this study of the Fuji Quicksnap camera, we learned how cold cathode tubes work and how they are practically applied as blinking indicators and flash tubes. For further discussion of the physics involved in cold cathode tubes, see J. B. Dance's Cold Discharge Tubes. For general discussion of a number of useful circuits, see Fisher and Gatland's Electronics - from Theory into Practice.

Bibliography

Dance, J. B. Cold Discharge Tubes. Life Books Ltd., 1967, London.

Fisher, J. E. and H. B. Gatland. Electronics - from Theory into Practice. Pergamon, 1966, Oxford.

Weston, G. F. Cold Cathode Glow Discharge Tubes. Life Books Ltd., 1968, London.