"The combination of high energy and sudden release thereof make CD circuits as tricky to handle as flash powder." - D.G.

Synopsis

Part 1 of this series broached the subject of converting the strobe circuitry from a disposable camera into a Capacitor Discharge firing box. It discussed the disassembly of a "Kodak FunSaver 35 with Flash", presented a reverse-engineered schematic, and went over the theory of operation. For some strange reason, a lot of space was spent reviewing how gas discharge tubes work.

This installment discusses a CD firing box that can be built from a modified FunSaver circuit board. This particular arrangement uses a gas discharge tube as an electronic switch to control the firing energy supplied to the electric match.

Safety precautions

The combination of high energy and sudden release thereof make CD circuits as tricky to handle as flash powder. Know and follow the precautions appropriate for high energy electronics.

Please read and understand all of part 1 of this series before proceeding further.

Circuit modifications

Now that we understand how the original circuit works, we will consider three sets of modifications. Two of the sets of modifications produce distinctly different CD firing boxes. One uses a mechanical switch to transfer the firing energy to the electric matches, the second uses a gas discharge tube as a switch. Both will be presented because each involves tradeoffs. The switch version delivers nearly all of the stored energy to the electric matches. In return, it demands a beefy switch. The tube version does not require much of a switch, and could even be adapted to firing by computer. In exchange, a substantial amount of energy will remain in C1 after the shot.

The third set of modifications applies to both versions of the CD firing box and is discussed in the remainder of this section.

Start by making sure that C1 is really still discharged by putting your clip leads and resistor across it. Measure the voltage across C1 until it reaches zero. Then solder a 1M resistor across it. This is a bleeder resistor to remove the charge when the unit is not in use. It is a rather poor bleeder, taking roughly 17 minutes to discharge C1 from a full charge to a reasonably safe level of 5V. Since the discharge rate of a capacitor is exponential, getting to zero takes an awful long time.

Note that the original circuit never actually disconnected the battery from the primary of T1. It leaves the battery in circuit at all times and puts "charge" switch S1 in series with base drive of Q1. I feel a lot more comfortable with positive, mechanical switching of primary power to a project like this.

In order to accomplish this, find the cheesie strip of springy metal that forms S1 on the non-component side of the board. Desolder it and replace it with an insulated jumper.

In the sample FunSavers that I used, the original circuit charged C1 to 343 volts. The capacitor is only rated for 330V. This makes me a little nervous. You might want to upgrade that capacitor or take steps to make sure that it doesn't charge too high.

Desolder and discard the two metal strips that form the battery holder. Get a battery holder of reasonable quality and a SPST NO pushbutton and put them in series where the strips once were. The short strip nestled in the cutout in the circuit board is soldered in two places, one of which is purely for mechanical support. Solder the new positive wire to the old connection that actually goes somewhere. The long strip extending out beyond the circuit board is soldered at two places, which are electrically connected. You can connect the new negative wire to either.

The "charge" switch operates at a low voltage and current. Almost anything will do there.

While you are installing a decent battery holder, you might want to use a larger power source, such as a "C" or "D" cell. This will provide more current and should shorten the charge time. But do not increase the voltage. Doing so would increase the output of the inverter, probably stressing C1 and other components beyond their limitations. In fact, even providing a better 1.5V cell increases the charge level. According to my tests using fresh Duracell brand cells:

	AA cell	C cell	D cell
0 to 300V charge time	14 sec	12 sec	9 sec
charge after 1 full minute	341 V	345 V	357 V
charge after 2 full minutes	343 V	346 V	358 V

Select your new power source according to your lust for short charge time and how far above the rated voltage you are willing to push a 330V capacitor. Over stressed capacitors have been known to explode.

One of the FunSavers that I dissected would sometimes not start to charge when power was applied. Perhaps the "switch in the base drive" trick makes it easier for the oscillator to start up. If this continues to be a problem, I might add a 22nF capacitor between the base and emitter of Q1 to give it a kick. I have seen such a capacitor on another gentleman's reverse-engineered FunSaver schematic and the circuit board is equipped to accept such a part, although my units did not include it. This particular inverter oscillator seems to be quite sensitive to stray capacitance in the circuit. When running on a test bench, with clip leads here and there, it sometimes failed to oscillate, or oscillated in such a way that charging time was in the order of minutes. Shortening the wiring helped a lot.

The half-wave rectification in the circuit cries out to be replaced with full-wave rectification. This should cut charge time in half. I've tried it, and it doesn't work well. The inverter circuit is very touchy and with this change either fails to oscillate, or oscillates at too high a frequency. I don't doubt that the circuit could be modified to overcome this problem, but rather than wade into that swamp, I'll simply note that it isn't as easy as replacing D1 with a bridge rectifier.

Either version of the CD firing box will need terminals to which the electric matches are connected via very long wires. Nice 5-way binding posts look classy, are flexible, and cost a bit of cash. Spring-loaded speaker terminals are inexpensive and convenient. You choose.

On shunting

A shunt is a deliberate short circuit imposed to keep stray electrical energy from unexpectedly igniting an electric match. Both variations on this CD firing box use a resistive shunt rather than a dead short.

The closer a shunt is to the electric match, the more likely it is to be effective. When a shunt is far away from the match, there is the possibility that the wire in between will act as an antenna, picking up induced energy. Shunting at the firing box with a dead short might actually complete the circuit, sending energy through the electric match.

I have chosen to shunt with a small resistor, in the hopes that it will reduce the current that might flow as the result of induced noise. This practice is open to debate.

I am certain that A.F.N. would welcome a thoughtful article on this subject.

The tube version

Disconnect the wire that goes from the flash lamp to the negative side of C1. Hook that side of the flash lamp to one of the firing terminals and the other firing terminal to the negative side of C1. This puts the electric matches in series with the flash lamp. If only we could get the tube to start ionizing, it will start to conduct, pulling most of C1's charge through the electric matches and the flash lamp.

The photo strobe trigger circuit remains largely undisturbed in this version of the firing box. Just replace S2 with a SPDT momentary-action switch, using one pole and the corresponding NO throw.

The other pole of the new S2, and his corresponding NC throw, is used in series with a 27-ohm 1/2-W resistor to shunt the firing terminals.

The "fire" switch in the tube version wants to be a little better than just any switch from the junk box, since it is switching 300 volts dumped out of the trigger capacitor, but it need not be a brute.

The circuit will function using the existing flash lamp to switch the power to the electric matches. You might consider replacing that lamp, though, with a quench tube.

A quench tube is yet another flavor of gas discharge tube. Like the flash lamp, it has three electrodes, two of which accept a potential and a third trigger electrode. In fact, a quench tube looks like a cross between a short, fat flash lamp and a NE-2 lamp. The true difference between a quench tube and a flash lamp is that the quench tube is optimized to conduct electricity instead of produce light when it is turned on.

The quench tube is used in photographic strobes that want to illuminate the scene with only as much light as is necessary for correct exposure. Such strobes measure the light returning to the camera and determine when there has been enough. But there is still energy in the storage capacitor and the lamp will continue to turn that energy into light until it falls below the turn-off threshold. So the quench tube is placed in parallel with the flash lamp and storage capacitor. When the strobe decides that the scene has had enough light, it triggers the quench tube, which shorts out the capacitor, wasting enough energy so that the voltage falls below the flash lamp's turn-off threshold and it goes out.

Since a quench tube is better at conducting electricity than a xenon strobe tube, more juice will get to your electric match if you replace the xenon tube for a quench tube.

All of these changes are summarized in figure 3.

Figure 3 - Tube variation of CD firing box.

The trade-off of this version is that electricity will stop flowing through the electric matches when the xenon strobe tube stops conducting. With this particular unit, that's around 38 volts.

The energy stored in a capacitor is:
E = 1/2 C V2
where:
E = energy in joules
C = capacitance in farads
V = electromotive force in volts
Capacitors are commonly rated in microFarads, using the symbol uF. A microfarad is 1x10-6 farads.

When the strobe is fully charged with a C cell, the energy storage capacitor holds:
.5 * (160uF * 1F/10-6uF) * 346V2 = 19J
After a shot, the capacitor holds:
.5 * (160uF * 1F/10-6uF) * 38V2 = .12J

The quench tube that I tested kept conducting down to 28 volts. After a shot with the quench tube, the capacitor holds:
.5 * (160uF * 1F/10-6uF) * 28V2 = .06J
Replacing the flash lamp with a quench tube squeezes an extra .06J out of the circuit. That's less that a third of one percent. Unless you have a quench tube or two sitting around the house, like I did, it's probably not worth the effort.

Please be aware that these numbers represent the energy expended from storage capacitor C1. The electric matches don't get all of that. You can see where a lot of the energy is going; it is turned into light by the flash lamp. After all, that's what a flash lamp is designed to do.

So how much energy actually does get to the electric matches? Several different experiments could measure it. The high tech way is to fire the CD firing box into a power resistor, use an oscilloscope to capture the voltage across the resistor and integrate it. The low tech way is to fire into a power resistor and note the increase in temperature. That's grist for some other article.

Continued...

The next installment in this series will present a design using a mechanical switch to apply the firing energy to the electric matches.

• CD Firing Box From Salvaged Parts - part 1, Basics
• CD Firing Box From Salvaged Parts - part 2, Tube Version [this page]
• CD Firing Box From Salvaged Parts - part 3, Mechanical Switch Version

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