"Amazing Electronic Fact: If you scuffed your feet long enough without touching anything, you would build up so many electrons that your finger would explode! But this is nothing to worry about unless you have carpeting." - Dave Barry, "What is Electricity?"

The problem

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.

Part 2 discussed a particular CD firing box that can be built from a modified FunSaver circuit board. That particular arrangement uses a gas discharge tube as an electronic switch to control the firing energy.

This final part discusses the use of a mechanical switch to control the application of the firing energy 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 and 2s of this series before proceeding further.

The mechanical switch version

Sacrifice a "Kodak FunSaver 35 with Flash" and perform the basic modifications that are common to both firing boxes.

Remove the xenon flash lamp and trigger transformer T2. Use them for another project some time, or mail them to me and I'll reuse them some day. Remove the trigger wires that went off to the camera shutter. We are interested only in the energy that is stored in C1.

Obtain a SPDT momentary action switch that is capable of switching high voltage and current (see the section entitled "A big honking switch"). This will be the new S2. Hook the pole to one firing terminal. Hook the NO throw to the positive side of C1 through an 8-ohm 20W resistor. Attach the other firing terminal to the negative side of C1. Also attach it to a 27-ohm 1/2-W resistor, the other end of which goes to the NC throw of S2.

Firing through the 8-ohm resistor increases the resistance through which C1 discharges. This, in turn, increases the time constant, providing a longer energy pulse to the electric matches at the end of the shooting wire. It also makes life easier on the firing switch.

The charge "ready" indicator circuit could be simplified, if you so desired - I didn't bother. You might try removing C2 and replacing the R2 and R3 combo with a single resistor calculated such that IL1 comes on when C1 hits firing voltage.

These changes are summarized in figure 4.

Figure 4 - Mechanical switch variation of CD firing box.

The one switch that must be capable of switching large amounts of energy is the "fire" switch in the mechanical switch version. There are plenty of switches at Radio Shack and other stores that won't do the job. Light duty switches are likely to fail, sooner or later. If you are lucky, the failure will be due to metal vaporized at the contact point and the switch will simply refuse to conduct any more. If you are unlucky, the contacts will weld themselves closed and when you press the "charge" switch, energy will go into your electric matches. This is a Bad Thing. So the switch must be rated for high current and voltage.

We can come up with a worst-case rating for the switch by assuming 346V fired into 10 ohms.

I = V / R
where:
I = current in amps
V = electromotive force in volts
R = resistance in ohms
Thus:
346 / 10 = 35A
I haven't noticed any switches at Radio Shack rated 35A at 350V. Your best bets are auto parts stores and places that carry appliance parts.

But do you really need that high a rating? The capacitor discharges rapidly, and you don't have that 346 volts for long at all! Maybe we can get away with something cheaper.

Some of the strongest inexpensive switches that I have found are snap-action switches of the type that has become synonymous with the name "micro switch". I bought one for $.76 at a surplus store marked as follows:
Matsushita
AV5450542H2
21A 250 VAC
1 HP 125 VAC
2 HP 250 VAC
5A 120 VAC "L"
The 21A looks impressive compared to a lot of other cheap switches, but we are looking for 35A and are using a voltage that's 40% higher. Worse yet, the DC ratings on switches are less than the AC ratings. The only up side is that I don't plan to fire into dead shorts very often. I suppose that this switch will last a bit longer than a lot of other switches, but even its tungsten contacts will melt some day.

So I used that switch anyway. It's an experiment. When it fails, I'll let you know how many shots I got out of it.

Conclusion

Capacitor Discharge firing might or might not suit your purposes, but by converting the strobe circuitry from a disposable camera you can try it out at little expense.

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

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