Pyrotechnics/Rocketry Ignition Systems

Basic ten-line ignition box.
Wiring inside box.
  The connectors for the cables to the field stations are 6-pin relay sockets
and male plugs.  Five are +12 V and one is ground.  The cables are 100 feet or
less and 18-gauge.
Basic system circuit.
  The diodes shown are all LEDs.
Launch pad continuity tester.
  There is an LED continuity tester at each field station and an LED on the
control box lights up when the field station LED lights up. Only one line can
be set to test at a time. The launch official tells the modeller what station
to use and flips that switch to Test. The modeller then goes out to that
station, prepares the rocket and connects the ignitor to the clips.  The field
continuity tester will show if the ignitor is good by the LED coming on, but
there may also be a short.  If the LED doesn't come on, the clips and ignitor
can be checked for the problem, or the modeller may be at the wrong station.
When everything is good, the control box switch is then put on Safe. The
control box switch can then be put on Test for other stations when other
modellers are ready.
                   HIGH-AMP TEN-LINE SYSTEM
The control box.
Wiring inside box .
The field/battery box.
Schematic diagram of the system.
   This system uses reliable solid-state devices to ignite fireworks, model
rockets or stage pyrotechnics. Each line can draw up to 30 amps.  Ignitors
draw from 6 to 15 amps cold inrush current. (See chart below.) So from 2 to 5
ignitors could be on each line and all ignite instantly if the battery charge
and capacity is adequate.
   There is a continuity-test LED for each line. However, if several ignitors
are paralleled on one line, the ones that are good and the ones that are bad
cannot be determined. So they need to be tested individually before twisting
their wires together. And continuity cannot be checked from the control box.
   There is an On-Off keyswitch, a red On LED, a selector switch for each
line and a momentary firing button on the control box. The meter reads battery
voltage as soon as it's connected out at the field box. The power needs to be
on only when checking continuity and when firing.  At all other times power
should be off because the field box relay draws 2 amps. But no ignitor can
burn unless the keyswitch is On, its line is selected (switch up) and the
firing button is pressed.
   The cable going from the control box to the field box needs to have at
least 14 conductors if 10 device lines are to be available. There is a ground
wire, a +12 volt wire, a field box relay activation wire, a wire that has +12
volts on it only when the field box relay is energized and one wire for each
of the ten ignition lines. There could be as many as 25 ignition stations if
there were 26 MOSFETs on the field box printed circuit board instead of 11,
since the firing button can switch up to 25.
   The field box contains a car battery, a car starter relay, the printed
circuit board, ten leads to the ignitor lines, a power-on switch for testing
continuity in the field and an On buzzer. If any line LED has continuity, it
means either that the clips are connected properly and the ignitor is good,
or that there may be a short.
   Turning on the keyswitch at the control box closes the starter relay in the
field box.  The gates of the MOSFETs are held low by 10K ohm pull-down
resistors when not pulled high to fire, so no current can flow  through them.
Pulling the MOSFET gate high will cause the MOSFET to conduct, similar to the
old relay's contacts closing.
   Any combination of the ten lines can be fired at the once by having their
line switches up when the big red Fire button is pressed.
   The only items not usually locally obtainable are the NTE 2395 MOSFETs
(50-amp, N-channel), 14-conductor cable and multi-pin connectors.
   For monster pieces or high-power rockets, there is one 100-foot extension
line that can be used.  The normal configuration is five in one group, four in
another group and one monster piece 100 feet away and at a right angle.
   Our 15-conductor, 22-ga. cable is 500 feet long.  The cable spool weighs
35 lbs. and is unrolled only as much as needed.
   The ignitor leads are 25 ft. long, soldered directly into the field box
printed circuit board for reliability.  They are coiled individually and
secured with twist ties.  The size 24 car battery has a carry strap and is
always inside a leakproof battery box inside the field box.
   The line continuity testers use a copper alligator clip as a normally-
closed switch. There are no fuses because a short circuit will draw 30 amps
at most due to lead resistance, which equals the burnout current of the
printed circuit foil and is well below the 50-amp capability of the MOSFETs.
  When used for model rockets, the ignitor clips should be copper, flat-tip
types because they are easily cleaned with almost anything. The kind with
sawteeth are impossible to clean properly when badly corroded by rocket
exhaust which has absorbed moisture. And the leads should always be masking-
taped to the launch rod for support and to avoid disconnection by wind.
       Control box: (10) SPST toggle switches
                    SPST keyswitch
                    (10) 1K ohm, 1/2-watt pull-up resistors
                    red "hot" LED
                    connector for multi-conductor cable
                    momentary 25PDT firing switch
                      (A/B printer selector w/ toggle catch removed)

        Field box:  car battery, battery cables, inner battery box
                    car starter relay
                    .22 mfd., 200-volt surge capacitor
                    NTE 506 diode
                    (11) NTE 2395 MOSFETs
                    (11) heat sinks, screws, nuts, heat sink grease
                    (11) 10K ohm, 1/2-watt pull-down resistors
                    (10) 470-ohm, 1/2-watt LED current-limit resistors
                    1K ohm, 1/2 watt pull-up resistor
                    (10) 25' 18-ga. zip cord lines
                    (10) RCA jacks
                    SPST slide switch (field continuity test)
                    ready buzzer (can be disconnected from inside box)
                    connector for multi-conductor cable

        Pad lines:  (20) copper flat-tip clips
                    (10) copper alligator clips (continuity test)
                    (10) bright LEDs (note polarity when soldering)
                    (10) double-sided printed circuit boards 1/4" x 1"
                    (10) 2' 18-ga. zip cord lines
                    (10) RCA plugs
A modified hand truck handles the 35-lb. cable
and the field box with a car battery inside.
Wire Resistance Info:
  18-gauge "zip cord" 100 feet long having two conductors would be one 200-
  foot-long wire. It has a resistance of 1.3 Ohms.  If it is connected to an
  Estes ignitor (.8 ohms), the total resistance is 2.1 Ohms.
  For example, using Estes ignitors and 12 volts:
   gauge  Total length  Total resistance   Current     Short-circuit current
    16      200 ft.        1.6 ohms         7.5 amps      15 amps
    16      400            2.4              5              7.5
    18       54            1.15            10             30
    18      200            2.1              5.7            9.2
    18      400            3.4              3.5            4.6
Ignitor Info (12 volts):
                         Cold resist.    Ignition     Inrush
     Copperhead:           .5 ohms        3 amps      24 amps (*)
     Estes:                .8             1           15
     First Fire:          1.0             2           12
     Magnalite (kit):     1.1             2.5         11
     DaveyFire 28 F:      2.0             1/2          6
     DaveyFire 28 B:      2.2             1/4          5.5
     Bernie Lalime's:     1 to 3
     (Bernie's ignitors have no bridge wire. The pyrogen is a conductive mix
     of powdered graphite, clear model dope, aluminum and pyro oxidizer.)
 (*) Copper foil can short-circuit AFTER ignition, so push and release button.
Nichrome wires, 36 ga. (coil) and 48 ga. ("hair")
     Magnalite kit ignitors use 36-gauge nichrome wire.
     DaveyFire 28 B ignitors use 48-gauge nichrome wire.
TIP: Dunk exhaust-blackened gator clips into boiling water and then dry them
     immediately to remove fresh exhaust deposits BEFORE corrosion can start.
                      USING COPPERHEAD IGNITERS
Gator clips adapted for Copperhead ignitors.
   These clips have a length of heat-shrink tubing on the tips so that when
they are turned the same way, opposite sides of the Copperhead ignitor are
contacted. A small bump of solder holds the heat-shrink on.
Gator clips on Copperhead ignitor.
Copperhead ignitor with 26-ga. soldered extensions.
   Tin the ends of the Copperhead foils with a small-tip iron. Tin the ends of
the extension leads. Next, solder lead and foil together. Now pull the foil
apart by pulling on the leads. (Solder may have bridged the foils.) Put a bit
of double-stick tape between the foils and squeeze them back together again.
   You can also solder extension leads to Estes ignitors to keep them out of
the exhaust blast, which quickly destroys alligator clips.
                       SIMPLE HOMEMADE IGNITORS
   Using tightly-twisted, solid-copper, 26-gauge wires about 12" to 18" long,
strip about 3/16" of the ends.  Move the ends parallel and about 1/64" to
1/32" apart.
Bare wires, after first dip and after final dip.
Improvised wires.
   On the left are solid 26-ga. "shooting wires" used for fireworks displays.
Next is one side of a solid 24-ga. security-wire pair with a bare 24-ga. wire
(for printed-circuit hookup).  Then an enamelled 22-ga. wire with a bare
24-ga. wire.  On the right is a 20-ga. stranded wire with a bare 24-ga. wire.
Graphite, dope, nail polish, stick and jigger.
   Put a small amount of powdered graphite, used to lube door locks, into a
shot glass. Into the graphite, put a drop or two of clear model plane dope or
clear nail polish and mix until it's a very thick syrup.  Dip all the bared
wires into this quickly, as many as you can. (It will dry fast!) Let these
dry thoroughly hanging down, preferably overnight.  Clean the jigger with
acetone.  When dry, this first dip should have a resistance of 3 to 7 ohms.
   For the next coating, you will need potassium perchlorate and powdered
aluminum.  Powder the "perc" very fine using the back of a spoon and a piece
of wood. Mix the perc with half it's amount of aluminum using a toothpick in
the shot glass. Put drops of dope or nail polish in to form a runny syrup.
Dip the ignitor ends into it. Shape the ends by rolling them on paper so that
they will fit into the rocket motor core. Let dry overnight.