bright enough to see in daylight. To accomplish
this, the circuit constantly trickles 5V through each
igniter in series with the LED and a 510Ω resistor.
Pressing a firing button overrides this series and
delivers full voltage to the igniter.
This is the neat part of the system; one DC wire
running to the launch bar serves as both the continuity check in one direction, and provides firing
12V DC current in the other direction when the
firing button is pressed. See makezine.com/20/
megalauncher for the circuit schematic. Naturally,
I went through a number of test igniters before
I found component values that would keep the LED
lit but did not spark the igniter.
I included the alarm, which sounds when the system is armed and the rockets are ready to fire, for
both fun-factor and to give adult supervisors some
warning to get everybody away from the launch
pads. The Lego sound brick I used only sounds
5 klaxon “beeps” per triggering, so I added a pulsed
trigger circuit based on a 555 IC chip to extend the
duration of the alarm and make it adjustable with a
100K potentiometer. I also added a yellow LED and
150Ω resistor that indicate when the 555 chip is
The circuit has two 5V DC voltage regulators
62 Make: Volume 20
because the peak draw of the continuity LEDs is
100mA and the regulators are rated to 1A. With a
10-pad system, this means dedicating one regulator
to the LEDs and using the other for the alarm.
I built most of the control circuitry on a piece of perf
board, separating the components to allow plenty
of space in back for the power and ground connections. I mounted the circuit board just behind the
Lexan control panel using 1½" pan head screws, so
you can see all the components up close but they
still have breathing room.
I mounted the switches, LEDs, and alarm speaker
in the Lexan panel, drilling LED holes with a #9 bit
and larger holes with a Forstner bit. The Arm switch
is a double-pole switch, and I used a military aircraft
weapons switch for the Mega Launch function, but
any SPST will work.
I arranged the 510Ω resistors that lead to the
continuity LEDs along the top edge of the circuit
board, allowing short connections to the row of
LEDs in the panel just above (Figure C). To connect
the alarm, I disassembled the original Lego brick,
soldered leads to the mini board inside for trigger
voltage and ground, and spliced in 2 more wires