R1
100K
Magnetic reed s witch,
left-hand side
N.O.
Single-pole triple-throw switches
Hallmark
electronic
LED card PCB
Trigger
Not used
SW1 SW2 SW3
Two AAA
batteries
Magnetic reed s witch,
right-hand side
N.O.
A
B
C
NOTE: I added the 100K pull-up resistor from power (+)
to the PCB trigger to avoid EMI noise (electromagnetic
interference). I also added a 0.33μF capacitor from the
trigger to ground; without this, the LEDs may activate
prematurely whenever the 3 switches are touched.
Fool Your Friends
Now just flip the switches in the correct pattern and
the LEDs light up and remain lit for several seconds.
Whichever way SW1 is toggled, SW3 must be in the
opposite direction (Figure D). SW2 isn’t used at all;
it’s just there, all wired up for show.
Practice your performance; the more confident
you are, the more your audience will be determined
to beat you at your own trick. Never display your
trick to the same people more than 2 or 3 times,
because someone will figure it out and reveal your
secret. I tell my audience it’s a mathematical algorithm and only I know the sequence to toggle the
3 switches, then I dare them to match my ability to
get the LEDs to light up. “Well, do you feel lucky?”
J. Tregre is an electrical engineer from New Orleans.
His interests are in professional model building and
robotics. Visit him at
buildingultimatemodels.com.
D
MATERIALS
AAA batteries ( 2)
2xAAA battery holder
Clear plastic antiperspirant container as big as
possible
Capacitor, 0.33μF Jameco part #609086,
jameco.com
Resistor, 100kΩ Jameco #691340
Electronic Hallmark Halloween or Christmas card
with LEDs You might want to buy a few different
ones; some circuit boards are easier to use than
others.
Magnetic reed switches ( 2) Jameco #171872
Single-pole triple-throw switches ( 3)
Magnetic ring Search online for Wizard PK rings.
These come in silver and gold and are available
in 8 different metric sizes (convert U.S. ring
sizes at
onlineconversion.com/ring_size.htm).
Soldering iron and solder
Hookup wire
Miscellaneous tools (optional) to disconnect the
circuit board and its batteries, if needed. You
might use needlenose pliers, wire cutters/
strippers, or a heat gun or hair dryer to melt
glue or shrink heat-shrink tubing.
Multimeter or ohmmeter (optional) but handy
61 Make:
