Fig. A: Vinyl 9-volt connectors are low-profile enough to
let everything fit. Fig. B: The completed circuit. Make
sure to orient the 7812 according to the schematic,
and don’t forget to slip on heat-shrink tubing prior to
soldering, to insulate all connections from the conductive metal tin. Fig. C: It’s a snug fit. Fig. D: Use light
pressure when cutting the openings; let the tool do the
work. The openings don’t have to be perfectly aligned.
7812 voltage regulator IC
Switch, SPST (single pole, single throw)
Case fan, 40mm square
9-volt batteries ( 2)
9V battery connectors ( 2) vinyl, not hard plastic
Pieces of screen, 50mm square ( 2)
Piece of carbon filter cut from a replacement filter
Insulated, threaded hook-up wire
Miscellaneous screws and washers
Dremel with cutoff wheel
Drill and small drill bits
CAUTION: Wear safety glasses
when drilling and cutting metal!
124 Make: Volume 19
9V batteries wired in series and steps the voltage
down from 18V to 12V, which is what the fan requires.
2. Solder the components.
Notice the battery connectors (Figure A); they’re
the flexible vinyl version, not the hard plastic type.
This allows both batteries to fit in the case. The vinyl
snaps are only minimally smaller, but it’s enough to
make the difference.
This is a very simple circuit. Solder it according to
the diagram, making sure to attach the component
leads to the 7812 properly (Figure B). Don’t forget to
use heat-shrink tubing on all connections; this is in
a metal box ... and metal conducts electricity!
3. Make sure it all fits.
It’s a snug fit, but you should be able to stuff everything into the tin, packing the batteries side by side
next to the fan (Figure C).
4. Cut and drill the holes.
I used a marker and a paper template for the fan
openings, making them 35mm square on each side.
After you cut the first fan hole, close the box and
use the template to align the second hole. You can
just “eyeball” the placement. There’s room for error.