■ FIGURE 4
one? Say, you start with a cheap
carriage lantern from a home improvement store, remove the glass and the
guts, install a UV LED, make two
cylinders out of wire mesh (one inside
the other), and then hook them up to
a high voltage DC-DC inverter. Would
it work?
Once the fly or mosquito gets
between “the fences,” it would
get zapped by a spark and fall down.
This is called a non-clogging zapper,
since the fly should not get baked to
the grid.
There may need to be some
calculations done on what the voltage
should be at different spacing and grid
sizes of the fence (bigger flies, small
mosquitoes, or both) and UV LED
would need to be covered somehow
so it cannot be looked directly into.
— Dusan
AYour idea is good. Get some
hardware cloth from the
local store and form it into
two cylinders, one 3/4 inches smaller in diameter than the other.
■ FIGURE 5
30
September 2007
To maintain the spacing, use 3/8 inch
wood dowels impregnated with wax.
Thirty thousand volts are needed to arc
3/8 inch at sea level. Adjust the voltage
(see Figure 4) so the arcing is only
occasional. Connect the outer screen
to a spike in the ground to insure that
curious fingers don’t get zapped.
In the circuit in Figure 4, R3
adjusts the duty cycle of the 555 to
change the voltage output. The nominal frequency is 20 kHz, but varies
with duty cycle. I chose a 500 volt, 14
amp transistor which may be overkill,
but I don’t want the transistor to
smoke when you are making adjustments. I chose the value of R1 by
guesstimating; the transistor drain has
to go to 300 volts in order to get
30,000 volts output. If you don’t get
enough output with R3 at maximum,
make R1 larger. R1 and C1 just
provide an upper limit to the drain
voltage. R1 will be dissipating nine
watts so a cement filled, 10 watt
resistor would be appropriate. All
other resistors are 1/4 watt. I expect
that Q1 will operate without a
heatsink because the normal current
should only be a couple of amps.
The voltage drop for blue LEDs is
about 3. 5 volts so you can have two in
series and two in parallel as in Figure 5.
RECTANGULAR HOLES
I have a very simple question. I
build a lot of simple nifty electronic
gadgets, many from Nuts & Volts and
usually put them in plastic project
boxes. I have never figured out how to
cut a clean rectangular or square hole
in one of these boxes. Usually, I need
round holes which are easily cut with
a hardware store type step drill, but
square or rectangular holes are needed for switches, transistor sockets, and
meters to name a few.
I lay out the hole, then drill four
small round holes at the corners, and
then try to saw or cut out the hole
with my Dremel tool. It usually ends
up a ragged mess. Stores want $500
for a punch. Isn’t there a better way?
— C. P. Furney, Jr.
AI have several Greenlee
punches. I don’t remember
what I paid for them but I
assure you that I did not pay
hundreds of dollars! When I make square
holes, I use your method: Drill a hole in
the center, cut the outline with a coping
saw or hacksaw and file to fit. I have flat
and square files for that purpose. I
found a URL that shows how to make
rectangular holes using several methods,
including using a chisel or punch:
www.makezine.com/pub/a/extras/15.
html?page=last&x-maxdepth=0. NV
