■ FIGURE 2
will have to provide isolation or it will
oscillate. The web article suggests
gluing a post to the speaker cone
and attaching it to the cover of a
glass jar. You can put the circuit and
battery in the jar to make a sensor
that can be partially buried in the
ground for good sonic conductivity.
BAND SAW BLADE
QI’m looking for a circuit that uses a 12 VDC car battery to butt weld 1/2” band saw blades.
I’m not sure if it can be used to
weld bi-metal blades, but the carbon
steel should work fine.
There is a commercial unit
shown at http://advancecarmover.
and PDF manuals there for
downloading and viewing. The price
seems a bit high for this unit.
Do you have any ideas on a
circuit that would allow me to safely
duplicate this blade welder?
— Larry Kraemer
AThere is no circuit; all you need is a switch capable of 500 amps to connect the battery to the blade. The jig
that keeps everything in alignment is
the key that makes it work.
QI’m repairing a portable DVD player and I want to replace the defective LCD screen.
I found that a 7” LCD display
from a digital photo frame 480 x 234
works great and will replace the
defective display in the DVD unit. I
believe that the photo frame LED
backlight works on approx. 12 VDC
@ 80 MADC and the DVD player
works on approx. 22 VDC 40
MADC. The problem is the original
LED backlight has 21 LEDs in series
and the replacement LED backlight
only has 15 LEDs in series.
My issue is that when the unit is
turned off, the power for running the
LEDs is 12 volts DC so the backlight
is lit up even when it’s off. When you
turn the unit on, the power goes to
approximately 22 volts DC (that’s the
reason the original LED stayed off). Is
there a circuit I could add that would
not allow 12 volts DC when it’s
turned off? And when it’s turned on
it would supply anywhere from 12
volts DC to 22 volts DC? This circuit
would have to be small as there isn’t
much room to add a lot of
Thanks in advance.
— Jeff Miller
AThe circuit in Figure 2 should do the job. Since the emitter and base are at he same potential when
the power is off, the transistor will be
off. When the voltage drive goes to
■ FIGURE 3
22 volts, 40 mA will be sent to the
LCD backlight. R2 allows the
backlight to go above 12 volts and
you can short it out to save room.
QI have what seems to be a simple problem, but my search for an answer has not given me good results.
I have six security cameras around
my yard and they all feed a DVR.
Every couple of years, I lose at
least one camera and the DVR to
lightning strikes. While replacing a
camera or two does not break the
bank, the cost of a DVR is quite a
Although I could probably just
use MOVs or some other passive
device, I thought a simple,
inexpensive, low power video buffer
would be in order; a sacrificial device
as it were. I say low power because
my electric bill is already high and I
am working to reduce it, not increase
it. The cameras are strictly video, no
modulation involved. Any ideas?
— Joe Heck
AI think metal oxide varistors at the camera power input and video utput, plus another MOV
at the DVR video input would
September 2010 25