Nuts and Volts - September 2010

■ 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 WELDER CIRCUIT

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. thomasnet.com/item/band-saw-blade-welder/band-saw-blade- welder-2/item-1017?&forward=1

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.

LCD BACKLIGHT

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 components.

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.

VIDEO BUFFER

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 different matter.

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?