Avoid disaster when drilling at homeBy Ben Godfrey
Description: Metal Detector Kit P/N 129091
Manufacturer: Velleman 7102
Time required: 1-2 hours
A simple home project can turn catastrophic fast if you come up against an electric cable, gas and water pipes, or your central heating system. With a handy metal detector you can check beforehand whether there are metal objects in a wall, ceiling or floor. An LED indicates if a metal object is in the vicinity and keeps your simple home projects, simple and safe.
Step 1 - Wrapping the Coils
Step 2 - Insert the DiodesFor this kit, there are two types of diodes: Silicon (P/N 179215) and Zener (P/N 782788). Because they look the same, read the outer casing to be sure that you are putting in the right diode. The silicon diode has 4148 on it, while the Zener's casing reads 3V9. I mistakenly exchanged the diodes, and it took a lot of time to figure out I had put them in the wrong holes. Also, make sure to match up the band on the diode to the band on the silkscreen.
If you suspect a misplaced or bad diode, measure the forward voltage drop across them, which should be about 0.6V for each diode. The reverse breakdown voltage for the Zener diode should be about 3.9V.
Step 3 - Adding Capacitors, Transistors and LEDs
1.2nF, 3.3nF (P/N 1947407), and 0.047µF (P/N 1947351) labeled 122, 332, and 473, respectively
(either 122 or 332 may be used for C1)
- It can be difficult to tell which capacitor is which. Make sure to check the number on the capacitor with the number in the instruction manual.
- As with the diodes, line up the transistors with the pictures on the silkscreen.
- Be sure to match the flat side of the LED with the flat side in the picture on the silkscreen. Since it can be hard to see the flat side, the short lead on the LED indicates the flat side. If the LED is not turning on, measure the voltage drop across the LED. If it is 0 Volts, you probably have put the LED in the wrong way.
Step 4 - Enclosing Your DetectorThe Metal Detector Kit needs a case, especially if you plan to move it around. I chose a Serpac enclosure (P/N 374635) because of its included battery bay, which keeps the battery as far away from the coil as possible. Below details the assembling of this box.
Drilling the PC Board
There are no mounting holes in the PC board, so I found the spot with the most space and made a hole and held the board to a bright light to see the traces. This ensures that you are not drilling through any of them.
The kit provides a small tube that fits into the large potentiometer for easy calibration. Enclosing the kits means you'll have to drill to accommodate that tube but lining up holes in both sides of the enclosure can be tricky. To make sure my drilled holes lined up properly, I put four nails through the screw holes on one half of the enclosure (see picture). Then I dusted the top of the calibration tube with chalk dust, positioned the second half of the enclosure onto the nails, and pushed. The result was a chalk mark showing where the tube was located and therefore where to drill. I used a 1/4-inch drill bit for this hole, and the tube fits quite snugly.
- Since the power button is rectangular, I used a 3/8-inch drill-bit, and a small file to cut the hole. It is not perfectly square, but you can comfortably push the power button down.
- To make the hole for the light from the LED to escape, I put a small piece of drinking straw on the LED and used the aforementioned nail trick. Another way to have done this would have been to extend the LED so it sticks through the hole.
TroubleshootingOne of the problems I had was getting the LED to turn off. I discovered that I had switched the diodes. This troubleshooting process can be made easier if you have some understanding of how the circuit works:
C1 and the 120-turn primary create a 200 kHz oscillator inducing varying voltages on T2 (like in the oscillator picture to the right). At the positive peaks of the oscillation, T2 is turned on causing the voltage on the collector of T2 to drop. These negative excursions also bias the base of T3, which causes T3, and therefore the LED, to turn off.
The reason why the LED does not turn on and off is that C2 and R5 create an integrator that integrates all the pulses making it look like a DC signal. When the coil is brought near a piece of metal, the oscillation frequency drops decreasing the number of negative "blips" raising the voltage on T3, turning it on (and the LED).
Sometimes the notation is a bit difficult to understand. If you see a value like 2K7, the first number is the largest power of ten's spot. The second number is what spot that actually is (in this case, 'K' stands for thousand). The second number is the next power of ten. So, in this case, 2K7 stands for 2700. Also, on the Zener diode 3V9 stands for 3.9 Volts.
Recommended power source 9V battery.
Finally, remember to have fun!
Ben Godfrey is an electrical engineering student at the University of California, Davis. Ben has written iPhone applications and is a Ham Radio operator.