1. R5 is the series resistance of L1 (value not critical). R10 is the series
resistance of L2 (value not critical).
2. C5 is several parallel capacitors chosen such that Q1 oscillates at about
650 kHz when C6 is in the middle of its adjustment range. Short out L2 and
use an AM radio to check the frequency.
3. C6 can be just about any small trimmer capacitor. However, use one that
doesn't change value when vibrated or jarred (i.e. bumping the sensing loop
against a rock).
4. C15 may need to be adjusted, depending on the capacitance of the shielded
cable. Temporarily short out L1, and check the frequency of Q2 with an AM
radio. The precise frequency is not important, but the reference oscillator
and sensor oscillator need to be at the same frequency when C6 is near the
middle of its range.
5. L2 is the sensing coil made by stringing 12 turns of #24 stranded hookup
wire inside a 3/8 inch copper tube bent into a 10 1/4 inch diameter circle.
Leave about a 1/4 inch gap where the ends of the tube meet. Drill a small
hole on the opposite side from the gap for the ends of the wire. Feed the
wire through the small hole to wind the coil. When the wire starts to drag,
push the wire bundle at the gap to aid in the winding. When doing the last
turn, bend the end of the wire inside the tube so that about 1/4 inch of the
end is perpendicular. This makes it easier to fish the end of the wire out
of the small hole when it comes around. Solder a short piece of hookup wire
to the copper tube near the small hole (before winding the wire inside the
tube, or you might melt the insulation). Trim the ends of the three wires
to about 1 inch. L2 is connected to the rest of the circuit by a length of
shielded twisted pair cable. The shield is connected only at the coil end.
Connect the short length of wire soldered to the copper tube to the shield
and to one end of L2, and to one of the twisted pair wires. Connect the
other twisted pair wire to the remaining end of L2.
6. The rest of the circuit is built on a 1x5 inch piece of perf board using
point-to-point wiring. The oscillator circuits are built on opposite ends
of the board, with the mixer and detector in the middle. This reduces
stray capacitance that might cause the two oscillators to synchronize,
which would desensitize the metal detector.
7. A handle is made from 1 1/2 inch PVC pipe, with a tee on each end. Overall
length is 4 1/2 feet. Insert 5 inch pieces of pipe into the sides of the
tees. Mount L2 on one end of the handle at a 45 degree angle with the wire
ends and tubing gap inside the PVC pipe (cut a slot with a round file).
String the shielded cable inside the handle from L2 to the circuit board
mounted inside the tee on the other end of the handle. Leave a little slack
in the cable so that the circuit board can be pulled out the side of the tee.
C6 is mounted in one side of the tee, a headphone jack in the other side.
A battery holder for 2 C cells is mounted on the outside of the tee.
8. R16 is a pair of headphones with 2000 ohm impedance.
9. The transistors Q1 through Q4 can be any high beta RF or switching
transistors. I used 2N3798 PNPs. If using NPNs (such as 2N3904) reverse
the battery polarity.
10. C6 is adjusted in the field while using the metal detector. With sensing
coil L2 at least 3 feet away from all metal, adjust C6 for a low frequency
tone in the headphones (about 50 to 100 Hz). The frequency of the tone will
shift as L2 is brought near metal.
Smaller items were found near the surface to 3 inches deep, larger items were found down to 1 1/2 feet deep.
FOUR DAY'S (Fall 96) FOUR DAY'S (Spring 97)
Backyard Frontyard Backyard Frontyard
Quantity Quantity Quantity Quantity
aluminum pull tabs 3 4 2 4
aluminum flashing 12 inch
10 inch
ashtray 6 inch 1
aviation snips 1
axe heads
partial 1
whole 1
batteries
c-size 1
bolts
2 inch 1 1
2 inch power line spice block 1
2 inch carriage w/washer,nut 2
3 inch carriage w/washer,nut 1
6 inch eyebolt w/washer,nut 1
12 inch 3
14 inch 1
bottle caps 2 9 1
buckles
2 inch 1
cans
aluminum 17.5 13 5.5 2
tin 1 21 2
spray 1
caulk tubes 1
chain
2 1/2 feet long with hook 1
9 inches long 1
2 feet long 1
clamps
1 inch stainless hose 1
4x4x7 inch (1/2 inch plate
with two large bolts) 1
coins
quarters 1 1
foil
1 inch 5 12 3 1
2 inch 2 5 1
3 inch 6 1 1
4 inch 2
5 inch 2 1
8 inch 1
horseshoes
partial 1
whole 1
hot rocks
1 inch 3 1
2 inch 3
3 inch 1 1
4 inch 1
5 inch 2 1
6 inch 1
10 inch 1
1 foot 2 3 1
2 feet 1
jar lids
2 inch 3 1 1
3 inch 1
knife
3 1/2 inch 1
license plates 2
misc metal (unidentified)
1/2 inch 2
1 inch 4 11 1 5
2 inch 3 8 5 5
3 inch 1 6 1
4 inch 2 3 2
5 inch 4 2
6 inch 1 2
9 inch 1
mousetrap 1
nails 11 2 2
nuts
1 inch tinnerman 1
lug 3
paint can lids
6 inch 1
paint cans
1 gal 1
pipe
1/2 inch diameter 3 inch
1 inch diameter 9 inch
razor blade single edge 1
scythe blades 11
shot gun shells 1 1
shielded 1/4 inch phone plug 1
squeeze tubes
5 inch 1
strainer 4 inch 1
straps
10 inch long with buckle
and 5 eyelets 1
toy trailer 8 inch 1
tree stand leg 10 inch 1
washers
1 inch 1 1 3
2 inch 1
2 inch square 1
wire
aluminum
#000 1.5 feet 1.5 inch
#00 1 foot
brass
#14 3 inch
steel
#14 2 feet
#18 6 inch
wire clothes hanger 1
wire rim glasses 1
Last Updated : Feb 7 1:04:01 EST 2018
Mail to jerryz@fishzee.com