Build A Four Transistor Metal Detector
By Paul Florian
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Want to be a treasure hunter? By discerning subtle changes in frequency,
this design is capable of detecting coins to a depth of three to four
inches.
The circuit can sense a soda can at a depth of six inches and metal pipes at
an even greater distance. The unit is powered by two 9V batteries in series.
The detector has a current draw of approximately 9 mA at 18 VDC. As a
result, the batteries should last a long time.
FIGURE 1. Four Transistor Metal Detector Schematic.
Theory The design consists of four parts:
- Q1 forms an oscillator whose frequency is determined by L1.
- Q2 is a reference oscillator whose frequency is set by C6.
- Q3 is a mixer that multiplies the outputs of Q1 and Q2.
- Q4 is a simple one transistor amplifier.
Construction The circuit was built on a 2.6” x 3.5” piece of perfboard cut from PC board specified in the Parts List. Make a copy of the schematic and cross off the components as they are installed. An adapter was used to mount C6 (refer to Photo 1).
PHOTO 1. C6 Adapter PCB.
To construct the board, cut a 1/2” by 3/4” piece of double or single
sided PCB material. Use a permanent marker to mask the areas where the
traces should go. Then, etch the board and clean the ink off with rubbing
alcohol. Finally, solder the capacitor and SIP pins to the PCB in surface
-mount fashion.
This adapter plugs into a two position SIP socket on the PCB. The pins of
the adapter can be bent to 90 degrees for top adjustment. The volume control
(R16) pin designations are shown in Figure 4.
FIGURE 4. Potentiometer pin designations.
\Break off the keying post of R16 with a pair of pliers. Instead of
soldering the connecting wires directly to the PCB, .100” male headers are
used. These headers are installed for the search coil, volume, S1, earphone,
and 18 VDC. Female crimp housing connectors are used to make the twisted
pair cable assemblies shown in Table 1.
FIGURE 2. Search coil form.
Now construct the wire coil. Wrap 21 turns of 22 gauge solid magnet wire
around a 4“ diameter tube (an oatmeal container works nicely). Once the
coil is wound, wrap it in electrical tape (see Photo 2).
PHOTO 2. Coil assembly.
Wrap 1” strips of aluminum foil around the coil, leaving space for the two
magnet wire leads. Strip three inches of insulation from a six inch red 22
gauge solid wire. Wrap the three inch bare end of this wire around the
aluminum foil (see Photo 3). Wrap the whole coil assembly with electrical
tape.
PHOTO 3. Shielded coil assembly.
Make the search coil strain relief bracket from a piece of aluminum sheet
(Figure 3).
FIGURE 3. Search coil RCA cable strain relief.
A 1/4” grommet is installed in the 1/4” hole. Cut off one end of the RCA
cable and feed it through the grommet. Remove 1” of insulation from the
end of the RCA cable. Cut the red wire from the search coil to 1/2” long
and strip the insulation to 1/4“. Scrape the enamel from the magnet wires
with a razor knife and tin with solder.
Solder the magnet wire adjacent to the red wire along with the red wire to
the shield connection of the RCA cable. The other magnet wire is soldered to
the center conductor of the RCA cable. The solder connections should be less
than 1/2” and carefully wrapped with electrical tape or the output of the
mixer may drift.
Next, mount the strain relief bracket to the wood slat with a 1/2” 6-32
machine screw, two external tooth lock washers, and a 6-32 nylon lock nut.
The coil is mounted to the coil form with three wire ties. Finally, attach
the 45 degree 7/8” inside diameter PVC elbow fitting to the coil form with
two #6 x 1/2” sheet metal screws. You must first drill 3/32” pilot holes
in the PVC joint to receive the screws. This 45 degree fitting receives one
end of the metal detector shaft (a 33” long piece of PVC tubing).
A photo of the completed search coil assembly with the bracket is shown in
Photo 5. PHOTO 5. Search coil assembly with bracket.
Two 10” and one 3.5” lengths of 7/8” diameter PVC pipe are used when
making the handle for the metal detector. Refer to Photo 4 for assembly.
PHOTO 4. PVC handle assembly.
The handle uses two 90 degree elbows and one 45 degree elbow. Two 5/32”
pilot holes and #10 1/2” sheet metal screws are used when connecting each
of these elbow joints to the PVC pipes.
Drill two 5/32“ holes in the bottom of the case as shown in Photo 6. These
holes are used to attach the enclosure to the handle assembly.
PHOTO 6. View of open case.
Use Photo 7 as a reference when mounting the following items to the front
panel: J2, S1, and R16. Drill appropriately sized holes in the lid for these
items.
PHOTO 7. Front panel view.
Next, drill a hole for J1 next to the 45 degree elbow joint where the 33”
PVC pipe shaft attaches to the handle. Separate the lower and upper 10”
lengths of PVC pipe. Drill mounting holes in the lower 10” piece of PVC
pipe to match the holes in the bottom of the case drilled earlier and mount
it with two 1-1/4” 8-32 machine screws, external lock washers, and nuts.
Install J2, S1, and R16 on the housing lid. Attach J1 to the case. Screw the
PCB to the front panel with four nylon 4-40 1/2” threaded spacers and
eight 4-40 screws. Connect the previously mentioned crimp housing cables
between their sources and destinations. Attach the two 9V battery holders to
the bottom of the case with double-sided adhesive tape.
Now, reassemble the upper and lower 10” PVC pipe sections. Insert the
33” main shaft into the handle assembly’s 45 degree elbow joint, drill a
pilot hole, and secure with a #10 1/2” screw. The cap at the end of the
handle is press-fit. Similarly, the search coil assembly is press-fit to the
end of the 33” metal detector shaft.
PHOTO 8. Completed metal detector.
After the unit has been tested and completely assembled, it’s time to look
for coins and jewelry! Good places to find coins at shallow depths are under
the swings at a playground and below bleachers. For very large metal objects
at close distances, the output pitch of the metal detector may be higher
than physically audible. Happy hunting with your four transistor metal
detector! NV