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For full article read Nuts & Volts Vol.29 No.2 - February 2008
Build A Four Transistor Metal Detector
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.
Theory
The design consists of four parts:
1) Q1 forms an oscillator whose frequency is determined by L1.
2) Q2 is a reference oscillator whose frequency is set by C6.
3) Q3 is a mixer that multiplies the outputs of Q1 and Q2.
4) Q4 is a simple one transistor amplifier.
L1 needs to be 100 µH. This inductor is created from 22 turns of 22 gauge magnet wire on a four inch diameter coil form (see Reference 1). However, I wound the coil with 21 turns, resulting in an inductance of 88 µH. The inductance measured into the phono cable with the search coil connected is 126 µH. In order to work properly, you must use the audio cable specified in the Parts List. The resonant frequency is F = 1 / (2p*SQR(L1*C)). C= 1 nF in series with 10 nF + 56 pF. Solving yields C = 965 pF. Calculating for F gives 456 kHz. Therefore, the oscillation frequency of Q1 is 456 kHz when no metal object is near the coil L1.
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PARTS LIST
R1,R4 15K 5% 1/4W
R2,R5 33K 5% 1/4W
R3,R6 1K 5% 1/4W
R7 ,R11 100K 5% 1/4W
R8,R13 4.7K 5% 1/4W
R9,R10 47K 5% 1/4W
R12 100 5% 1/W
R14 1M 5% 1/4W
R15 2.2K 5% 1/4W
R16 5K audio taper pot
C1,C7 100 nF 50V ±10%
C4,C9 10 nF 50V ±10%
C3,C10,C13 1 nF 50V ±10%
C2 56 pF 100V ±20%
C8 470 pF 100V ±20%
C5,C11,C14-15 10 µF 50V ±20%
C12 1 µF 50V ±20%
C6 15-150 pF variable cap
L1 See text
L2 100 µH ±5%
Q1-Q4 2N3904 transistor
J1 RCA jack
J2 3.5 mm phone jack
S1 SPST switch
Aluminum sheet K&S Metals
Wood slats Forster 6-Count Slats
PVC pipe 6’ 7/8” OD PVC Pipe
Audio cable 6 ft cable
Case 6 x 4 x 2
Battery snap 9V snap (2)
Earpiece Ceramic earphone
PCB Prototype board
Battery holder 9V holder (2)
Tape Two-sided adhesive tape
Batteries 2 x 9 VDC
Build A Four Transistor Metal Detector
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.
Theory
The design consists of four parts:
1) Q1 forms an oscillator whose frequency is determined by L1.
2) Q2 is a reference oscillator whose frequency is set by C6.
3) Q3 is a mixer that multiplies the outputs of Q1 and Q2.
4) Q4 is a simple one transistor amplifier.
L1 needs to be 100 µH. This inductor is created from 22 turns of 22 gauge magnet wire on a four inch diameter coil form (see Reference 1). However, I wound the coil with 21 turns, resulting in an inductance of 88 µH. The inductance measured into the phono cable with the search coil connected is 126 µH. In order to work properly, you must use the audio cable specified in the Parts List. The resonant frequency is F = 1 / (2p*SQR(L1*C)). C= 1 nF in series with 10 nF + 56 pF. Solving yields C = 965 pF. Calculating for F gives 456 kHz. Therefore, the oscillation frequency of Q1 is 456 kHz when no metal object is near the coil L1.
...
PARTS LIST
R1,R4 15K 5% 1/4W
R2,R5 33K 5% 1/4W
R3,R6 1K 5% 1/4W
R7 ,R11 100K 5% 1/4W
R8,R13 4.7K 5% 1/4W
R9,R10 47K 5% 1/4W
R12 100 5% 1/W
R14 1M 5% 1/4W
R15 2.2K 5% 1/4W
R16 5K audio taper pot
C1,C7 100 nF 50V ±10%
C4,C9 10 nF 50V ±10%
C3,C10,C13 1 nF 50V ±10%
C2 56 pF 100V ±20%
C8 470 pF 100V ±20%
C5,C11,C14-15 10 µF 50V ±20%
C12 1 µF 50V ±20%
C6 15-150 pF variable cap
L1 See text
L2 100 µH ±5%
Q1-Q4 2N3904 transistor
J1 RCA jack
J2 3.5 mm phone jack
S1 SPST switch
Aluminum sheet K&S Metals
Wood slats Forster 6-Count Slats
PVC pipe 6’ 7/8” OD PVC Pipe
Audio cable 6 ft cable
Case 6 x 4 x 2
Battery snap 9V snap (2)
Earpiece Ceramic earphone
PCB Prototype board
Battery holder 9V holder (2)
Tape Two-sided adhesive tape
Batteries 2 x 9 VDC
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