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Hammerhead PI metal detector

“Hammerhead” is a pulse induction (PI) metal detector design. It is intended primarily as a learning platform for experimenters, and as a base design on which to expand. This design is very much like other commercial PI detectors, which tend to have similar circuit designs, so it has the potential for good performance.
This project includes a layout for a simple single-sided PCB using common through-hole components, as well as a much smaller two-sided surface mount PCB for those with steady soldering irons.
Flexibility is achieved with several build options. They include:
      • Passive or active MOSFET turn-off (through-hole only)
      • Generic preamp support for NE5534, LM318, and others
      • Single-ended or differential integrator
      • VCO or non-VCO audio •Extra component pads around the  preamp to support experimentation (through-hole only)
This design also makes most of the  important circuit parameters variable:
      • Transmit pulse rate
      • Transmit pulse width
      • Integrator sample delay
      • Integrator sample pulse width
      • Secondary integrator sample delay
      • Threshold
      •  Sensitivity
      •  Autotrack speed
      • Volume

Circuit diagram

A simplified diagram of the circuit is shown in Figure. It can be divided into five sections: power supply, clocking, transmit plus receiver front-end, receiver back-end, and audio. Each section can be built and tested sequentially. An oscilloscope is useful for probing the circuitry to see what is going on, but is not absolutely necessary, providing the circuit is built correctly. A digital multimeter (DMM) is needed.
The complete circuit is shown in the next Figure and, at first glance, appears to be complicated because of all the options that were designed in. However, the design options really don’t add much complexity, and allow for different con- figurations and performance comparisons. Simplified circuits will be shown in the descriptions of the hook-up options.
Power for Hammerhead is provided by a single 12-volt battery pack, prefer- ably an 8-pack of AA’s. A 9-volt pack can be used with a reduction in battery life, and even a 9-volt transistor-radio- type battery can be used, with a significant reduction in battery life, providing a proper supply tank is used at the coil switch (I’ll discuss this later).

HammerHeard PI PC Boards

Although this project can be built on perf board using point-to-point wiring, it is much easier to use a PC board, and the layout is already done. Figure shows the PCB layout; because it is single-sided, it is very easy to fabricate. However, a ready-made PCB is available, see the Sources section at the end of the article.
The next Figure shows the parts placement for all the possible parts. Remember, though, that some parts are omitted, and some shorted, depending on the option.

Parts List (Opt. 1)

Resistors (5% 1/4-watt)
R156k
R4, R42a1.5k
R5, R6, R15, R37, R43,
R44, R45a, R47, R48, R5010k
R9220
R101
R11See Text
R12, R24, R25,
R28, R36, R391k
R13, R16, R17, R191M
R20, R2127
R17, R19, R26, R27, R29,
R32, R33, R34a, R38100k
R30150k
R46a330
R403.3k


Potentiometers
R2, R18, R31100k
R3, R42, R4610k
R23100
R341M
R355k
R415k Audio Taper
R4550k

 

Capacitors
C1100µF elect.
C322µF elect.
C4, C5, C6,
C7, C14, C15, C2747µF elect.
C8, C24, C25, C2610nF poly.
C9, C17,
C18, C22, C230.1µF poly.
C101000µF elect.
C111nF
C19, C20, C210.47µF poly.

Diodes
D1-D4, D6, D71N4148

Transistors
Q1, Q5, Q6, Q72N3906
Q2, Q8, Q9, Q112N3904
Q3IRF740

Integrated Circuits
IC1ICL7660(Voltage inv)
IC278L05 (+5v ref)
IC3, IC479L05(-5v ref)
IC5, IC9NE555 (Timer)
IC6NE5534(Opamp)
IC74066 (Bilateral switch)
IC8TL072 (Opamp)
IC11, IC1274HC221 (Multivibrator)

Misc
J1Audio jack
SP1Speaker
SW1, SW2SPST switch
SP1Speaker
Search coil (see text), 12v battery,
battery clip, housing, knobs, coax, etc.

Coil sizing
 
This design, like most other PI designs, is set up to use a simple mono coil. A mono coil has only one winding, as opposed to coils with multiple windings, which often require delicate alignment. Important coil parameters are coil diameter, number of windings, and wire gauge. For all-purpose coin hunting, a good coil size is 10 inches (25cm) in diameter. A smaller coil will be more sensitive to smaller targets, but will achieve less depth. A larger coil will gain depth, but lose sensitivity to small targets. One popular setup for PI detec- tors is to use a very large coil, such as 1 meter, for deep cache hunting. This design can be adapted for this; see the Substitutions section. The standard coil for this design has a diameter of 10-inches (25cm), with 26 turns of 26 AWG6 enamelled wire. A simple way to wind a coil is to drive a circle of nails of the correct diameter into a piece of plywood, and wind the wire around the nails. Then, pull out a few nails and slide the coil off the jig. Before removing the coil, ensure that both ends have loose leads (pigtails) at least 2 to 3 inches (5-8 cm) long. Also slide some short pieces of tape under the wire, between nails, and wrap tightly to keep them bundled after the coil is removed from the jig. It is best to overlap the pigtails and wrap a piece of tape at the overlap, as shown in Fig. 11. This is really all that’s needed to wind a mono coil, and the pigtails can be soldered to a length of coaxial cable that feeds the main circuitry. However, a critical addition is a ground shield which, for PI detectors, is mostly 6.26AWG = 27SWG = 0.4mm. A slightly larger wire gauge is fine. needed to shield from outside electrical interference.
Adjustment & Usage
 
With the detector completely assembled and packaged, make sure the volume is turned down and apply power. SW2 should be closed. You should be able to adjust the Threshold and vary the audio tone from dead silence to a high pitch. Set the Thresh- old to get a very low frequency puttering audio. Wave a metal target close to the coil and the audio frequency should increase. If the metal is held absolutely still over the coil, the audio should retune itself back to the threshold tone.
With all the other pots set to the positions prescribed in the Construction & Calibration section, you should be able to detect a medium-sized coin several inches from the coil. Increasing the Sensitivity (by decreasing R35) might slightly improved depth, but the detector quickly becomes unstable, especially in noisy environments 7 . Increasing the 7.Such as inside a house, which is a poor environment for testing a PI detector. You might experience significant instability due to interference. Pulse Width also improves depth slightly, at the expense of power consumption. The most interesting controls are the Sample Delay (R42) and Sample Pulse Width (R46). Varying these can change sensitivity, even to different types of metal. In general, a short delay (around 15−20µs) gives the best sensitivity to low-conductance metal like gold. Once the detector is operating and stable, you can turn SW2 off and test the non-autotrack mode. Now when a metal target is held absolutely still over the coil, the audio should not retune to the threshold tone. This is normally used for pinpointing targets, not for general searching. In fact, unless a CMOS opamp is used for IC8, it is likely that this mode will become unstable after 15 seconds or so. Even JFET-input opamps have a small input bias current that will quickly charge C19. This design is intended to be a learning platform for how a PI detector works, so it has a lot of knobs to turn. Thus, it is quite easy to loose track of how adjustments interact, so when testing different settings it is best to keep a good record. As you find an optimal setting for a certain knob, mark the position on the panel.

Troubleshooting and solutions
Here are some common problems and  potential solutions.
Output voltage of the 7660 is not right.
                   • Check the polarity of the diodes.
                   • Check the polarity of the electrolytic cap C3.
                   • Try disconnecting C11.
No pulses or strange pulse widths coming out of the 74221’s.
                   • Make sure they are CMOS versions  of the 74221 (C, HC, AC). TTL
versions such as the 74LS221 will not  work.
                  •  Check polarities of D6 & D7.
Coil pulse is ringing badly
                  •   Oscilloscope probe capacitance is  loading the coil. Use a low-
capacitance probe.
                • Incorrect damping resistor value.
                • Excessive cable or shield capacitance.
Valid pulse on the coil, but no output on opamp IC6:
               • Misadjusted offset R18 can saturate  the output.
               • An incorrect (low) value for R16 can  saturate IC6.
               • Bad clamping diode D3/D4 can fry  the opamp.
Calibration of IC8a via R23 doesn’t work:
               • For this procedure, R44 & R48  should be installed, but Q8 & Q9  should NOT be installed. If Q8 & Q9  have been installed  already, simply  short R44 & R48.
               • R5 (or R15) should not be installed. If  they are installed and Q3 is being  pulsed, then the coil should be  disconnected. There will be a slight  (but acceptable) error in calibration.
               • Is R22 installed (low-pass filter) or  shorted (no filter)?
               • If R22/C16 was installed as a highpass filter, temporarily  hort C16.
Audio is not working or is erratic:
              • Make sure TP9 is close to zero volts  for no target.
              • Try reducing sensitivity via R35.
              • If testing indoors, try going outside,  well away from noise sources.

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