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easure – r t d e i r u ble! or b nce a f e u i g l r n a e i v p h x y c g or l e r n l i g a t c e n i r Sea e d p g r s n a o i eth rew ou go pr y e can be ally if you find som m i t t nex especia this metal locator Try out mbing. beachco arke l by John C S EARCHING FOR BURIED TREASURE is a popular pastime for many people. For some it's a dream. For others it's a full-time (and occasionally lucrative) occupation. Some comb the beaches for dropped coins and jewellery (have you ever noticed when you drop a coin at the beach how the sand seems to eat it immediately, even if you see exactly where it lands? That’s one of Murphy’s corollaries and is one of the reasons metal detectors were invented!) Others try the goldfields, hoping to “strike it rich” either from a nugget left undisturbed over the centuries, or perhaps overlooked in the tailings, or spoil, from earlier gold mining. Who knows, there could be another “Welcome Stranger” just waiting for you to claim it. (Let us know if you do!!!!) Types of detectors There are many types of metal detectors on the market today ranging 36  Silicon Chip from the simple and low-cost amateur variety up to the very complex professional units costing many hundreds, sometimes thousands, of dollars. While they’re all designed to perform the one function, to detect metal objects, they do this in different ways. Some are able to differentiate between non-magnetic metals (such as gold, silver and copper) and magnetic metals comprising iron. These are called discriminating metal detectors and are usually complex in their operation and of course are expensive. Professional treasure hunters usually use this type because it saves them lots of digging – to find nails, metal cans and ring-pulls from old aluminium cans! (Ring-pulls haven't been around for more than a decade but our experience is that every single one of them was discarded exactly where we wanted to seach . . .) Other metal detectors are simply designed to react in the presence of any metal. The metal locator described here is of this type. It simply gives an audible indication whenever it detects any type of metal. Whether you’ve found gold or garbage, well, that’s pure luck! It is very easy to use and gives a change in the audio frequency as the search head is swept across any metal. It is good for detecting small objects at a moderate depth and large objects at a greater depth. Pin-point accuracy is quite good and with a bit of practice you can locate an object to within a few centimeters very easily. How it works This detector uses the principle that the inductance of a coil changes when a piece of metal is brought near to it. The coil is a part of a free-running oscillator with its the coil inductance and added capacitance setting the operating frequency. The coil is located in the search head which is swept over the ground. When the coil encounters metal, the oscillator changes in frequency. This change is detected and converted to an audio signal which the operator can hear via an inbuilt speaker or headphones. Block diagram Fig.1 shows the general operating principle of the metal locator. There are two oscillators: the search oscillator and a second fixed oscillator. A comparator monitors both oscillator signals and when the search oscillator shifts its frequency, the comparator's output changes audibly. The fixed oscillator runs at nine times the frequency of the search oscillator and so a 1Hz change in the search oscillator will give a 9Hz change in the audible output, making it very sensitive. This can be regarded as a modified beat frequency oscillator (BFO) circuit except that instead of two oscillators being very close in frequency, one is nine times the other. How can this be? The secret lies in the comparator which is really a D-type flipflop. The output is buffered and amplified to drive a loudspeaker. In operation the search oscillator frequency is adjusted via coarse and fine tuning controls so that there is no sound, or a very low frequency growl, comming from the loudspeaker. When the search head is brought near metal, the frequency will rise rapidly. The circuit is shown in Fig.2. It comprises three low cost ICs, three transistors, a regulator and the search coil, along with several resistors and capacitors. The search oscillator is in a Colpits configuration with the coil in the collector of Q1. The .001µF capacitor between collector and emitter provides feedback. The oscillator frequency is set by the search coil inductance, the paralleled .001µF capacitors across the 1kΩ emitter resistor and the .001µF capacitor between collector and emitter. Small changes in the base voltage of Q1 change the collector capacitance which in turn alters the oscillation frequency. The oscillator must be stable (that is, with minimal drift) so that the frequency controls will not constant adjustment. To ensure this stability we have specified polystyrene capacitors for the oscillation setting components. Features * Audible metal detection * Loudspeaker or headphones * Course and fine controls * Volume control * Stable circuit * Battery operated * Low cost * Ground capacitance effect eliminated with shielding * Ideal for finding small objects near soil or sand surface Fig.1: block diagram of the metal detector. The text above explains the theory of operation. DECEMBER 1999  37 Fig.2: the circuit diagram. The signal at the collector is coupled via a 100pF capacitor to the gate of JFET Q2. Its gate is biassed at half supply using by two series connected 150kΩ resistors. The output at the source follows the gate signal and effectively buffers the oscillator signal from the next stage, an amplifier based on NAND gates IC1a and IC1b. These normally digital gates are operated in a linear mode by the 100kΩ feedback resistor between the output (pin 8) and the input (pins 12/13). The 10kΩ input resistor and 100kΩ feedback resistor set the gain at 10. The resultant signal is “squared up” by gates with the IC1c and IC1d which are connected as inverters. The output from IC1d is applied to the clock input of the D-flipflop, IC3b. The fixed oscillator is based on a 2MHz crystal and IC2a, a NAND gate with its two inputs tied together so that it becomes an inverter. The 1MΩ resistor between the output (pin 6) and the inputs (pins 4 & 5) sets the inverter as a high gain amplifier and provides drive to the crystal on the input side. The 4.7kΩ resistor driving the crys38  Silicon Chip tal and the 68pF loading capacitors form a low pass filter, preventing the crystal from oscillating at a spurious frequency. The output of IC2a (pin 6) drives another inverter, IC2b, which squares up the waveform. IC2c and ICd are connected in parallel and further buffer the signal and provide drive to the the clock input to of IC3a, a D-flipflop. The flipflop divides the 2MHz input by two to give 1MHz at the Q output. This is applied to the D input of comparator flipflop IC3b. Oscilloscope Traces Operation of this flipflop as a comparator is best described by the accompanying oscilloscope waveforms. The top trace in Fig.4 is the clock input from the search oscillator after it has been squared up by IC1c and IC1d as described earlier. The centre trace is the 1MHz signal from IC3a. Note that the search oscillator has been adjusted so that it is a precise sub-harmonic of the 1MHz oscillator. This means that the rising and falling edges of both waveforms will remain fixed relative to one another and so the rising edge of the top waveform which clocks IC3b will occur when the 1MHz waveform (the data input to IC3b) is either always high or always low. The Q output of IC3b is latched to the logic level on the D input on each rising edge of the clock input. Thus if the level on the D data input is always the same when the clock goes high we will have no change at the Q output. The waveforms in FIg.4show the fixed oscillator and the search oscillator signals and the resultant mixer output when the frequencies are in an exact 9-times multiple. The waveforms are in phase. At top is the search oscillator running at 111kHz. The middle trace is the 1MHz fixed oscillation frequency. Below it is the mixer output which remains low. This is because the positive edges of the search oscillator always find a low on the fixed oscillator and so the Q output of IC3b stays low. Now if the search oscillator changes in frequency (hey! you’ve found gold!) the clock signal to IC3b will not be in phase with the 1MHz input. We therefore have a slow drift between a Fig.3: the component overlay. Only the headphone socket is mounted off the board – even the speaker is glued in place using silicone sealant. Compare this layout to the photograph overleaf when assembling the board. high and a low voltage at the D input as the clock is sent high. The Q output thus goes high and low in response to the changing data pattern. The oscilloscope waveforms in Fig.5 show what happens when the search oscillator is slightly slower than the 111kHz in phase frequency. The positive edge of the search oscillator finds a low on the fixed oscillator first and then finds a high two cycles later. This is shown as the lower trace and has a frequency of about 32.5kHz (the beat between 1MHz and 107.5kHz). Output The output signal is fed to Q3, an emitter follower amplifier, via thevolume control potentiometer. This Fig.4: the top trace is the clock input from the search oscillator after it has been squared up. The centre trace is the 1MHz signal from IC3a. transistor drives the internal speaker or the headphones. Plugging in headphones automatically switches the internal speaker off. Power for the circuit is derived from a 6V battery comprising four AA cells. The audio amplifier is powered directly from the 6V rail but the rest of the circuit runs from a regulated 5V rail provided by REG1, an LM2940T-5. Fig.5: this shows what happens when the search oscillaor is slightly slower. The mixer output now shows a frequency of 27kHz. DECEMBER 1999  39 Two views of the disassembled case showing the PC board from above and below. In the photo above, note the way the speaker is glued to the board using silicone sealant. The battery case (left photo) needs to be of the “long skinny” variety to fit under the PC board. This is a low dropout regulator which will continue to regulate even if the battery voltage is close to 5V. Current consumption of the circuit is 15mA when the volume is turned fully down, rising to 25mA when there is a loud tone in from the loudspaeker. Much care has been taken to ensure that the various stages are isolated from each another. This prevents the search oscillator from being “pulled” by the fixed oscillator to lock onto a sub-harmonic. This would cause reduced sensitivity. The search oscillator is decoupled from the 5V supply via a 220Ω resistor and 47µF capacitor in parallel with a 0.1µF capacitor. The 0.1µF capacitor is there to compensate for the fact that the 47µF electrolytic capacitor is not as effective at high frequencies. IC1 is also decoupled with a 220Ω resistor and 47µF capacitor, while the fixed oscillator (IC2) is decoupled with a 10Ω resistor and 47µF and 0.1µF capacitors. Construction Most of the components for the Fortune Finder are mounted on a PC board 40  Silicon Chip coded 04303001 and measuring 132 x 87mm. It mounts in a plastic case 157 x 95 x 54mm and a label measuring 154 x 90mm is affixed to the lid. Begin construction by checking the PC board for shorts between tracks, breaks in tracks and hole sizes. Larger holes are required to mount the regulator (3mm hole using an M3 screw and nut), for the switch S1 lugs (1.5mm each) and the cutout required for the loudspeaker magnet to pass through (about 35mm). You can make the cutout for the loudspeaker using a series of small holes around the perimeter and then filing to shape. Some commercial PC boards may have this hole punched. Begin by inserting the resistors and links using tinned copper wire. The resistors can be selected using the colour code table and/or measuring each value with a digital multimeter. The capacitors are inserted next, taking care to orient the electrolytics with the shown polarity. The accompanying table shows the possible markings on the low value capacitor. Apart from the five .001µF capacitors which must be polystyrene types for stability, the low-value capac- itors can be either monolithic or ceramic types. Next, insert and solder the crystal and semiconductors (transistors, and crystalICs and regulator), making sure you insert the semiconductors with the correct orientation and position. The regulator is mounted lying down and secured with an M3 screw and nut. The leads are bent down 90° at the appropriate points, inserted through their holes and soldered into the PC board. Switch S1 mounts with the terminals inserted directly into through the PC board holes and soldered on the other side.. Cut the pot shafts to about 10mm long, suitable for the knobs used. Note that there are five pins used for each potentiometer with three pins for the terminals and two pins for securing and earthing each pot body. Immediately before inserting and soldering the pots, scrape the plating off the pot body alongside where the two pins will be located to make soldering these pins to the body easier. Each pot is mounted about 2mm above the PC board, with the PC stakes soldered to the terminals. Solder the scraped pot body to the PC pin along- The handle assembly prior to mounting the box. Note the rebates in the dowel for the saddle clamps. . . side. You will need a good, hot iron for this operation. The lid of the case requires drilling for the loudspeaker holes, the pot shafts and for the power switch. Use the front panel label (or a photocopy) as a guide to the locations of the holes. Attach the label after drilling and cut the holes in this with a sharp knife. Attach a 130mm length of hookup wire to one terminal of the loudspeaker and a 60mm length to the second terminal and insert the loudspeaker through the hole in the PC board. . . . and an “above” and “below” view showing how the box is fixed to the dowel. Secure the PC board to the lid of the case with the pot bushes and power switch. Turn the assembly over with the pot shafts resting on a table. Centre the loudspeaker and secure it to the rear of the PC board with some silicone sealant. Allow to cure. Drill holes in the case for the cord grip grommet at the front edge and for the headphone jack socket on the side. Search Coil Assembly Fig.4 shows the coil plate assembly. It consists of a baseplate, coil assembly, brackets for the handle and a cover plate. The coil assembly is attached to the baseplate with silicone sealant. The brackets are attached with wood glue (PVA) and with holes in the side suitable for the wooden or plastic dowel. A slot is cut in the plastic plate for the broomstick to pass through and attach to the angle brackets. The plate is then held in place with two 4G self-tapping screws into the brackets. The use of such small metal screws does not affect the metal locator operation. Fig.8: complete details of the various components of the Fortune Finder metal locator. We have specified a broom stick instead of dowel because broom sticks are usuall more durable timber than ordinary dowel. DECEMBER 1999  41 Parts List 1 PC board, code 04303001, 132 x 87mm 1 label, 154 x 90mm 1 plastic case, 157 x 95 x 54mm 1 57mm 8Ω loudspeaker 1 SPDT toggle switch, S1 1 4 x AA cell holder (2 x AA long x 2 x AA wide) 4 AA cells 1 2MHz parallel resonant crystal, X1 2 10kΩ 25mm log potentiometers, VR1,VR3 1 100kΩ 25mm linear potentiometer, VR2 1 stereo switched 6.35mm jack panel socket 3 knobs 1 small cord grip grommet 21 PC stakes 6 6g x 10mm self tapping screws 2 4g x 10mm self tapping screws 1 wood or plastic dowel 8mm diameter 40mm long 1 300mm x 300mm piece of aluminium foil 1 roll of insulating tape 1 20mm x 60mm x 1.5mm aluminium for battery holder bracket 1 plastic cable tie 1 container of silicone sealant (non-acid cure ­– eg, roof and gutter sealant) Hardware and wire 1 base of 3mm MDF, 170mm diameter or thicker lightweight timber 1 plastic plate 170mm inside diameter (size to suit base) 2 timber or plastic angle brackets (20 x 20 x 20mm minimum) 2 2m long 20-22mm diameter broom handles 2 dual mounting 20mm conduit plastic saddle clamps 3 90 degree 20mm pvc elbows (Clipsal 245/20 or equiv) 1 32m length of 0.4mm enamelled copper wire 1 1m length of 0.8mm tinned copper wire 1 2m length of single core shielded cable 2 M3 screws x 6mm 4 M3 screws x 10mm 6 M3 nuts 1 6g x 30mm wood screw The search coil is made using 0.4mm enamelled copper wire. It has 70 turns, wound to make a 140mm inside diameter circle. The accompanying panel shows how it is done. If you don’t have a 140mm diameter former to wind it on, the simplest way of winding the coil is to find a 215mm x 30mm length of wood or plastic. Wind 70 turns around this, slide the coil off the wood or plastic and then open this rectangle into a 140mm circle. Wind a layer of insulating tape tightly around this, with the two start and finish wiresleads exiting at the same point. Now cut aluminium foil into strips 20mm wide and wind these around the coil, starting at the wire exit point. Cover the start end with insulating tape for the first 20mm or so. Completely wrap the coil in the aluminium foil until it reaches the wire exit point and continue to cover the insulation-taped coil for the next 10mm. Make sure the finish end of this aluminium does not come in contact 42  Silicon Chip Semiconductors 2 74HC00 quad NAND gates (IC1,IC2) 1 74HC74 dual D-flipflop (IC3) 1 BC558 PNP transistor (Q1) 1 2N5484 N-channel JFET (Q2) 1 BC338 NPN transistor (Q3) 1 LM2940T-5 low dropout 5V regulator (REG1) Capacitors 4 47µF 16VW PC electrolytic 1 10µF 16VW PC electrolytic 1 0.22µF polyester or ceramic 4 0.1µF polyester or ceramic 5 .001µF (1000pF) polystyrene 1 100pF NP0 ceramic 2 68pF NP0 ceramic Resistors (0.25W 1%) 1 1MΩ 2 150kΩ 3 10kΩ 2 4.7kΩ 2 220Ω 1 10Ω with the start end or the coil will be shorted. Next, wind on 15 turns of 0.8mm tinned copper wire evenly spaced around the aluminium foil. This shorts to the aluminium foil, giving something to solder to (you cannot solder to aluminium foil). Solder one end (only) of this tinned copper wire to one end of the 70-turn coil underneath. You will need to remove some of the insulation from the enamelled copper wire. Fortunately, enamelled copper wire is normally coated with a heat stripping coating insulation which can be removed with a hot soldering iron. Capacitor Codes       Value   IEC EIA 0.22µF 220n 224 0.1µF 100n 104 .001µF   1n 102 100pF 100p 101 68pF   68p   68 1 100kΩ 2 1kΩ This “earthy” end of the coil connecting to the tinned copper wire can be terminated to the shield of the connecting cable. The shielded cable core attaches to the other end of the 70-turn coil. Insulate the terminations and the whole coil assembly in with another layer of insulation tape. Note that one end of the tinned copper wire coil you wound does not connect to anything. The coil assembly needs to be mounted onto a wooden baseplate using silicone sealant. We used some scrap western red cedar and routed a channel for the coil to sit into. Alternatively, you could use 3mm MDF but this would be more likely to suffer water damage. We used a plastic dinner plate as a cover for the coil/baseplate assembly. The baseplate is made to suit the diamater of the plastic plate – ours was about 170mm. The plate can be obtained from stores selling plastic dinnerware. The support stick and handle are made with broomstick timber (dowel) Winding The Search Head 1 Wind 70 turns of 0.4mm enamelled copper wire onto either a 140mm diameter former or a length of thin wood or plastic about 215mm long. Remove the coil from the former and pull it into a circular shape. Leave about 100mm of wire protruding and cover the complete coil with a layer of insulation tape. 2 Cut aluminium foil into strips 20mm wide and wind over insulation tape, overlapping each turn slightly. Cover the first 20mm or so with insulation tape to hold it in place. Continue winding the foil on right around the coil and onto the insulation tape but do not let the finish of the foil touch the start of the foil. and 20mm PVC conduit fittings. The stick may need to be filed down where it enters the conduit elbows and for the saddle clamps if it is the standard 22mm diameter. We painted the broomstick handle before assembly. The final 100mm length of handle is attached at a right angle to the main stick using a wood screw. Each elbow is locked to the stick with 6g self tapping screws. Saddle clamps secure the detector box to the handle with M3 screws and nuts. After the silicone sealant has cured (for both the search head assembly and where it holds the speaker onto the PC board), you can continue the wiring for the headphone jack socket and attach this to the side of the case. Attach the search head to the broom handlein place and wire the shielded The search coil assembly viewed from underneath . . . 3 Wind 15 tight turns of 0.8mm tinned copper wire directly over the aluminium foil. Solder one end (only) to one of the wires of the inner coil. Solder these to the shield wire of the shielded cable which goes to the detector electronics. Connect the inner conductor of the shielded cable to the other wire protruding from the inner coil. The remaining wire from the 15 turns is not connected. Cover the whole coil with a layer of overlapping turns of insulation tape. 4 Secure the coil assembly to the base with non-acidic silicone sealant. The base can be made from lightweight timber with a 150mm diameter groove routed into it, or can be a 3mm MDF sheet with the coil glued to the inside surface. When the silicone sealant has dried, give the whole base several coats of oil-based paint to make it as weatherproof as possible. . . . from above showing the handle mounting brackets . . . . . . and finally with the “dinner plate” cover in place. DECEMBER 1999  43 cable to the PC board via the cord grip grommet in the case. This cable should be wrapped around the main stem several times and tied in place with a cable tie. Note that any movement of the lead will alter the search head frequency. Place the pot knobs on and connect the AA battery pack to the 6V supply terminals on the PC board. (Some battery holders can be screwed directly to the base of the case but some will require a bracket to be made). Apply power – you should be greeted with an audio tone. If not, adjust the volume control fully clockwise and adjust the coarse knob until a sound is heard. Extreme left and right settings of the coarse control should prevent oscillator operation. Check the power supply using a multimeter. There should be 5V between the metal tab of REG1 and the output. This 5V should also be on pin 14 of IC3. Check the voltage between pin 14 of IC1 and earth or 0V, and also pin 14 of IC2 and 0V. These should be just a little less than 5V. Adjust the controls until the frequency becomes a very low growl or stops completely. You will find that there are several positions on the coarse control where the output tone reduces to a low frequency but there will be one position which gives the loudest tone. Use the dominant tone to begin with, although you may find another position is better for some types of ground or metal. Now bring the search head near a metal object and check that the frequency increases. Note that the fine control is logarithmic and will give very fine adjustment at the anticlockwise position and coarser adjustment toward the clockwise position. This means that the adjustment of the coarse control should be done with the fine control past its halfway anticlockwise position. When you bring the search head near the ground you may find that the frequency changes, requiring readjustment of the controls. There is also the possibility that this adjustment was made in a location where metal was located and so it is a good idea to sweep the ground and find a good compromise adjustment. While the search oscillator has been designed to be stable in frequency with minimal drift, it will be far more stable after about a 15-minute warm-up with the power switched on. Also it will work best if it is given the chance to stabilise in the environment in which it is to be used. So do not store it in some cool dungeon and then expect the search oscillator to be stable when it is brought out No.  1  1  1  1  1  1  1  1 Value 1MΩ 150kΩ 100kΩ 10kΩ 4.7kΩ 1kΩ 220Ω 10Ω 44  Silicon Chip into the midday heat. The metal locator can be used with headphones that are high impedance types, as used with hifi systems and personal stereos. These are typically 32 ohms. Using these will reduce current consumption from the batteries and enable the locator to be used in noisy environments. Also the sensitivity to metal detection will appear to be better due to the closer proximity of the sound to your ear and the your ability to concentrate on the Resistor Codes sound more effectively. You can experiment with the 4-Band Code (1%) 5-Band Code (1%) locator by burying various items brown black green brown brown black green yellow brown in dirt or sand to learn how the brown green yellow brown brown green black orange brown metal locator responds to various brown black yellow brown brown black black orange brown items at different depths. Typical brown black orange brown brown black black red brown detection depths are 40cm for a tin can, 10cm for a wedding ring and yellow violet red brown yellow violet black brown brown 14cm for a 10 cent coin. brown black red brown brown black black brown brown You are now ready to tackle red red brown brown red red black black brown the beaches and exploration brown black black brown brown black black gold brown SC goldfields.