Circuit Circus Charles D. Rakes - Geotech - … movement can come from an older volt-ohm meter or from some other piece of retired gear. Several dif-ferent 455-kHz ceramic filters

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Here are some metal detector circuits from three separate Circuit Circus columns.

ics can be built on perfboard and shouldbe housed in a metal cabinet.

Capacitor C1 can be any variablecapacitor that you happen to find inyour junkbox or one removed from anold broadcast-radio receiver. The 50-µAmeter movement can come from anolder volt-ohm meter or from someother piece of retired gear. Several dif-ferent 455-kHz ceramic filters weretried in the circuit and all seemed towork just fine. If you can’t locate aceramic filter, just send an SASE. (self-addressed, stamped envelope) to me at“Circuit Circus,” Popular ElectronicsMagazine, 500-B Bi-County Blvd.,Farmingdale, NY 11735 and I’ll sendyou one. [Note: Popular Electronics isno longer in business. For ceramic fil-ters, try Digi-key Corp.]

The loop should be located at leastone foot away from the locator’s cabi-net, separated by a non-metal support.A wood dowel rod is a good choice.Run a twisted pair of unshielded wiresbetween The loop and the circuit board.

If for some reason you don’t get ameter reading when turning C9 throughits rotation, it could be that the oscilla-

METAL DETECTOR (March 1991)The next entry is one of my favorite

gadgets - a simple two-transistor metaldetector - which you can put together inan evening or two and enjoy using forhours on end. The circuit (see Fig. 3)probably won’t lead you to a pot ofgold, or any other treasure for that mat-ter. But it can help locate wiring in thewalls or pipes in the floor, and will costyou next to nothing to build. And if youhappen to have a youngster under footwith nothing to do, this circuit justmight be the one gadget that will getjunior outdoors and into a fun hobby.

In Fig. 3, transistor Q1 (a 2N3904NPN device) is connected in a simpleLC oscillator circuit with the values ofL1, C3, C4, and C9 determining the cir-cuit’s operating frequency. Theoscillator’s output is fed through C1 andR4 to a 455-kHz ceramic filter. Whenthe oscillator is tuned to the filter’s cen-ter frequency, the filter acts like aparallel tuned circuit and produces ahigh level 455-kHz signal at the junc-tion of R3 and R4. The 455-kHz signalis fed to the base of Q2, which is con-figured as an emitter follower. The

output of Q2 (taken from its emitter) isthen converted to DC by D1, and fromthere, is applied to M1 (a 50- to 100-µAmeter).

With The oscillator operating at, orvery near the filter’s center frequency,the meter will read somewhere in thevicinity of mid-scale. But when anymetal object larger than a BB is broughtnear the loop, the meter’s reading willeither increase or decrease, dependingon The type of metal. The circuit willdetect a penny two inches away or a“D”-cell battery at about five inches inopen air.

The search loop is wound on a smalldiameter form that’s best suited forlocating smaller objects at close range,but a larger loop may be built to detectlarger objects located at greater dis-tances. A plastic end cap for a 4-inchPVC sewer pipe (which can be pur-chased at just about any plumbing-supply shop) can be used as the coilform for The search loop. The searchloop should be ten close-wound turns ofnumber-26 enamel-coated copper wirewound around the bottom of the end capand taped firmly in place. The electron-

Charles D. Rakes

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tor just isn’t tuning to the filter’sfrequency. A frequency counter can beconnected to the emitter of Q1 to seewhat signal (if any) is present. Or, if acounter isn’t available, use a standardBC receiver and tune to the oscillator’ssecond harmonic. If the oscillator isoperating at 500 kHz, tune your radio to1 MHz and you should hear the carrier.

If the oscillator’s frequency is too high,add capacitance across C9. If the fre-quency is too low decrease C3 and C4.Also if the meter won’t quite make it tofull scale, R4 can be reduced in value; ifthe needle bangs full scale, R4 can beincreased.

Through a little experimenting, you’llsoon determine the best method to use

in tuning the locator for detecting thesize and type of desired metal objects.The circuit is more sensitive when thetuning is adjusted so that the meter is atabout half scale when no metals arepresent; at that setting, the circuit willindicate ferrous and non-ferrous metalsby causing the meter to increase withone and decrease with the other.

METAL DETECTOR (June 1991)A while ago we discussed a simple

metal detector circuit and, judging fromthe response, it was obvious that a num-ber of you were very enthusiastic aboutthe subject. So the next circuit that we’lldiscuss is one that is designed to do thesame job, but in a different way.

One of the most sensitive and inex-pensive metal detectors that you canbuild is a variation of the VLF TX/RX(very-low frequency transmitter/receiver) detector, which is a two partapparatus. Such double-box detectors— which would not respond to any-thing smaller than a pound coffee can— were generally designed to detectlarge metal objects buried deep, beneaththe ground.

Our’s is a mini-version that can detectcoin-sized objects from a few inchesaway or larger objects at a distance ofover two feet. The sensing loops (coils)

on both the transmitter and receiver por-tions of our detector are slightly over 4inches in diameter and are separated byabout 12 inches. The operation of theTX/RX metal detector is based on thedirectional properties of the magneticfield produced by the transmitter loopand the reception properties of thereceiver loop.

In such circuits, the majority of themagnetic energy flows from the trans-mitter loop in an edgewise directionwith almost no radiation perpendicularto the loop. The receiver loop offers thesame directional properties as the trans-mitter’s loop, but since it is positionedperpendicular to the transmitter loop,almost no energy is detected. When ametal object is placed within the field ofeither loop, the loop’s magnetic field isslightly distorted, allowing the receiverto detect a small part of the redirectedenergy.

The VLF receiver, see Fig. 4, is builtaround an LM1458 dual op-amp and asingle 2N3904 general-purpose NPNsilicon transistor. Coil L1, the pick-updevice, is a homebrew inductor (100-turn loop) that is tuned to approxi-mately 7 kHz by C6. Any 7-kHz signalpicked up by the loop is fed to U1-a,which provides a gain of 100. The sec-ond op-amp is also configured for again of 100. The two op-amps producea combined gain of 10,000, dependingof the setting of R8. The output of U1-bat pin 7 is fed to a rectifier circuit thatconverts the 7-kHz signal into a posi-tive DC voltage.

That DC voltage is then fed to thebase of Q1 through R5, causing Q1 toturn on. With Q1 turned on, BZ1 soundsto indicate that metal has been detected.Power for the receiver is supplied by asingle 9-volt transistor radio battery.

The transmitter portion of the circuit

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(see Fig. 5) is built around a single tran-sistor that’s configured as a Colpittsoscillator. The transmitter’s sensingcoil, L1 (another 100-turn loop), istuned to about 7 kHz by capacitorsC2—C4. Transmitter power is suppliedby a 9-volt battery.

Assembling the circuit is a snap. Theloops are wound on plastic end caps(that are made to fit on 4-inch plasticpipe) with an outside diameter of 4-1/2inches. The coil is made by jumble-winding 100 turns of number-26enamel-covered copper wire around thecenter of each end cap. The ends of thecoil are then taped in place. The loops

are then mounted to opposite ends of awood dowel (about 12 inches), and ori-ented perpendicular to each other.

The receiver and transmitter circuitrycan be built on perfboard and mountedinside the end caps on which the loopsare formed, or placed in separate plas-tic enclosures and positioned away fromthe dowel mounted loops.

Tuning up and checking out thedetector is easy. Turn both units on; thebuzzer (BZ1) should sound. Turn thereceiver’s gain down until the soundjust about ceases, and then slowly rockthe transmitter’s loop back and forthuntil a perfect null is obtained. Keep

increasing the receiver’s gain and repo-sitioning the transmitter for the deepestnull. If everything is working correctlythe null (at full receiver gain) will besharp. If not, the receiver and transmit-ter may not be tuned to the samefrequency.

To tune the receiver to the transmit-ter’s frequency connect a DC voltmeterto the cathode of D1 and vary C6 for themaximum output voltage at the diode; 4to 5 volts is normal.

The detector is most sensitive whenthe circuit is operating at maximumgain and off null just enough to pro-duce a low level output from BZ1.

METAL DETECTOR (August 1991)Our next entry see Fig 2, places the

NE602 at the center of a simple yet sen-sitive metal detector. Transistor Q1, a2N3904 general-purpose NPN transis-tor, is connected as a Colpitts oscillator,operating at a frequency of about 250kHz. The oscillator’s inductor, L1,serves as the metal sensor. When theloop is brought near a metal object, theloop’s inductance changes, causing ashift in oscillator frequency which istransmitted to pin 1 of U1.

Integrated circuit U1’s internal oscil-lator is also operating at a frequency ofabout 250 kHz. When the two oscilla-tors are operating at, or about the samefrequency U1’s mixer output at pin 4, isan audio tone that equals the differencefrequency of the two oscillators. If theloop oscillator is operating at 250 kHz

and the local oscillator is operating at250.5 kHz, the audio tone would be thedifference of the two, or 500 Hz.

The audio tone passes through a low-pass filter, made up of L3 and C8, andthen travels to the headphone jack (J1)through coupling capacitor C12. Whenthe sense loop is passed over a metalobject, the Colpitts oscillator’s fre-quency is shifted, causing the audiotone to change, thereby indicating thatmetal has been detected.

Inductor L1 is a homemade coil,made from 5 turns of #20 enamel-coated copper wire wound on a 9-inchdiameter wood or plastic form. Afterwinding the coil, tape the windings inplace and attach a non-metallic handleto the search loop. The coil should beconnected to the circuit via shieldedmike or mini-coaxial cable. To obtain

the best operating stability, the metaldetector circuit should be neatly assem-bled (keeping the component leads asshort as possible) and housed in a metalcabinet. Transistor Q1 and its associ-ated components should be locatedaway from U1 and its support compo-nents, so that the two oscillators won’tlock together when the circuit is tunedfor a very low-frequency, audio-outputtone. A standard 9-volt transistor radiobattery will do for the power source.

To use the circuit, position the searchloop away from any metal object andadjust C9 for a low-frequency audiotone. It is much easier to detect metalobjects at greater distances from thesearch loop if the output-tone’s fre-quency is very low. That’s because it ismuch easier to detect a two- or three-hertz change at 15 Hz than at 150 Hz.

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Therefore, it is wise to set C9 for thelowest possible output frequency formaximum sensitivity. When searchingfor buried objects, position the searchloop parallel to the ground and aboutone inch above its surface. Then simplysweep over the desired area.