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Note by Colin Mitchell: The first part of this discussion is a very old article using US imperial measurements, by Charles D. Rakes. A table of wire gauges is provided at the end of the article. The rest of the circuits are from different sources....

Note by Colin Mitchell: The first part of this discussion is a very old article using US imperial measurements, by Charles D. Rakes. A table of wire gauges is provided at the end of the article. The rest of the circuits are from different sources.

All these circuits have about the same sensitivity as the single transistor circuit shown in fig 7 of Part II (shown below), using an AM radio as the receiver. They have been included to show the ingenuity of design-engineers, in an attempt to improve the performance.

Here is a reference from another website with exactly the same views as myself:
The Beat-frequency oscillator (BFO) is the simplest (and oldest) type of metal detector technology and is a good starting point for learning how metal detectors work. The basic beat-frequency metal detector employs two radio frequency oscillators which are tuned near the same frequency. The first is called the search oscillator and the other is called the reference oscillator.

The outputs of the two oscillators are fed into a mixer which produces a signal that contains the sum and difference frequency components. This signal is feed to a low-pass filter removing the harmonics. As long as the two oscillators are tuned to the same frequency, the output will have no signal.
When a metallic object disturbs the magnetic field of the search coil, the frequency of the search oscillator shifts slightly and the detector will produce a signal in the audio frequency range.

Although once popular, BFO's are no longer being made by professional metal detector manufacturers. They are simple and inexpensive, but do not offer the accuracy and control of modern PI or VLF detectors. Attempts have been made to add new features such as discrimination and more advanced models were produced in the 1970s, but they were soon replaced by recent, more sophisticated technology.

BFO designs are still used in cheap hand-held devices and in low quality, toy type detectors.

The Simplest Metal Detector Circuit is also shown below and it only requires 4 components.

Using a Faraday Shield around the search coil will reduce the effect of the ground altering the frequency if the ground has a large amount of iron in the rocks. Simply wind aluminium foil around the turns of the search coil and leave a small gap where the wires exit.

It is pointless going to a lot of work building a complex receiver (as shown in a number of the circuits below) as the result will be no better than the simplest circuit.

All these circuits are limited to picking up a coin at 90mm to 150mm. Basically, a 90mm coil with pick-up to 90mm and 150mm coil will pick-up to 150mm.

An AM radio will detect the change in frequency of a few cycles at 150Hz and you cannot get better than that.
To get a deeper penetration, you need to deliver very high energy to the coil to produce magnetic flux that enters the ground and gets stored in the gold nugget.

The coil is then turned off and the circuit listens for the collapsing energy from the gold nugget being released and detected by the coil.
This is called Pulse Injection technique and will be covered in later circuits.

For now, here are some simple circuits:
Metal Detection Basics
One of mans greatest challenges throughout history is to see what can­not be seen, to detect what is hidden, and to reap riches from these treasures. This visit were going to look at some very basic metal-detection circuits. Now don't get me wrong; the circuits we'll share here most likely will never locate a valuable treasure, but they can be put to use performing other more practical applications. However, in the early days of the last century, even the simplest of metal detectors were successful in dis­covering some very valuable buried treasures. Simplicity often is the best route to take in solving a seemingly difficult task. Never give up on an electronic adventure because you don't have the latest and greatest equipment.

Ferrous Ferrets
ferrous ferrets.gif
Our first example of a ferrous detector is a simple mechanical device shown in Fig. 1.
The detector is a modified balanced scale, which indicates ferrous objects and magnetized items. A mag­net is attached to one end of the arm and a simple north/south scale is attached at the opposite end. A pivot is located near the magnet end of the arm and a slide balancing weight is on the opposite end.

The magnetic scale should be balanced with no ferrous items near by. Any non-magnetized ferrous object positioned below and close to the magnet will cause the pointer to go up due to the magnetic attraction.

The magnetic scale should be balanced with no ferrous items near by. Any non-magnetized ferrous object positioned below and close to the magnet will cause the pointer to go up due to the magnetic attraction.

A magnetized object with the south pole facing up will cause the pointer to go down, and when the north pole faces up the pointer will rise. This ultra-simple magnetic detector is very sensitive and will easily determine what objects are ferrous and the polarity of magnets.

Electronic Ferrous Ferret
Our first electronic metal detector circuit, see Fig. 2, uses a Hall Effect sensor to detect weak permanent magnetic fields. Almost all ferrous objects retain some degree of magnetism, and those that do are easily detected with our Hall Effect ferrous-detector circuit.
electronic ferrous ferret.gif
Fig. 2. The electronic version of the Ferrous Ferret uses a simple Hall Effect IC.
Weak magnetic fields can be detected with this easy-to-build device.
The HAL 115UA-C IC Hall Effect sensor is the heart of the weak-field detector circuit and is available for less than a buck from Digi-Key. This Hall Effect sensor is a bipolar device that is sensitive to a magnet's north pole on its branded side and to the south pole on the opposite side. The branded side, see Fig. 3, is the side that displays the part number.
hal 115ua c ic.gif
Fig. 3. Let's get up close and personal with our friend—the HAL I15UA-C.
The branded side-where the part number is displayed—is sensitive to a magnet's north pole,
while the opposite side is sensitive to a magnet's south pole.

The sensor's output (pin 3) is normally low when no external magnetic field is present. Placing a magnet with its north pole facing the branded side of the sensor will cause the output at pin 3 to go high. Placing a magnet with its south pole facing the non-branded side will also cause the output to go high.
hall effect ic.gif
Fig. 4. Utilizing some skill and patience, inductors can be hand-wound.
Here is a simple diagram showing the typical inductor needed for metal-detector circuits.
Here's how the circuit operates. Two gates of a 4093 quad, 2-input, NAND Schmitt trigger IC are connected in a low-frequency square-wave oscillator circuit operating at about 100 Hz. The output of gate "C" drives the base of Q1, which is connected in an emitter-follower circuit supplying the 100-Hz signal to L1. Inductor L1's drive level is set by R6. The output (pin 3) of IC2 is connected to an LED and a metering circuit.
Inductor L1 supplies a low-frequency AC bias to the backside of the Hall Effect sensor, IC2. This AC bias in effect increases the Hall Effect sensitivity many times over and also allows it to detect both north and south pole magnets from the branded side; however, the circuit is much more sensitive to north pole fields. The arrangement of L1 and the Hall Effect sensor is shown in Fig. 5.
the hall effect ic works.gif
Fig. 5. The Hall Effect IC works in conjunction with the inductor.
A low-frequency AC bias is supplied to the backside of the IC via the inductor.
The Hall Effects output waveforms are shown in Fig. 6. The waveforms are observed at pin 3 of the Hall Effect IC. Output waveform "A" is set by adjusting R6 for a symmetrical output without any ferrous metals in the pick-up area. If a scope is not handy, a DC voltmeter can be used to set the output to about 4.5 volts. This setting will produce an output waveform very close to the one shown in Fig. 6A. The "B" output waveform occurs when the north pole of a magnet is brought in proximity of the Hall Effect sensor. The south pole of a magnet produces the output waveform shown in Fig. 6C.
utilizing some skill and patience.gif
Fig. 6. Here are the waveforms that might come from pin 3 of IC2.
Resistor R6 can be adjusted to calibrate the circuit.
Winding L1
Inductor L1 (see Fig. 4) is fabricated by jumble winding 500 turns of #32 enamel-covered copper wire on a ¼-inch diameter ferrite rod. The rod's actual diameter and length are not critical, and any size rod material from ¼inch to ½inch in diameter will do. The rod's length can be anything from 1 inch to 3 inches. The type of rod material suitable for this application can be salvaged from an old AM transistor radio or some older TVs.
If the rod material cannot he located, don't give up 'cause there are other paths to take. A relay coil with a resistance of 10 ohms or greater will generally work for L1. Some miniature audio transformers have straight sections of laminations that can be used in place of the rod material. Most of the rod material I've used and have recommended here is actually designed for much higher frequency use. As a last ditch effort, try a number of small nails taped together as a core for L1 and see what happens. Here's a great place to experiment with various coil core materials and windings to improve or vary the circuit performance. Keep me informed on your efforts.

Try This One
Something else came to mind after disassembling the circuit, and due to time restraints I was never able to check it out. I would like to challenge you to do so. What if a second Hall Effect IC sensor was added to the circuit but placed beside IC2 with its branded side facing L1's core?

Fig. 5. The Hall Effect IC works in conjunction with the inductor. A low-frequency AC bias is supplied to the backside of the IC via the inductor.
Duplicate IC2's circuitry with the new IC, but leave out the metering circuit. See Fig. 7 for details. Try to get like waveforms from both circuits by adjusting R6 and positioning the two ICs on the end of L1. Connect one lead of a digital DC voltmeter to pin 3 of IC2 and the other meter lead to pin 3 of the added IC. If I'm correct, the circuit should be as sensitive to the south pole of a magnet as the original circuit was to the north pole. If not, try connecting a DC voltmeter to the output of IC2 and another voltmeter to the output of the added IC. IC2 should remain more sensitive to the south pole of a magnet, and the added IC should be more sensitive to the north pole.

All Metals Detector Circuit
Our next metal detector circuit takes us back into the early years of the last century where tubes were king and semiconductors were only diodes. It was discovered early on that any metal object placed near the tank circuit of an oscillator would shift its frequency either up or down. A tank circuit is the combination of an inductor and capacitor that make up a tuned circuit. Ferrous metals near the inductor of a tuned circuit cause the oscillator's frequency to go down and non-ferrous metals cause the frequency to increase. This is the basic effect that the Beat Frequency Oscillator (BFO) type of metal detector uses to detect all metals. Figure 8 shows a block diagram of the circuits making up a typical BFO detector. A search loop is usually wound in a circular fashion to serve as the inductor in the search oscillator's tank circuit. The reference oscillator is very similar to the search oscillator with a much smaller inductor, which is usually shielded from the search loop. RF signals are taken from both oscillators and fed to a common mixer, where the sum and difference frequencies of the two oscillators are mixed. The sum frequencies are filtered out, leaving only the audible difference frequencies to pass on to the amplifier and headphones.

Fig. 7. Increase your odds at detection with this simple modification. An additional Hall Effect IC is added to balance the circuit's sensitivity to the north and south magnetic poles.
Fig. 7. Increase your odds at detection with this simple modification.
An additional Hall Effect IC is added to balance the circuit's sensitivity to the north and south magnetic poles.

As a practical example, we'll set the search oscillator up to operate at a frequency of 100,100 Hz, and the reference oscillator to a frequency of 100,000 Hz. The difference frequency between the two oscillators is an audible 100 Hz that is fed to the headphones. The search coil is then moved over a small ferrous metal object causing the oscillator to drop in frequency to about 100,050 Hz. The audible 100 Hz tone drops to 50 Hz indicating a metal object is located somewhere near the search loop. A non-ferrous object near the loop will cause the oscillator to increase in frequency and produce a higher audio output tone. A carefully adjusted BFO metal detector can be used to discriminate between ferrous and non-ferrous metals.
Fig. 8. The popular loop-detector circuit has been a mainstay for many treasure hunters.
A set of headphones allows the user to hear an indication of ferrous material and magnetic fields.

Two-Transistor BFO Detector
One of the simplest BFO metal locators to build is the two-transistor circuit shown in Fig. 9.
waveforms pin 3 of ic2.gif
Fig. 9. This is the schematic for a Beat Frequency Oscillator metal-detector.
Two transistors are used as the oscillators in this circuit.

The circuit may be set up to operate on any frequency between 50,000 Hz to over 1 MHz by selecting the tank circuit components. Just about any good general-purpose NPN transistor suitable for low RF applications will work just fine. The search loop can be as small as a dime or three feet or larger in diameter. A small loop works best for small objects buried shallow and a large loop works best for large objects buried at greater depths. The two oscillator circuits should be separated and shielded from each other to reduce frequency pulling between the two. A really well constructed BFO detector will be able to operate with a difference of less than 100 Hz between the two oscillators. The lower the audio output tone the easier it is for the ear to tell a small frequency shift. The detector's maximum sensitivity is obtained when the two oscillators are operating just a few cycles apart. Believe me, this is not an easy task to accomplish, but one well worth the effort.

Here's how the simple BFO detector operates. Transistor Q1 along with its associated components make up a Colpitts oscillator circuit with the search loop, C1 and C3 forming its tuned circuit. Transistor Q2 with its associated components make up another Colpitts oscillator circuit with L2, C2, and C4. forming the tuned circuit. The emitters of Ql and Q2 are coupled together through R1, R2, and the low-impedance headphones. This circuit arrangement functions as a simple RF mixer circuit. The audio frequencies are fed to the headphones, and the RF frequencies are bypassed to ground through C8.

Winding And Scrounging
The loop may be wound on almost any round insulated non-metallic form, such as wood or plastic. Inductor L2 can be an old ferrite rod antenna coil removed from an AM transistor radio, or one can be made by winding a coil on a round insulated form. Let me offer the following winding suggestion to get you going on building the BFO circuit. Locate a 10- to 12-inch wood or plastic hoop that's about 3/4-inches wide and close wind ten turns of #18 to #22 enamel-covered copper wire evenly around the forms. Tape over the wire with plastic electrical tape and connect to the circuit with a length of two-wire zip cord. If an antenna coil cannot be found for L2, then close wind about 80 turns of #22 enamel-covered copper wire on a 1-inch plastic pill bottle or plastic pipe.

One important thing to do in selecting the two inductors is to be sure that the reference oscillator can tune to the same frequency as the search oscillator. If a frequency counter is available then the chore will be super easy. If not, some experimenting with different pairs of capacitors (C1 and C3 or C2 and C4) will be necessary to bring both oscillators to the same frequency.

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