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It probably won’t find gold nuggets in the bush but it will demonstrate how a metal detector works. A Detector For Metal Objects This simple project will demonstrate how metal detectors work. It uses the principle whereby the inductance of an air-cored choke changes in the presence of metal. By JOHN CLARKE Metal detectors are used in many applications. These include motor vehicle detectors at traffic lights, detecting unwanted metal objects in food processing, as process counters in industry and as treasure locators for fossickers. As you might expect, they vary widely in circuit complexity and function. A vehicle detector can easily detect a large piece of metal (ie, a car or truck) above it but it is quite a lot harder to detect metal fragments in food or coins and other items such as 36  Silicon Chip ring pull tabs from drink cans buried in beach sand. Some metal detectors can even discriminate between ferrous metal (ie, those with iron such as steel, cast iron, wrought iron, etc) and non-ferrous metals (aluminium, zinc, tin, lead, copper, mer­cury, silver, platinum, gold, etc). The metal detector presented here is of the simple variety and it only detects large items of metal at close range. It does not discriminate between ferrous and non-ferrous metals. Most metal detectors depend on the principle that an air-cored choke will change its inductance when brought into close proximity with a piece of metal. If the metal is ferrous (ie, if it has magnetic properties), the change in inductance will be considerably greater than for a non-ferrous metal and this fact can be used in circuits which can discriminate between metals. In our circuit, we have used an aircored inductor (choke) as the variable element in an LC oscillator. Block diagram Fig.1 shows the block diagram of the Metal Detector. There are two frequency sources, called oscillator 1 and oscillator 2, which are monitored with a NAND comparator. Oscillator 1 is adjusted using VR1 so that its frequency is exactly the same as for oscillator 2 when no metal is close to ductance increases. When non-ferrous metals are in close proximity to L1, the inductance decreases and so the frequency increases. This is the basis of discriminating metal detectors, as mentioned above. Circuit diagram Fig.2 shows the circuit which is based on two CMOS logic ICs. NAND Fig.1: block diagram of the Metal Detector. gates IC2a and IC2b form The two oscilla­tors beat together in a NAND oscillator 1 while IC1a gate comparator which then drives a loud­forms oscillator 2. -speaker. Oscillator 1 is a standard two-gate circuit with the 390pF capacitor alternateinductor L1. When metal is brought in ly charged and discharged proximity to L1, the frequency of os- via the 1kΩ resistor and series concillator 2 changes and this is detected nected trimpot VR1. in the comparator. When power is first applied, IC2b’s The comparator produces a tone in output could be either low or high. the loudspeaker whenever it detects When its output is high, IC2a’s output a difference in frequency between is low and the 390pF capacitor charges the two oscillators. The tone varies so that the junction of the two 1kΩ depending upon how large the metal resistors drops toward 0V. When this item is and how close it is to induc- junction voltage reaches IC2a’s lower tor L1. threshold, its output at pin 11 goes An interesting property of this high and so IC2b’s output goes low. type of oscillator is that when metWhen this happens, the 390pF als containing iron (ie, ferrous) are capacitor charges in the op­ posite brought close to L1, the frequency direction via the 1kΩ resistor and of oscillation falls because the in- trimpot. When the upper threshold of IC2a’s input is reached, its output goes low again and the cycle repeats. The frequency is varied by means of trimpot VR1. LC oscillator The LC oscillator works by successively charging and dis­ charging a .022µF capacitor via inductor L1. When power is first applied, pins 1 & 2 of IC1a will be low (because the .022µF capacitor is discharged) and the pin 3 output of IC1a will be high. The capacitor is then charged through inductor L1. When the voltage at pins 1 & 2 reaches the upper threshold of IC1a, its output goes low and discharges the .022µF capacitor via L1. This cycle repeats endlessly. The 180pF capacitor at pin 3 of IC1a makes the oscillator immune to variations in capacitance across the inductor. This means that the oscillator is insensitive to hand capacitance. Without the capacitor, just moving your hand close to L1 would change the oscillator frequency. NAND gate comparator IC1b inverts and buffers oscillator 2 and then NAND gate IC2c monitors both oscillators. Pin 2 monitors pin 10 of IC2b while pin 1 of IC2c monitors IC1b. The output of IC2c is then inverted using IC2d. Fig.2: the circuit is based on two CMOS logic ICs. IC1a is an LC oscillator using an air-cored inductor. If metal is brought close to the inductor, the oscillator frequency changes. May 1998  37 Fig.3: the top trace is the oscillator waveform at pin 10 of IC2b (oscillator 1) while the second trace is the output of oscillator 2 at pin 4of IC1b. This is what happens when there is no metal close to the inductor. 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The upper trace frequency remains the same as expected at 299kHz, while oscillator 2 frequency shifts, as shown on the middle trace. The resulting inverted NAND com­parator output on the bottom trace is a varying pulse width waveform. A NAND gate has a high output unless both inputs are high. The oscilloscope waveforms of Figs.3, 4 & 5 show what happens. On Fig.3, the top trace is the oscillator waveform at pin 10 of IC2b (oscillator 1) while the second trace is the output of oscillator 2 at pin 4 of IC1b. The bottoms trace is an inverted NAND gate output at pin 4 of IC2d. Note how the oscillator frequencies and waveforms are virtually the same. Fig.4 shows what happens when a metallic object is brought close to inductor L1. The upper trace frequency remains the same as expected at 299kHz, while oscillator 2 frequency shifts, as shown on the middle trace. The resulting inverted NAND com­ parator output on the bottom trace is a varying pulse width waveform. “So what?”, you might say. Well, these waveforms do not tell the whole story because the oscillator frequencies of around 300kHz are totally inaudible from the loudspeaker. But when we wind down the timebase on the oscilloscope we see what is happen­ ing at audible frequencies. Fig.5 shows the output of IC2d with a metallic object near inductor L1. See how it consists of bursts of signal at a rate of about 1.5ms. This is equivalent to a signal of about 670Hz and is quite audible although the signal is a bit weak at this point in the circuit. Therefore the pulsed signal from IC2d drives the base of transistor Q1 via a 10kΩ resistor. The transistor in turn drives the 8Ω loudspeaker via a 100Ω resistor which provides current limiting. The circuit is shown as being powered from 12V DC but in practice it can be powered from a 9V battery or 9V DC plugpack. Diode D1 protects against reverse polarity connections while LED1 indicates when the power is on. Fig.5: this shows the output waveform from IC2d when a metallic object is brought near induc­tor L1. It consists of bursts of signal at a rate of about 1.5ms. This is equivalent to a signal of about 670Hz and is quite audi­ble through the loudspeaker. Construction All the parts are mounted on a PC board measuring 104 x 69mm and coded 04405981. If need be, the PC board can be mounted into a plastic Table 1: Capacitor Codes ❏ Value   IEC ❏ .022µF   22n ❏ 390pF 390p ❏ 180pF 180p EIA 223 391 181 Fig.6: this is the component layout for the PC board. The loudspea­ker’s magnet is fixed to the PC board using super glue. Table 2: Resistor Colour Codes ❏ No. ❏  1 ❏  1 ❏  2 ❏  1 Value 10kΩ 2.2kΩ 1kΩ 100Ω 4-Band Code (1%) brown black orange brown red red red brown brown black red brown brown black brown brown 5-Band Code (1%) brown black red brown brown red red black brown brown brown black black brown brown brown black black black brown May 1998  39 Parts List 1 PC board, code 04405981, 104 x 69mm 1 40mm diameter Mylar 8Ω loudspeaker 1 DPDT miniature slider switch (S1) 1 560µH (0.56mH) air-cored choke (L1) 1 4.7kΩ miniature horizontal trimpot (VR1) 6 PC stakes 1 40mm length of hookup wire Semiconductors 2 4011 CMOS NAND gates (IC1,IC2) 1 BC548 NPN transistor (Q1) 1 1N4004 1A 400V diode (D1) 1 5mm red LED (LED1) Fig.7: this is the full-size etching pattern for the PC board. utility case measuring 130 x 68 x 41mm but that won’t be large enough to accommodate the air-cored choke. Fig.6 shows the parts layout for the PC board. Begin construction by checking the board for shorts or broken tracks. This done, insert the PC stakes for the supply input (+12V and 0V), inductor L1 and for the loudspeaker. The resistors can be insert­ed next, using Table 2 as a guide to the values. Alternatively, check each resistor with your multi­meter before it is soldered into the board. The capacitors can be installed next. Take care with the 100µF electrolytic which must be inserted with the correct polar­ity. This done, insert the two ICs making sure that they are oriented correctly. Trimpot VR1, LED1, transistor Q1 and diode D1 can then be installed and soldered in place. When you mount switch S1 you will need to crimp its eyelet terminals so that they will fit into the PC board holes. The loudspeaker is wired to its terminals on the board and then secured with some super glue on the back of its magnet. Finally, connect the air-cored choke to the PC board. Capacitors 1 100µF 16VW PC electrolytic 1 .022µF MKT polyester 1 390pF ceramic 1 180pF ceramic Resistors (0.25W, 1%) 1 10kΩ 2 1kΩ 1 2.2kΩ 1 100Ω Testing Apply power to the circuit and check that LED1 lights when S1 is on. Adjust VR1 until no tone is heard from the loudspeaker. Now bring a metallic object close to the coil and check that a tone is heard. You may need to readjust VR1 for best results. You may also want to experiment with a larger search coil of say 150mm Miscellaneous Solder, super glue diameter and about 50 turns. With the larger diamet­er coil, the detector will be more sensitive to smaller SC metal items. 14 Model Railway Projects Our stocks of this book are now limited. All we have left are newsagents’ returns which means that they may be slightly shop soiled or have minor cover blemishes. Otherwise, they're undamaged and in good condition. 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