p10 metaldet g4 finalreport

7/27/2019 P10 MetalDet G4 FinalReport

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YTE - Department of Electrical and Electronics Engineering

EE 316 - Electronic Design Project, Metal DetectorProject Final Report 1

EE 316 - Electronic Design Project

Project: P10

Metal Detector

Final Project Report13 May 2012

Objective

Metal detectors are widely used equipments around the world for differentpurposes. They are used for security purposes such as finding mines in mine fieldsor detecting metal objects on people in airports or such buildings. They are used forarcheology. Treasure hunters also use them widely to detect precious metals undersoil. Even food technology benefits from metal detectors to inspect different materialsin food.

Our aim is to design a metal detector that rapidly responds to metal objectsentering a region of 100cm2. Whats more, it should be able to detect a 1 TL coin at adistance of 10 cm. The type of metal detector we chose to accomplish this task isBeat Frequency Oscillator(BFO) type metal detector.

Group Members

Common efforts: All efforts are common.

zgrhan rek 160206022

Onur nolu 160206010


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Revision History

Week-8: The oscillators consisting of JFETS are replaced with BJT oscillators due todifficulties to find JFET transistors on market.

Week-9-10: The frequency of operation of the search and reference oscillators arechanged from 650 kHz to around 380 kHz.


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1. Introduction

The metal detector we are designing is a BFO type metal detector. There arethree important challenges in making a BFO type metal detector.

1. Detecting a metal object: In detecting a metal object, a circular coil will be used.The change in the inductance of the coil is the defining part that an object is detected.

2. Generating oscillations: Two LC oscillators are used to generate two ACsignals with very close frequencies. One of the oscillators include the circular searchcoil which detects the metal objects.

3. Finding the difference between AC signals: A frequency mixer circuit willgenerate a signal consisting of the difference and sum of the frequencies theoscillators produced. A low pass filter is used to remove the high frequencycomponent and acquire the low frequency part.

Following paragraphs summarize the necessary background information for thekey technologies utilized in this project.

1.1. Electronic Oscillators

Electronic oscillators are circuits those generate an oscillating voltage outputat a frequency. There are many types of oscillators. They can generate sine, square,saw-tooth, triangle or even complex waveforms as outputs. They are widely used dueto the huge requirement of oscillating waveforms in electronic applications. Aparticular type, L-C oscillator is crucial for a metal detector. An oscillator is simply an

amplifier with a positive feedback.


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1.1. Inductance of a Coil Near a Metal Object

The equation of an inductance of a coil is given below.

L=(0KN2A)/l

where L = Inductance (H)

0=permeability of space(H/m)

K=Nagaoka coefficient (changes with the geometry of the coil)

N=Number of turns

A=Area of the cross section of coil (m2)

l=Length of coil (m)

The inductance of a coil is dependent of the factors given above. As it can beseen, permeability of space that the coil is in effects the inductance. Magneticmaterials in space cause the permeability of the space to change, thus changing the

inductance of a coil. The inductance decreases when a metal object is nearby, thusincreasing the frequency of the search oscillator circuit which will be discussed later.

1.3. Frequency Mixers

Frequency mixers are non-linear devices. They take two input signals andproduce outputs at different frequencies. Usually this output is composed of the sumand difference of frequencies of inputs. Frequency shifting is necessary for someapplications in signal processing, so frequency mixers basically shift the frequency ofinput signals to another frequency range. This process is called heterodyning.


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2. Technical Description

Figure 1. Block diagram of the metal detector.

Two identical Colpitts Oscillators are used for search and reference(local).When the search coil L1 is brought near a metal object its inductance decreases.This causes an increase in the frequency of the signal V1. This signal is mixed withthe one coming from local oscillator, V2. The resulting signal, Vm includes both thesum and difference frequency components of V1 and V2.Feeding Vm to a low passfilter removes high frequency components. Remaining signal Vf is in audible range .Vf is then amplified with an audio amplifier. The amplified signal is fed into atransducer to produce sound.

The needed frequency of operation for the oscillators are set around 350 kHz.The oscillators produce signals with very close frequencies. The frequency of thereference oscillator can be adjusted to set the frequencies similar to each other, orwith a slight difference.


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2.1. Search Oscillator

Both search and local oscillators are Colpitts Oscillators and built with Bjttransistors. The diameter and turns of the search coil effect the inductance it

provides. Our frequency of operation is meant to be around 380 kHz. But after furthermeasurements and tests, we set both of the oscillators around 343 kHz.

The variable capacitor is used to set the circuit in conjunction with the variablelocal oscillator. The output of the search oscillator is a 2.5 Vp-p, 8 V mean witharound 350-400kHz frequency signal. The capacitance and the resistor values arecalculated according to the oscillation frequency formula of Colpitts oscillator:

f0 = 1/(2*sqrt(L*C1C2/(C1+C2))


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Substituting the values of capacitors and inductor in the above formula gives

the frequency of operation around 388 kHz. But since the search coil and the

reference coil must be set to same frequency, slight changes in diameter and turns of

the coil will effect the frequency of operation.

After testing and measuring the turns and diameter of the coil is changed andthe new operation frequency is changed to 340 kHz.

2.2. Search Coil

Search coil is an important part of the metal detector. The inductance of the

coil decreases near a metal object, which causes the frequency of the oscillator

circuit to increase. This change in frequency is used to detect the metal objects.

Radius of the coil is a critical part in design. Because increasing the radius of coil,

makes the detector to be able to detect from more distance, but makes it less

sensitive to smaller objects. That means that we had to find a balance between

distance of detection and the size of the metal objects. The search coil denoted as

L1 is meant to have around 80 uHs of inductance. Using the formula below the turns

and diameter of the coil can be adjusted.

L = r2N2/(9r+10l)

Where L=inductance (uH)

r=radius of coil(in)

l=length of coil(in)

N=number of turns

It turns out that, in order to have 80H of inductance, we need around 18-19

turns of 12 cm diameter, 0.55 mm thickness enameled copper wire. After practising

the circuit, we found that 12 cms of diameter didnt give enough detection distance.Thats why we wound another coil of 14cms diameter. To keep the frequency same

the turn count of the coil must be lowered. After inspecting the performance of the

detector, we decided to keep the turn count at same value which gave us an

inductance of105 uH.

Design specifications of the coil:

Number of turns: 19

Diameter(cm): 14

Inductance(uH): 102


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2.3. Reference Oscillator

The local oscillator(reference) is also a Colpitts type Oscillator. It is prettymuch identical to search oscillator except that there is another capacitor parallel tothe inductor. This is a trimmer capacitor which means that it can be adjusted. Weneeded a variable capacitor with around 50-500pF changing value. The best wecould find was a 10-60pF. This capacitor is used to make fine adjustment of thefrequency of operation. Since we had such a small interval of capacitor wesometimes changed turns of the search coil and changed the values of the capacitorsused to set the frequencies to the same value. The values shown on the schematicsare result of setting the frequencies equal to each other. Coupling capacitors of 82nFare used for capacitive coupling between parts of the circuit. The frequency of V2 is

around 343 kHz with an adjustable interval of around +5kHz -5kHz. Theamplitude of the signal is 8 V mean with 2.5 V peak to peak value.


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2.5. Signal Amplification and Producing Sound

The output of low pass filter, Vf, is very weak to be able to produce sound. So,before we can produce a sound effect to notify the user that a metal is detected. Weneed to amplify the signal. For this process, a JFET pre-amplifier and a standardLM386 Audio Amplifier is used.

As previously mentioned, the power of the signal Vf is not enough to drive asound transducer. Thats why it is first amplified. The basic JFET pre amplifieramplifies the signal to around 1 V mean with 100mV peak to peak value. After this,the signal is fed to LM386 audio amplifier through a 10k potentiometer that adjust thegain. The output of the LM386, Va is enough to drive a small buzzer connected to the

output. The gain of LM386 is maximum 20. 10 uF capacitor between legs 1 and 8of LM386 is used to increase to gain to 200. Adjusting the potentiometer controlsthe volume coming out of buzzer by changing the gain from 0 to 200.


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3. Test Results

3.1. Search Oscillator

The output of the search oscillator, V1 is measured as a sine wave with 344.3kHz and 7.8 mean value with 3 V peak to peak voltage. This is almost similar with thecalculated value.

3.2. Reference Oscillator

The output of the reference oscillator, V2 is around 343 kHz(adjustable) with 8V mean and 2.2 V peak to peak value.

3.3. Frequency Mixer and Low Pass Filter

The inputs to the frequency mixer was V1 and V2. The output of the frequencymixer without low pass filter is not measured separately. Output of the low pass filter,Vf is a sine wave with 67 mV peak to peak value and 2.2 mV mean. The frequency ofthis signal is the same with the difference between oscillators. At the time of themeasurement this difference was around 1kHz.These low values were expected.

3.4. Signal Amplification and Producing Sound

The input to the JFET preamplifier is Vf. The output of the pre amplifier Vp is asine wave with 1.2 V mean and 120 mV peak to peak value. Output of the LM386audio amplifier couldnt be retrieved with our measuring equipment. The reason forthis is that, even though we didnt really need it, we used a 10 uF capacitor betweenpins 1-8 of LM386 to hear the buzzer sound better. This caused the gain to be veryhigh. Because of this high gain even a slight adjustment of the volume controlpotentiometer caused the output signal to go very complex and unmeasurable. Oneother possible cause is that, we saw that when we tried to measure a part of thecircuit with an oscilloscope, the sound of buzzer was considerably lowered.

LM386 Specifications

Operating Supply Voltage 4-12 V or 5-18V

Quiescent Current (IQ) Vs=6V => max(IQ)=8mA

Voltage Gain 20-200(10 uF capacitor)


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4. Conclusion

The type of the metal detector we designed is the simplest of all. It is easy to

design, but has some drawbacks. One drawback is that, if distance of detection isincreased, then the object must be bigger so that it can be detected.Aside from that, the metal detector project was a success. Even though we

couldnt achieve detecting a 1 TL coin from exactly 10 cms. We got pretty close toaround 7-8 cms. The metal detector is stable, which means that it doesnt respond tonon-magnetic objects or doesnt change the tone of the buzzer without a reason. Itdetects an object entering the region of detection rapidly and notifies the user with aclear change in the sound buzzer produces. The adjustment

The detector we designed is not yet ready for practical use. Coil must bewounded with proper isolating band, and then shielded with Faraday shield to reduceground capacitance effects. Faraday shield is simply winding the coil with aluminum

paper. We didnt need that because we didnt use the device for practical purposes.Also the device doesnt have any handle or box to contain the circuit. These are allfor practical purposes and can be easily added after the project is finished.


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5. Component List

Component List Part Number Manufacturer SupplierAudio Amplifier LM386 National

Semiconductorwww.national.com

n-channel RFAmplifier

BF256A FairchildSemiconductor


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EE 316 - Electronic Design Project, Metal DetectorP j t Fi l R t 15

References

1. "How Metal Detectors Work"

http://electronics.howstuffworks.com/gadgets/other-gadgets/metal-detector.htm

2. Wikipedia article on "Metal Detectors"http://en.wikipedia.org/wiki/Metal_detector

3.Article on Metal Detectorshttp://www.biltek.tubitak.gov.tr/gelisim/elektronik/dosyalar/14/14.pdf

4. Metal Detectorhttp://electronicsfreelancer.wordpress.com/2011/10/13/metal-dedektoru/

5. Donald A. Neamen, McGraw Hill, Microelectronics Circuit Analysis andDesign Third Edition, pp:1069-1078

6.Wikipedia article on Electronic Oscillatorhttp://en.wikipedia.org/wiki/Electronic_oscillator

7.How to Make a Metal Detectorhttp://www.easytreasure.co.uk/bfo.htm

8.Wikipedia article on Frequency Mixerhttp://en.wikipedia.org/wiki/Frequency_mixer

9.LC Oscillatorshttp://www.electronixandmore.com/articles/oscillators.html

10.Introduction to Mixershttp://michaelgellis.tripod.com/mixersin.html

11.Metal Detectorhttp://www.bobsdata.com/metal_detector

12. Treasure Finderhttp://makeprojects.com/Project/Treasure-Finder/1113/1

p10 metaldet g4 finalreport

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