Hand Held Metal Detector

 they emerge from a freezing tunnel than they do after sitting twenty minutes on a conveyor line.
We have signals from product, large pieces of metal, small pieces of metal, and vibration all exactly the same as the last. To identify a metal contaminant, the detector must remove or reduce this product effect.vying for the attention of the detector. We also have varying product signal because each product is not exactly the same as the last. To identify a metal contaminant, the detector must remove or reduce this product effect.
Signals from ferrous metal are larger than signals from the same size piece of nonferrous or stainless metal. Signals caused by vibration are always along the same lines as non-ferrous metals. Special circuits can be used to amplify the signals by differing amounts, according to phase, to improve sensitivity of the metal detector to the stainless signal, and reduce the sensitivity to vibration. The detector can be adjusted to “rotate” the signals from the product in a way that maps out an area where the product is expected to be. This allows the product to be ignored and emphasizes analysis of signals outside that area to identify metal contaminant. The technique to create this detection envelope is called product compensationthe product effect. 
Product effect is caused by various factors and does not always generate the exact same signal. The “detection envelope” must be large enough so a product signal is normally within the envelope. Metal contaminate generally produces a different signal which would not appear within the “detection envelope,” and the detector would trigger for product reject when it sees the signal.
Sensitivity 
In theory, aperture size determines the sensitivity of a given metal detector. Smaller apertures generally allow smaller pieces of metal to be detected. The smaller dimension of rectangular apertures is used to calculate the sensitivity, although the length also contributes. 
The smallest size aperture should be selected to maximize the sensitivity of a detector, however, there are some exceptions including metalized film and highly conductive product.
	In production, the sensitivity is affected by product effect, type and orientation of contaminant, and other ambient factors. Sensitivity is also affected by the position of the contaminant in the aperture. The least sensitive point is the centerline axis of the aperture, and this is where to test performance. As metal gets closer to the sides , the signal it generates gets larger, making it easier to detect.
	Types of metal
Metal detector sensitivity is not the same for all types of metal. The ease of detection depends on how easily they are magnetized (the magnetic permeability), and the electrical conductivity of the metal. Metal detectors are calibrated for sensitivity to ferrous metals, non-ferrous metals, and stainless steel.
Ferrous:
Ferrous materials are any metal easily attracted to a magnet (e.g. steel, iron, etc.). Typically, ferrous metals are easiest to detect and usually the most common contaminant outside of food processing plants.
Non-ferrous:
Non-ferrous materials are highly conductive non-magnetic metals (e.g. copper, aluminum, brass, phosphor bronze, etc.). When inspecting non-conductive products, these metals produce almost the same size signal as ferrous metals because they are all good conductors. When inspecting conductive products, increasing the test sphere size by at least 50% is a good practice.
Non-magnetic stainless steel:
High quality 300 series stainless steels (e.g. Type 304, 316, etc.) are the most difficult metals to detect due to their poor electrical conductive qualities and, by definition, have low magnetic permeability. These are commonly used metals in the food processing and pharmaceutical industries.
When inspecting non-conductive products, a stainless steel test sphere typically needs to be 50% larger than a ferrous sphere to produce the same size signal. When inspecting conductive products, a stainless steel test sphere needs to be 200% – 300% larger than a ferrous sphere to produce the same size signal.
Design 
Transmission/ Receiver coils 
Our purpose is creating a metal detector device which is one of the security applications. The resistive component of the received signal is measured for different frequencies (stepped frequency like, ~30 frequencies in the 0.1-10 kHz band). The measurement points are plotted in a diagram with the resistive component divided by the frequency against the frequency. The diagram is characteristic for a kind of metal and an approximate cross section. Furthermore for a given cross section andmetal type there exist a frequency at which the resistive component reaches a maximum. Different cross sections lead to different maximum frequencies. This peak frequency is proportional to the sample’s resistivity divided by its cross-sectional area. Actual signals for complex objects (e.g. revolvers) are also discussed, as well as phase accuracy and system stability issues. Object orientation effects do however not seem to be addressed.
The first one and also the most important part in this project is balanced coil (transmission and receiver coil). This is only tricky part. The search loop (transmission coil) is best wound on to plywood former. There are several methods to make a search coil. It depends on the frequency expected.
Method 1: cut three circles from some 3mm plywood, one 15cm diameter and two 16 cm diameter. Using wood glue to make a sandwich with 15cm diameter in the center. When the glue has set you can wind 10 turns of 0.25mm enameled cooper wire around the groove in the edge of the former. 
Method 2: cut a 16mm diameter circle from some 10mm plywood. Then with this circle clamped in a vice run a saw around the edge of the circle so as to make a slot about 5mm deep and 2 mm wide around the edge to accommodate the windings. 
Ideally this coil will be oscillating at about 104 kHz, with amplitude about 0.5v peak to peak. The second oscillator need to be made much smaller and if possible attached to the control box, so it can be adjusted. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal. It has been found in practice best to make the search oscillator fixed say at 100 kHz and to arrange for the reference oscillator to be adjustable 100 kHz plus or minus 250 Hz. This gives a beat note of 250 Hz to 0 to 250 kHz. The beat note disappears or nulls when the two oscillators are about equal. This type of detector is most sensitive when the beat note is close to zero, about 5 Hz any slight change being noticeable. 
The reference coil itself is wound on a piece of wood or plastic about 10/12mm diameter and about 50mm long The actual number of turns of this coil depends on the diameter of the former and can only be found by experiment. Start with about 125 turns, 25 enamelled copper wires (this coil when finished has to fit inside the plastic tube) and remove turns until the two frequencies are close. This coil is attached to the circuit board at points marked coil 2. If all is well the detector should be howling at this point. When the two oscillators are well matched it should be possible by adjusting the brass nut in or out to bring the beat note to a halt or null.
Practical Detector Circuit
There are several ways to build a metal detector circuit. Base on the parameters which we mentioned, we searched for some real circuits and chose the best one for our project:
By using Proteus software to draw schematic of the circuit, we have a circuit as below:
Coil A = search coil: Coil B = reference coil: B + = The red battery lead from 9v. 
2 off 220uf / 16v Electrolytic : These are 220 microfarad / 16v working voltage. You can use a higher working voltage but not less. Higher working voltage capacitors work just the same but they get physically bigger. They have a negative lead that must be connected to the battery - track. These components must go in the correct way round. 
5 off .1 and .01 polyester : These also have a working voltage. 63 volt in quite common and will be ideal. If you want to use the pcb layout above you will need capacitors with 5mm lead spacing.     .1 can be marked as .1 or 100n or sometimes 104 :   .01 can be marked as .01 or 10n or sometimes 103. These components can go in any way round.
All resistors 1/4 watt 5%: These are general purpose carbon film resistors with a 5% tolerance and rated at 1/4 watt. You could use resistors of a higher wattage as this does not affect the working they just get bigger. 1 watt or bigger will not fit on the board. These components can go in any way round.
Transistors: The bc 2N222 transistor is described has Audio, low current, general purpose NPN.
All components that we use are shown as below:
Test 
Metal Test Sample
Historically, metal detectors have been tested with a ferrous and non-ferrous, and sometimes a stainless steel test sample. More recently there has been a trend toward using a single preferably stainless steel) test sample in order to simplify the test process. The size of the test sample must be established so that it can be reliably detected inside product passing through the detector, down its centerline – the least sensitive point. Every application will be different and samples should be tailored to each application and detector. Samples too small for the application will cause unnecessary test failures, and create frustrated test operators. If the sample is too large, the performance of the detector will not be accurately tested. Establish a realistic, repeatable operating performance level using a selection of test sample sizes. Then choose an appropriate test sample(s) for testing each application type.
Typical guidelines for sensitivity:
Conclusion