Metal Detector Circuit Pcb

[Rosamunda Froats]

Metaldetector is a very common device that is used for checking persons, luggage or bags in shopping malls, hotels, cinema halls, etc. to ensure that person is not carrying any metals or illegal things like guns, bombs etc. Metal Detectors detect the presence of metals.

There are different types of metal detectors like hand held metal detectors, walk through metal detectors and ground search metal detectors. Metal detectors can be created easily and the circuit for a basic metal detector is not that complex.

TDA0161 Proximity Detector IC: TDA0161 is a Proximity Detector IC manufactured by STMicroelectronics. It can be used detect metal objects by detecting the slight changes in the high frequency Eddy current losses.

The TDA0161 IC acts as an oscillator with the help of externally tuned circuit. The changes in supply current will determine the output signal i.e. current is high when a metal object is near and it is low when there is no metal object.

There are three main parts in the metal detector circuit: the LC Circuit, the Proximity Sensor , output LED and the Buzzer. The coil and the capacitor C1, which are connected in parallel, will form the LC circuit.

LC Circuit: LC circuit has inductor and capacitor connected in parallel.This circuit sarts resonating when there is same frequency material near to it. The LC circuit charges capacitor and inductor alternatively.When the capacitor is charged fully ,charge is applied to inductor.

Inductor starts charging and when charge across the capacitor is nil, it draws charge from the inducutor in reverse polarity. Then inductor charge is reduced and again the process repeats.Note inductor is a magnetic field storage device and capacitor is electric field storage device.

Proximity Sensor: The proximity sensor can detect the objects with out any physical interference. The proximity sensor will work same as infrared sensor, proximity also release a signal, it will not give output unless and until there is no change in the reflected back signal.

If there is a change in signal it will detect and give the output accordingly. There are different proximity sensors for example to detect plastic material we can use capacitive type proximity and for metals we should use inductive type.

The LC Circuit, which consists of L1 (coil) and C1, is the main metal detector part of the circuit. With the help of this LC Circuit, which is also called as Tank Circuit or Tuned Circuit, the TDA0161 IC acts as an oscillator and oscillates at a particular frequency.

When the LC circuit detects any resonating frequency from any metal which is near to it, electric field will be created which will lead to induces current in the coil and changes in the signal flow through the coil.

Variable resistor is used to change the proximity sensor value equal to the LC circuit, it is better to check the value when the coil is not near any metal object. When the metal is detected, the LC circuit will have changed signal.

The changed signal is given to the proximity detector (TDA 0161), which will detect the change in the signal and react accordingly. The output of the proximity sensor will less than 1mA when there is no metal detected and it will be around 10mA (usually greater than 8mA) when coil is near to the metal.

I built it and it works pretty good. Not sensative enough to use for digging up treasures at the beach, but I think it will be useful for finding that lost bolt or screw in the grass. Next step will be building a soldered up version and mounting it on a handle. No problem finding the IC on ebay.

I have a need for similar circuit but the only difference is, the circuit I need should function the same way as this one does IN MAGNETIC FIELD. Do you know if this should function the same way under the influence of magnetic field?

You never know what knowledge from your youth will be useful later in life. My experience with metal detectors helped solve a problem that my colleagues and I had in growing large crystals of an important semiconductor, gallium arsenide. We needed to produce crystals of uniform diameter, and the usual approach, weighing the crystal as it grew, was not an option. That was because the process of creating the crystals involved solidifying them from a crucible of molten material in a pressure chamber at about 1,250C and 25atm.

Instead of using an electronic balance to measure the mass gained by the crystal, I used a position-sensing inductance coil to sense the conductive surface of the molten gallium arsenide. A motor in a feedback loop raised the crucible to achieve a constant gap between the sensing coil and the molten surface, as the crystal was solidified from the liquid by pulling. The coil used in this case was 17 turns of molybdenum wire wound on a 10mm diameter form to a length of 13mm [1].

There are many published circuits for beat frequency metal detectors that use transistors to realize the oscillators, mixer, and amplifier. Unfortunately, simple transistorized radio-frequency mixers are inefficient, so I decided to update this type of circuit with a digital mixer (Figure 3). The sensing and reference oscillators are also implemented using digital circuitry in the form of CMOS logic gates, some of which are biased to act as analog amplifiers. The circuit is powered by a 9V battery.

The essential problem with this circuit and its analog cousins is the need to adjust the reference oscillator, to match its frequency to that of the sensing oscillator, each of which is about 500kHz to 700kHz. This demands either a variable inductance, as shown in the circuit diagram, or a variable capacitance. The needed adjustment is distracting for adults, and it might be too difficult for young children. This problem inspired me to design an advanced metal detector using a PIC microcontroller.

My advanced metal detector uses the PIC16F688 (PIC16) microcontroller as a frequency counter and to monitor changes in the search oscillator frequency from a baseline value. To make this metal detector behave like a traditional metal detector, the difference frequency was reconstructed, using a voltage-controlled oscillator (VCO) to drive the loudspeaker or headphones. A block diagram is shown in Figure 4. You might think that the microcontroller could just output this difference frequency on a pin, thus eliminating the VCO. However, there would be too many simultaneous processes happening to do this effectively.

The circuit diagram shown in Figure 5 details the construction. The sense coil oscillator connects to the Timer1 internal counter of the PIC16. Software captures the count value 10 times each second. When power is applied, or when the zero switch is depressed and released, the sense coil frequency is sampled and stored. This allows a difference frequency to be calculated upon subsequent counts. The potentiometer provides an analog signal that selects a multiplier value for this difference frequency, and a voltage value of this is created by the DAC to drive the VCO.

The sensitivity function operates by converting the analog voltage across the potentiometer to a digital multiplicand with value of 1, 2, 4, 8, or 16, as derived from a logic chain that translates the linear voltage to a stepped value. The frequency difference count, multiplied by this factor, is then sent to the DAC, where it is converted to a voltage, and then to a tone by the VCO. The front panel is shown in Figure 7 along with the completed metal detector.

The metal detector was designed to be easily held by both adults and children. As noted in the coil construction description, it was designed to be lightweight, with a fiberglass rod connecting the coil to the electronics. As shown in Figure 7, the rod was connected to the control box via a fixture that allowed setting the height of the coil. This fixture was built around a short piece of PVC pipe that was filled with a fast-setting polyurethane material, and then drilled to accommodate the rod and a tapped 1/4-20 hole for a steel bolt with a knob to tighten the rod in place.

The metal detector was tested by my 10-year-old grandson in the backyard of his house. To our amazement, he quickly found a die-cast zinc toy buried a few inches underground near a garden bed. This toy had been lost decades earlier by a previous occupant of the house.

Remember this: almost all circuits (up to about 4 - 6 transistors) have the same capability: detecting a 20mm coin at about 100mm. It doesn't matter if the circuit is simple or complex, The sensitivity revolves around the circuit driving the coil. Some circuits are more sensitive to "interference" and we have demonstrated this in Circuits 1 - 8 at the end of the page. The most-sensitive circuit is a 100mm dia coil with just 12 to 20 turns and operates at about 200kHz as shown in circuit 8. The frequency of the circuit will change by one Hertz and this can be detected on an AM radio. You cannot get better than this.

The simplest circuit is shown below:Another Simple Metal Detector Circuit

To learn more about the basics of circuit-design and recognise components, component-values, go to our: Basics Electronics 1A

We also have two Metal Detector Projects:

Metal Detector MkI

Metal Detector MkII

BASIC CIRCUITRY of

METAL DETECTION

byCharles D. Rakes

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.

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