How to make metal dector

Google is your friend John: here's one.

https://nicholsonelectronics.co.uk/build-a-metal-detector/

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How to make metal dector with 555 ic

https://www.instructables.com/Metal-Detector-Variable-Frequency-Using-555/

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How 555 inductor work with metal dector 

From the linked article:

The heart of the electronics design is two 555 Timer IC's. IC1 is used to generate a square wave used in the Tx coil and IC2 is used to detect any signals picked up by the receive coil.

The Tx (transmit) section of the circuit generates a square wave that is fed to the Tx coil via a MOSFET. A MOSFET is utilised to allow more current to be driven through the Tx coil. The equation for calculating the magnetic force on a wire is Force (N) = magnetic flux density (T) × current (A) × length (m) therefore allowing more current through the coil increases the magnetic force. The maximum output current from the 555 is about 200mA however if we use this to drive the IRF510 MOSFET it is then possible to drive a current up to about 5A through the coil, which is then limited by the impedance generated in the coil.

The Frequency at which the Tx coil operates is calculated using the equations shown below:

Minimum Frequency = 1 / (((R1 + 2 x (R2max + R3)) x C4) / 1.44)

Minimum Frequency = 1/ (((1000 + 2 x (100000 + 75000)) x 10x10^-9) / 1.44) = 410 Hz

Maximum Frequency = = 1 / (((R1 + 2 x (R2min + R3)) x C4) / 1.44)

Maximum Frequency = 1/ (((1000 + 2 x (0 + 75000)) x 10x10^-9) / 1.44) = 953 Hz

 

The Rx (receive) section of the circuit acts as the detector. In this circuit the 555 Timer IC is setup as a Schmitt Trigger. If an imbalance occurs between the RX and Tx coils it is fed initially to the transistor Q1 via capacitor C2 which amplifies the signal. The base of the transistor is biased using R5 and R6 and is tuned using R5. The signal is basically a weaker version of the square wave generated by the Tx coil. A Schmitt Trigger works by setting the output high if the upper threshold of the input is reached and then setting the output value low if the lower threshold of the input is reached. The nature of a 555 Timer means that the lower and upper threshold limits are set at 1/3 and 2/3 of the supply voltage accordingly. This means with a 9v supply the lower threshold is 3v and the upper threshold is 6v. The threshold and trigger pins of the 555 timer are connected together and the voltage at this point is set by a voltage divider by the three potentiometers, R7, R11 (Fine Adjust), and R10 (Course Adjust). By setting up these three potentiometers we are aiming to set the midpoint of the input to somewhere between 3v and 6v (around 4.5v). The output of the Schmitt trigger is from pin 3 which can drive 200mA therefore is capable of directly driving the piezo buzzer and indicator LED.

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How unbalance frequency is Dected what is the use of metal dector in railway station airport mall metro station 

All that is covered in the piece, but it's not a simple circuit to describe. They are used in railways, airports and so on to detect knives, guns and other offensive weapons that could be smuggled into a mass transit system by bad actors.

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How to make scan bag in metro station railway station what is name of electronic machine 

That would be an X-ray machine, we can't make these as they are extremely dangerous.

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How x ray machine are made for checking price of x ray machine checking in metro station