Arduino Project help request (using a metal detector to stop motors)

me and a group are constructing a minesweeping robot that uses a metal detector attached to the front that will continuously search for metal that comes within range of the coils.

What i am trying to achieve is to have the robot immediately stop once metal is detected. From my understanding i would be able to do this by using if statements that simply take an input signal from an external metal detector going in to the digital pins. So i think the code would go something like if metalinput is 1.5v> then send a signal to the motor controller to stop the motors.

However i am unsure of how exactly i would be connecting the metal detector up to the arduino and in what format the code would be.

I have built a small prototype of what i think it would look like which is in the images, but if i could get some direction on where to go from here that would be a great help.

Back up a bit. My metal detector only has an audio tone output to either a speaker or headphones. IS yours different and outputs a digital signal?

Paul

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Why have you put that overlay over the chip and another part of the circuit. Is that a real Uno or a forgery?