Need help with VHDL code for metal detector

I am working on a metal detector at the moment but cant figure out how 
to implement it my VHDL code.

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ENTITY sensor IS

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     port ( metaldetector : in std_logic;

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            metal : out std_logic;

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          );

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END ENTITY sensor;

As long as there isnt any metal close to the sensor 'metaldetector' gets 
pulses on a 6.1kHz frequency. So as long as the 'metaldetector' keeps 
getting pulses the out port 'metal' should be '0'.

When there is a pulse missing (or multiple pulses) 'metal' should become 
'1' until the next pulse.

It shouldnt be that hard to make a code that can do that, but i just 
cant figure it out. Any help would realy be great!