Since you didn't see fit to add your sketch to the post and you didn't clearly describe what the LT????? thing is... It looks like it should work. The 'sensor' has an output at some unstated condition... and you are using a part of that output to trigger the FET... OK as far as it goes however the Vgth of a fet is generally about 2 volts. If you need sensing at a volt or less (> .8V) you should use an NPN type transistor, a 2N3904 should work very well and you are missing one part, Fet or Bipolar transistor THERE must be a pull-UP on the drian or collector lead (the one that goes to the '328 input) unless you have set the pin as an input and set it true (high) without that it cannot work and without your sketch there is no way to tell. IMO
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