Detecting tones from PC audio output

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This is w*rk related. I will apologize in advance!

I want to detect the presence of a ~1 second tone coming from the audio output jack of a PC. I don't need to hear the tone, just detect it's presence and control a relay. Clearly, I can record any tone and assign it to the application interface.

From the top down, RCA into the output jack, into a magic box to detect the signal and then control a relay. This is important because it provides the income for this crack addict hobby!

(If you are interested, this is to detect failure of statistical control and capability in a real time manufacturing process. We will use the signal to interrupt the operation.)
 
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