I have a project which uses an Arduino to detect tones generated by a laptop. The Tone Detector (TD) circuit is a very simple LM567 implementation designed to detect 10kHz. The circuit is powered by the Arduino, connected to the laptop headphone socket via a standard 3.5mm stereo audio cable. In testing, with Arduino externally powered, the circuit works fine - the laptop generates tones, which the TD detects and raises a signal to the Arduino.
The problem is that it doesn't work if a USB cable is connected between the Arduino and the laptop. I have tried all configuration variations I can think of, but the common factor is when the USB cable AND the audio cable are connected it doesn't work, and it seems the audio is simply not generated in this configuration. The target Arduino is a Leonardo, but the same problem occurs with a Uno.
All suggestions very welcome!
In response to questions, yes there is a common ground. The audio input for the TD is connected to 0v.
Tone Detector circuit:
Audio input is connected to Input and 0v; Arduino to 5v, 0v & Output.
It is clear from the comments that the issue is around connecting the audio and digital grounds together, the question now is, how can I solve this?