Get the signal when pressing mouse left key

A Few weeks ago, I had a project which I need to synchronize two data acquisition system. One is recording force, the other is EMG data. Originally, I hope the force acquisition system can send the analog output to the EMG system, so I can record both data on EMG system. However, the force recording system is too difficult to send the analog output because the manufacture close all the lines in the box.

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Another way coming in my mind is to use a analog signal sending to both system. However, as I mentioned before, the force recording system closes all the lines in the box and I can't open it. I was forced to think another solution. Finally, I come up with sending a signal by the mouse when pressing the start of the force recording system so my EMG system can record the time of the beginning of force recording.

However, I know nothing about printed circuit board. I relied on the spirit of try and error to find the two position which can generate 5 voltage when pressing the mouse left key and then wield it.

I know it is nothing for many people, but for me, it's a good experience to do the thing that I have never done before although I accomplished it by try and error.