Hi,
I am experimenting with the LM567 / NE567 tone decoder IC.
I am taking the audio output from the PC headphone jack and using an PC application to generate a 12kHz tone. Using an LED connected to the LM567, the LED lit everytime there was a 12 KHz tone - freq calibrated using a POT. Using a scope the input voltage to the 567 was about 68mV when it triggers.
The problem I get is that after working fine at the beginning, it stopped working. It still triggers but only then at higher mV - several hundred mV. I cannot get it back to the state where it generated the lock at the lower level of 68mV. Even after re-adjusting the POT.
Could the input part of the 567 been damaged at all? Trying to think what the setup was when it stopped working and maybe because I was using a 3.5mm mono plug plugged into the stereo jack of the PC to introduce the input signal to the 567 and plugging/unplugging it, somehow a high voltage spike was input to the the 567 by accident. I am using a coupling capacitor at the 567 input.
I tried this with a second 567 chip and looks like I'm getting the same result.