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LM567 tone decoder faulty?

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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.


New Member
Did you sweep for centre or tune it?
Hello Tony,

This time, I used a POT ( multiturn variable resistor) to sweep to the frequency of 12kHz as it was coming out of the PC. Basically turning the POT (adjusting fo) until the LED lights. I used the circuit from this link: http://www.scary-terry.com/more_stuff/tonedet/tonedet.htm

The first time, I actually calculated the component values to lock onto a frequency of 12kHz, also using the Vrms - BandWidth graph in the datasheet and then made fine adjustments to the frequency until the LED came on (which it did). Then the same thing happened and it stopped working.

Maybe the 567 is drifting maybe due to temperature or maybe I am trying to capture a small signal in very tight bandwidth, so perhaps its not necessarily broken.

Tony Stewart

Well-Known Member
Most Helpful Member
NE567 has a sensitivity of about 20mV at centre freq. so either source or NE567 is drifting with voltage or temperature.
Can you add 10x gain or use the CD4046 instead?

I once did a project using 2nd order adjustable BPF with adjustable Q and relatively flat gain feeding a precision rectifier. it was part of my grad thesis design in 1975.

The result was I could tune it to any note on the scale and I could whistle with an electret mic and detect each note with a switched output.

There is a trade-off between simplicity and sensitivity and accuracy.
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