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Alarm sound detector schematic


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An LM567 or CD4046 Phase-locked-Loop circuit can be used to detect a certain frequency and give a high output.
But your alarm is audio. Many sounds contain 3kHz and 4Khz frequencies, including noise.
A 1kHz square-wave contains plenty of 3kHz.
A frequency detector would give an output when a baby is crying and when a jet plane flies past.

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Hi Pramono,
I don't think a 4kHz filter with only a 5% bandwidth is practical. Its amplitude and frequency will drift and also probably the alarm's frequency. There are T-filter circuits on the web that narrow their bandwidth by injecting feedback to a point in the filter that is normally grounded. They use many matched parts to determine their amplitude and frequency.

There are many sounds at 4kHz. The harmonics of many other sounds are at 4kHz.
Recently my daughter brought over a Keys Finder gadjet that is supposed to beep when you whistle. I can make it beep only when I whistle at exactly one octave below its frequency (my whistle must have a strong 2nd harmonic, and I can't whistle as high as 4kHz) but it goes off all the time when my wifey and daughter are talking quietly and whenever the TV is on.

Telephones don't use a single frequency. They use two frequencies that aren't related. It is called Touch-Tone or Dual-Tone-Multi-Frequency. The tones are detected only when their frequencies are correct and also only if their levels are about the same. They shut-off their microphone when dialing to avoid interference.

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Hi there Guru...
Im thinking to use some kind of bandpass filter with 4 Khz as the center frequency and use high value of Q.
I got the article from http://www.electronics-tutorials.com/filters/active-bandpass-filters.htm
but i think the opamp used is double supply opamp (+ gnd -).
i dont know how to convert this circuit for a single supply opamp such as LM358.
can any body give me a hand?

thank you

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The bandpass filter circuit biases the non-inverting inputs (pin 3) of the opamps to half the supply voltage so they work with opamps usually needing a dual supply.

The MFBP circuit doesn't have a high Q nor high gain and any kind of bandpass filter needs an opamp with a wide bandwidth for low phase-shift.
The old LM358 is low-power (the 1st one, it is half an old LM324) so its bandwidth is reduced.
I would try the circuit with a TL072 wide-bandwidth dual opamp.

I have seen very high-Q bandpass filter circuits with a T-filter and even a Wien-bridge filter inside a feedback loop which sharpens the Q.

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