Detecting narrow-band audio signal 1

I'm trying to design a circuit that will re-act to a narrow-band audio signal. Say, if a door switch was wired to make a short 2,600 hz chirp when it opened, how can I ignore ALL other audio noise and accept only 2,595hz - 2,605hz more or less? The audio amplifier circuit and so on is absolutely no problem. Even audio filters (wide-band) I can do. But this ULTRA-NARROW-Band filter is giving me fits.

Would I be better working with, say a MC4044 frequency detector? If I remember correctly, those need a 5v signal input. I can't alway guarantee that the chirp will be the same strength.

Any suggestions are appreciated!

Sincerely,
Jethro

 

[peebee]

I'm thinking you might want to try comparing the detected audio to a reference signal and count the beat frequency. Cheap metal detectors work similarly, look at some of them for a beat counter. Frequency drift could be a problem, though.

 

[cbarn24050]

Hi, it seems like you are trying to do things the hard way. If you mix the audio with a say 2580Hz then you get an output of 15-25Hz which is easier to filter.

 

[fred29653]

Look at telephone tone decoder circuits they do just what you want.

 

[repoman]

2600Hz LOL!

Thanks captain crunch whistle. The 2600Hz tone has long been disabled by the phone company, so forget about phreaking.

Anyway, why not grab a simple TL074 Op-amp and making 2 x 12db/octave notch filters. Look up Delyanis filtering, or 4th order active bandpass filters.

Does this device have to be battery operated?

 

[GLANGWELL]

I've built similar circuits using the old LM567 PLL chip. It's cheap and VERY easy to use (8pin). It's been a while since I worked with it, so there may very well be a newer (better?) replacement, but the LM567 is still widely available and should do the job. In fact, I have one sitting here now that I plan to use in a very similar circuit any day now.
Over a decade ago, I designed a HAM radio morse code "regenerator" which relied on this IC. It worked well under conditions close to what you describe here.
There are limitations whereby the bandwith you choose will affect other parameters (etc.), but at a quick glance I think you will find it suitable. You can do a search for "LM567 application", or try this link to a PDF app page:

http://www.ee.nmt.edu/~wedeward/EE382/SP00/LM567.pdf

On that PDF sheet, there is an image called "typical applications". At the bottom of that image you'll see a simple circuit named "AC test circuit". That "AC test circuit" is in fact a PLL tone decoder, and will work over a wide range of voltages (with limitations - see specs).
The formula and example there should make it easy to design it to your specs.
I'd suggest that you breadboard the above circuit first, while you tweak it to fit your needs. Maybe install an LED as the breadboard "load", to observe it's action.
Fast, easy, simple, cheap. (less than $0.50 US).
Hope that helps.
GL

 

[Chancy99]

EPE (Everyday Practical Electronics) magazine has a neat project in the November 2001 issue called a Pitch Switch.

It did exactly what you are looking for. Very neat and interesting application of non-microcontroller circuitry.

I have that issue in PDF format - 4.7meg. Shoot me an email address that can take it and I'll send it along.

Dean.