Detecting narrow-band audio signal
Detecting narrow-band audio signal
(OP)
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
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





RE: Detecting narrow-band audio signal
RE: Detecting narrow-band audio signal
RE: Detecting narrow-band audio signal
RE: Detecting narrow-band audio signal
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?
RE: Detecting narrow-band audio signal
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:
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
RE: Detecting narrow-band audio signal
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.