I hate vague requrests, please give more information!
The **broken link removed** PLL tone decoder is designed for this, but we need more info like the input, are you planning to use a microphone or will it interface with another circuit, you might have to build an amplifier?
Usually, a tone dectection circuit is built using an LM567 IC. It only takes a couple of external components to do what you want. Here's a link to a data sheet: **broken link removed**
(Actually, the LM567 is discontinued and has been replaced by a low-power CMOS version, the LMC567, which does the same thing.)
JB
this looks like just what i need for my audio cable tester 500hz to 3000hz, but havn't a clue about the additional components required?? can anyone help me out??
there is no contact with the cable, i've bought a cable tracer today from maplins, does kinda what i want it to but i would ideally want to increase the sensivity between 500hz and 3000hz, do get a bit of 'ac' fuz on it too, would be nice to try and reduce that if possible