Recognizeing a frequency

[555 timer chip guy]

Dos any one have a circuit that can recognize a specific frequency.

[Kohanbash]

do a google search for a notch filter or bandpass filter.
you can use an op amp or a simple RLC circuit.

[robot321]

the G8870 decodes DTMF Freqs.

[555 timer chip guy]

It needs to be able to recognize square wave forms.

[Kohanbash]

maybe a DAC before the notch filter. Or use a cap in parallel with the receiver to convert to sinusoidal. I don't know of any offhand but they make tone decoder IC's which also might work

[trigger]

why don't you ADC your signal, and then filter in software? (e.g., use DSP).  Much easier to find a square wave. 

[555 timer chip guy]

thanks but I don't know how to do that yet.

[Kohanbash]

You will need to play with the values a lot but here is a basic circuit that should work.

[555 timer chip guy]

You will need to play with the values a lot but here is a basic circuit that should work.

Thanks for that schematic I was looking at some thing very similar a littel wile ago
do you haze any basic starting values I could use for maybe 100Hz , and also I heard you could use a comparator and a crystal to recognize a frequency.

[ed1380]

<-noob
Isn't that what an osciliascope is for?

[zamboniman60]

well, if you don't have an extra $160 laying around for an oscillosope, you could use the "inductor in series with an LED brightness test" or the "speaker and function generator guesstimation".