Hi I'm italian. I have a project to do: I want to turn on a LED only if the frequency in input is less than 100hz (for example).
I can only use RC filter.
Thanks for your help!!

 

any chips you can use like phase locked loop (pll) or op amps?

 

Hi I'm italian. ...
I can only use RC filter.
Thanks for your help!!

Italians can only use R's and C's?

 

there is no resonant point with rc circuits like lc circuits. you have to have active elements and use the phase shift of the rc circuit or several rc circuits across an active element to use to detect frequencies. see "ACTIVE FILTERS".