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http://forum.allaboutcircuits.com – Hi all I just need some help on my project. what it will take to make a circuit that indicates by turning an led on. when a particular Ac signal is applied. For eg. 2 + A(Sin(2pi*f*t) where 1<=A<=4 and 20hz <= f <= 20k Hz. and also switch on another led when the frequency is between 970Hz to 1.03khz. i only know that a narrow band pass filter will be required to pass the desired frequency. (Embedded)