|In a frequency detector, you won't be turning on and off an LED, so you
|would just test the input and increment a counter
|(once or a number of times, depending on required noise immunity and
|processor speed) for a period of time each time a
|high is seen. You would repeat the process for the same amount of time and
|counts, only decrementing the counter each time
|a high is seen. The cycle rate of incrementing and decrementing would be
|the target trequency. The number of samples and the
|trigger threshold (count) would set the bandwidth. Noise would be present
|both during the incrementing and decrementing and
|would be cancelled (averaged) out.

The problem you're apt to run into is that the phase of the input
signal is unknown.  Consequently, if you just use a single detector
you'll get a signal that may be anywhere from full-value positive
or full-value negative.

There are two ways around this problem:

[1] Use two or three dectors, whose phases are set either 90 or 120
    degrees apart, and use a suitable weighting function to combine
    their results.

[2] Use a reference frequency which is "off" by a known amount; this
    will cause the output of the detector to produce a wave whose
    frequency is the difference between the source and reference
    frequencies.

For DTMF dection, [1] is the way to go although most TV's and FM
radios use [2] in their tuning circuits.