RC Circuit
From Wikiversity
RC Circuits
RC circuits are circuits that contain a resistor and a capacitor. These circuits are primarily used as frequency filters. There are two basic arrangements: high-pass and low-pass. A high-pass filter allows frequencies above the cut-off frequency to pass, while a low-pass filter allows frequencies beneath the cut-off frequency to pass. The arrangement of the resistor and the capacitor is what determines their behaviour.
Note that at a particular frequencly, called the cut-off frequency, the Capactive Reactance is equal to the Resitance value. (There is also an associated phase shift of 45 degrees.)
R = Xc
Substituting 
we then have:
The cut-off frequency, defined as the frequency at which the signal power is attenuated by 50% (or 3.01 dB), is a function of the resistive and capacitive values. We can rearrange the above formula to solve for f as follows:
When the capacitor is in parallel with the load while the resistor is in series with the capacitor and load, this creates a low pass filter. 
When the resistor is in parallel with the load and the capacitor is in series with the resistor, a high pass filter is created. 
A single RC circuit creates a filter with a 20.0 dB/decade, or 6.02 dB/octave, slope.
