Capacitor Charge and Discharge
Time constant equals resistance times capacitance.
The RC time constant
A capacitor charges and discharges through a resistor at a rate set by the time constant, tau = R × C. After one time constant a charging capacitor reaches about 63 percent of the supply, and after five it is nearly full.
Discharging
A discharging capacitor falls to about 37 percent after one time constant and is almost empty after five.
Example
100 kilohms and 10 microfarads give a time constant of 1 second. This timing is what sets the blink rate in many flasher circuits, and with an LED it creates a soft fade when a button is released.
Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.