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From the PyroEDU tutorial for the 555 Timer
Interval for the blinker is based on the time it takes for the capacitor to discharge. The more energy (greater farad value), the longer the blinker takes to charge and discharge.
Additionally, increasing or decreasing the resistance on the two large resisters will increase the amount of time it takes to discharge the capacitor, and thereby, keep the light on for longer periods of time.
Try playing with some of the switches to see what happens, particularly, how the switches affect voltage (signal), to the LED.
The Math
from PyroEDU:
Let:
Time On = T(ON)
Time Off = T(OFF)
Resist#1 Resistance = R1 oms (top)
Resist#2 Resistance = R2 oms (bottom)
C = Capacitor Farad Value in F
Let:
ln(2) = 0.6931471806... = 0.693
T(ON) = ln(2)*(R1+R2)*C
T(OFF) = ln(2)*R2*C
Period = T(ON)+T(OFF) seconds
Frequency = 1/[T(ON)+(OFF)] Hz (hertz)
Using a bit of algebra, you can calculate the time off and time on you want in addition to period and frequency.
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