Many LEDflasher projects require that you adjust how fast or how slow the LEDs will blink on and off. A slowflashing railroadcrossing light in a toy town, for example, will need a slowflashing LED.
Adjusting the value of the resistors and the capacitor in the flasher circuit will let you vary the flashing rate. For flasherLED circuits based on the 555timer circuit, a formula is used to calculate the flash frequency
Things You'll Need
 Resistors
 Capacitors
 Calculator
 LED flasher kit based on 555 timer

Obtain an electronicsflasher hobby kit that uses a 555timer chip. Build the circuit according to the instructions given.

Locate the 555 timer on your circuit board or schematic. Now locate the timing resistors (R1 and R2) and timing capacitor (C) that are connected to the 555timer component. Look at the components connected to the 555 timer: between pins 8 and 7 for R1, between pins 7 and 2 for R2 and between pins 6 and 1 for C. Write down the value of the resistors and capacitors. Use the values for R1, R2 and C given on the schematic or the electronic parts list, or the value that is written on the resistors and capacitor components.

Calculate the frequency of operation using the formula given in the 555timer data sheet. Use 1000 ohms for R1, 2000 ohms for R2 and 10uF (0.000010) for the capacitor C. Multiply the value of R2 by 2, then add that to the value of R1. Next, multiply the result by the value of C. Finally, divide 1.44 by the last result. Verify your calculation with the formula:
f = 1.44/(R1 + 2R2)C = 1.44/(2000 + 2*2000)0.000010 = 24 blinks per second
Select a higher value of R1 to lower the blink rate. Use 20,000 for this example. Calculate the blink rate to be 0.6 blinks per second, or 6 blinks every 10 second.
Tips & Warnings
 The time the LED is on and the time the LED is off will also be affected by the values selected for the resistors and capacitor. The formula for the on time is 0.693(R1+R2)C and the formula for the off time is 0.693*R2*C (0.693 multiplied by the value of R2 multiplied by the value of C).
References
 Photo Credit LED image by URIO from Fotolia.com