How to Make a Slow-Flashing LED

Updated April 17, 2017

Many LED-flasher projects require that you adjust how fast or how slow the LEDs will blink on and off. A slow-flashing railroad-crossing light in a toy town, for example, will need a slow-flashing LED.

Adjusting the value of the resistors and the capacitor in the flasher circuit will let you vary the flashing rate. For flasher-LED circuits based on the 555-timer circuit, a formula is used to calculate the flash frequency

Obtain an electronics-flasher hobby kit that uses a 555-timer 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 555-timer 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 555-timer 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.


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).

Things You'll Need

  • Resistors
  • Capacitors
  • Calculator
  • LED flasher kit based on 555 timer
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About the Author

Mark Stansberry has been a technical and business writer over for 15 years. He has been published in leading technical and business publications such as "Red Herring," "EDN" and "BCC Research." His present writing focus is on computer applications programming, graphic design automation, 3D linear perspective and fractal technology. Stansberry has a Bachelor of Science in electrical engineering from San Jose State University.