Blinking an LED (555 Timer)

Blinking an LED is a simple project that illustrates how to use the 555 Timer in it's astable configuration. You could also buy a stand alone blinking LED but then it would not be a project anymore!

=Bill Of Materials=


 * 555 Timer
 * R1 1500 Ohm resistor
 * R2 820 Ohm resistor
 * R3,R4 180 Ohm current limiting resistors
 * Two LEDs
 * C 68 µF capacitor
 * 0.01 µF capacitor
 * Batteries or other DC 4.5 volt power source

Astable mode


In astable mode, the '555 timer ' puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:


 * f = 1/(ln(2) * C * (R1 + 2R2))

The high time from each pulse is given by


 * high = ln(2) * (R1 + R2) * C

and the low time from each pulse is given by


 * low = ln(2) * R2 * C

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

=References=


 * http://en.wikipedia.org/wiki/555_timer_IC
 * http://www.horrorseek.com/home/halloween/wolfstone/TechBase/com555_555TimerCalc.html