PAGE 077 OF 150 · TIMERS, FLASHERS & DIMMING
555 Astable LED Flasher
Two resistors and a capacitor set blink speed.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The circuit
In astable mode the 555 repeats high and low pulses without a trigger. Two resistors (R1 and R2) and a capacitor (C) set the timing.
Formulas
- Frequency f = 1.44 ÷ ((R1 + 2×R2) × C)
- Duty cycle = (R1 + R2) ÷ (R1 + 2×R2)
Example
R1 = 1 kilohm, R2 = 47 kilohms and C = 10 microfarads gives about 1.5 flashes per second with near 50 percent duty cycle. Connect the LED and a 470 ohm resistor from output to ground.
REMEMBER
Switching an LED on and off quickly can look like steady dimmer light.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.
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