Build a blinking LED with a 555 timer

The 555 timer is one of the most popular chips ever made, and the blinking LED is the circuit everyone builds first. Two resistors and one capacitor decide how fast it blinks.

Change the values and the LED below blinks at the real speed.

What's happening inside

The capacitor charges through R1 and R2. When it reaches two thirds of the supply voltage, the output goes low and the chip starts emptying the capacitor through R2 only. At one third it flips back, the output goes high, and the capacitor charges again. Round and round.

It's the capacitor charging curve, used as a clock.

The formulas

• On time: 0.693 × (R1 + R2) × C
• Off time: 0.693 × R2 × C
• Frequency: 1 ÷ (on time + off time)

With R1 = 1 kΩ, R2 = 68 kΩ and C = 10 µF: on for about 0.48 s, off for about 0.47 s. Roughly one blink per second.

Notice that the on time always includes R1, so in this basic circuit the LED is always on a bit longer than it's off. Make R2 much bigger than R1 and it gets close to 50/50.

Wiring all 8 pins

• Pin 1: ground
• Pin 2 (trigger) and pin 6 (threshold): joined together, and to the plus side of the capacitor
• Pin 3 (output): to the LED's resistor, LED to ground
• Pin 4 (reset): to plus. If you leave it floating the chip may randomly stop.
• Pin 5 (control): a 10 nF capacitor to ground keeps it stable. Often works without.
• Pin 7 (discharge): between R1 and R2
• Pin 8: plus supply
• R1 from plus to pin 7, R2 from pin 7 to pins 6 and 2, capacitor from pins 6 and 2 to ground

Common mistakes

• R1 too small. When the chip discharges, pin 7 connects to ground. A tiny R1 then lets a big current flow straight from plus into pin 7. Keep R1 at 1 kΩ or more.
• No LED resistor. The output can push a lot of current. The LED still needs its resistor, see the LED resistor calculator.
• Too fast to see. Above about 20 Hz your eyes see a steady light. Use a bigger capacitor or bigger resistors to slow it down.
• Electrolytic in backwards. Minus stripe goes to ground.

Make it yours

Swap R2 for a potentiometer and you can turn the speed up and down. Put a small speaker or buzzer on the output, speed it up to a few hundred Hz, and you have a tone generator. Same chip, same formulas.