A blinking LED with the 555 timer

The 555 is an integrated circuit, a complete small circuit packed into one chip with eight legs. It has been made since the 1970s and is still one of the most used chips in hobby electronics. Its best-known use is making an LED blink. This page combines two earlier ones: how a capacitor charges and choosing a resistor for an LED.

Change the values in the tool. The LED blinks at the speed the real circuit would have.

How it works

In this circuit the 555 runs in a mode called astable, which means it never settles: it switches on and off by itself for as long as it has power. A capacitor charges through two resistors, R1 and R2. When the capacitor reaches two thirds of the supply voltage, the chip switches its output off and starts discharging the capacitor through R2 only. When the capacitor has dropped to one third of the supply, the output switches on again and the capacitor charges once more.

The output pin is on while the capacitor charges and off while it discharges. The LED connected to the output therefore blinks, and the speed depends on how long the charging and discharging take.

The timing

• On time: 0.693 × (R1 + R2) × C
• Off time: 0.693 × R2 × C
• Frequency (blinks per second, in hertz): 1 ÷ (on time + off time)

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

The share of time the output spends on is called the duty cycle. In this basic circuit the on time always includes R1 and the off time doesn't, so the output is always on a little longer than it is off. When R2 is much larger than R1, the duty cycle gets close to 50%.

Wiring the eight pins

• Pin 1 goes to ground.
• Pin 2 (trigger) and pin 6 (threshold) are joined together and connected to the plus side of the capacitor.
• Pin 3 is the output. Connect the LED's resistor to it and the LED from the resistor to ground.
• Pin 4 (reset) goes to plus. Left unconnected, it can stop the chip at random.
• Pin 5 (control) can stay unconnected, but a 10 nF capacitor from it to ground makes the timing more stable.
• Pin 7 (discharge) connects to the point between R1 and R2.
• Pin 8 goes to plus.
• R1 goes from plus to pin 7, R2 from pin 7 to pins 6 and 2, and the capacitor from pins 6 and 2 to ground.

Common problems

• R1 too small. When the chip discharges the capacitor, pin 7 is connected to ground inside the chip. A very small R1 then lets a large current flow from plus into pin 7. Keep R1 at 1 kΩ or more.
• No LED resistor. The output can deliver more current than an LED should get, so the LED still needs its resistor.
• It seems to stay on. Above about 20 blinks per second, the eye sees a steady light. Use a larger capacitor or larger resistors to slow it down.
• The capacitor is reversed. An electrolytic capacitor's minus stripe goes to ground.

Variations

Replacing R2 with a potentiometer in series with a small fixed resistor gives a blink speed you can adjust with a knob. With a much smaller capacitor, the 555 switches hundreds of times per second, and a small speaker or piezo sounder on the output plays a tone instead of blinking a light.