555 Timer Calculator

Astable: the blinker

In astable mode the 555 charges C through R1 and R2, discharges it through R2 alone, and flips its output each way. Frequency is 1.44 ÷ ((R1 + 2×R2) × C), and because charge and discharge take different paths, the duty cycle is always above 50%. Want close to an even blink? Make R2 much larger than R1.

Monostable: the one-shot

Ground the trigger pin for a moment and the output goes high for exactly 1.1 × R × C seconds, then drops. That's a debounced button, a staircase light, a "hold for three seconds" behavior, all from one chip, one resistor and one capacitor.

The pin labels on the diagram follow the standard DIP-8, pin 1 at the notch. A fuller version lives on the 555 pinout page.

The parts that keep 555 circuits from working

Three faults account for most of them. A missing decoupling capacitor across the supply lets the chip's own switching current upset its comparators, so add 100 nF at the pins. A floating reset pin, number 4, makes the output stop at random, so tie it to the supply rail unless you are actually using it. And leaving pin 5 open lets noise onto the control voltage input, which is why a 10 nF capacitor from pin 5 to ground appears in nearly every published schematic.

Bipolar 555 or CMOS

The original NE555 is a bipolar part that drives 200 mA and produces a sharp current spike on every transition. The CMOS versions, sold as 7555, TLC555 or LMC555, run from lower supplies, draw microamps instead of milliamps and behave far better on a battery, at the cost of a much weaker output. For blinking an LED either works. For a battery-powered timer that must idle for months, the CMOS part is the only sensible choice.