Source: https://mokxi.com/learn/555-timer-simulator Updated: 2026-10-05 Learn # A 555 timer simulator, blinking an LED Beginner Written by the Mokxi team, updated October 5, 2026 Start building free Open the editor Or see every lesson. - 261 parts on the bench - 31 boards running now - 1.00x real time, on every board Press Run, then click the slide switch A 555 in astable mode blinks an LED; the slide switch swaps the timing resistor, from 1.4 Hz to 7.4 Hz. Press Run, then flip it. The 555 is the chip that taught a generation to build oscillators. Wired as an astable it needs two resistors and a capacitor to blink an LED, beep a buzzer or clock a counter, with no microcontroller and no code. It is still one of the first chips on any electronics course, and it is a good test of whether a simulator really simulates a circuit or only draws one. The circuit above is a 555 in astable mode on a breadboard. The capacitor charges through two resistors until it reaches two thirds of the supply, then the chip discharges it through one of them until it falls to one third, and the output flips each time. The LED on the output blinks at the rate those parts set. A slide switch swaps the timing resistor, and the rate changes from 1.4 Hz to 7.4 Hz. Press Run, watch it blink, then flip the switch. The timing comes out of the circuit, not out of a number stored in the part. The capacitor’s voltage is solved in time with the rest of the circuit, so change the capacitor or a resistor in the properties panel and the frequency moves to where the formula says, f = 1.44 / ((R1 + 2 x R2) x C). Put a scope probe on the capacitor and you see it ramp between one third and two thirds of the supply; put one on the output and you see the square wave, high a little longer than it is low, because the capacitor charges through both resistors and discharges through only one. From here the 555 does more. Drive a piezo from its output and it is a tone generator. Feed its output into a decade counter and you have a light chaser with no code at all. Wire it as a monostable and one press gives one pulse of a set length. The 555 timer calculator page works the formula both ways, from parts to frequency and from a frequency you want back to the parts, and the 555 projects page collects more builds to open and run. ## Questions Can I simulate a 555 timer online for free? Yes. Mokxi runs the 555 in your browser with no install and no account, in astable and monostable circuits, with the capacitor’s voltage solved in time so the timing comes from the parts you choose. Why is the 555 output high longer than it is low? In the basic astable the capacitor charges through both resistors but discharges through only one, so the high time is always the longer of the two. Related ## Keep going The 555 Astable: Timed by a Capacitor, Not a Crystal The 555 Monostable: One Press, One Timed Pulse 555 Timer Projects for a Science Fair (No Coding) Every analog circuit Sweeps and frequency response, in the docs Mokxi and Wokwi, compared The project page, with the full sketch Beginner ## Build this for real Open the editor, change a value and watch the number move with it. Nothing to install, and no account needed. Start building free Open the editor