555 timer simulator
555 timing, from the formulas to a live circuit.
The 555 is timing distilled to an RC network and a couple of thresholds. Learn the astable and monostable maths, run the numbers in the calculator, and build the timing loop on the canvas.
Straight talk: there is no drop-in 555 chip in the palette yet. What this page gives you is the real timing theory and a calculator that matches it, plus the RC timing network you build from a source, resistors and a capacitor.
Two modes, two sets of timing
Astable
Free-running oscillator. The capacitor charges through R1 + R2 and discharges through R2, giving a continuous square-ish wave.
f = 1.44 / ((R1 + 2·R2)·C)
t_high = 0.693·(R1 + R2)·C
t_low = 0.693·R2·C
Monostable
One-shot pulse. A trigger sends the output HIGH for a single timed interval set by one resistor and one capacitor, then it returns LOW.
t = 1.1·R·C
Questions
- Is there a ready-made 555 chip in the simulator?
- Not yet — the 555 is not a first-class part in the palette. You build the timing behaviour from primitives: a source, resistors and a capacitor, and reason about the charge and discharge thresholds. The formulas on this page and in the calculator match that circuit.
- How do I calculate the astable frequency?
- Use f = 1.44 / ((R1 + 2·R2)·C). The 555 timer calculator does it for you and also returns the period, the high and low times, and the duty cycle.
- Why is the duty cycle always above 50%?
- Because the capacitor charges through R1 + R2 but discharges through R2 alone, so the high time is always longer than the low time. Making R1 small relative to R2 pushes it closer to 50%.
- What about the monostable one-shot?
- A monostable 555 produces a single pulse of width t = 1.1·R·C after a trigger. Pick R and C for the pulse length you want, then verify the RC timing on the canvas.