Try it
Keeps pulsing
Runs automatically: HIGH → LOW → repeat.
HIGH 762 µs · LOW 693 µs · repeats continuously
Illustrative playback; each displayed cycle is 1 second. HIGH/LOW proportions follow your inputs.NE555 circuit & pin connections
Wiring notes & pin reference
8 (VCC) and 4 (RESET) → supply positive. 1 (GND) → supply negative. 3 (OUT) → output. 5 (CTRL) → 10 nF → ground. Add 100 nF supply decoupling close to pins 8 and 1. All ground symbols connect to the same return.
VCC → R1 → pin 7 (DISCH) → R2 → pins 6 (THRES) and 2 (TRIG) → C → ground.
Logical schematic, not physical package layout. TI LM555 datasheet, figures 11 and 14.
Understand the result
What's going on
In astable mode, a 555 timer never settles - it charges a capacitor up, discharges it back down, and repeats forever, producing a square wave with no input signal needed at all. Two resistors and a capacitor set the pace.
The formula
t_high = 0.693 × (R1 + R2) × C
t_low = 0.693 × R2 × C
f = 1 / (t_high + t_low)
Show the derivation, mnemonic, and worked example
Build it up
The capacitor charges through both R1 and R2 together, which sets how long the output stays high. It then discharges through R2 alone, which sets how long the output stays low. Because charging always uses more resistance than discharging, the high time is always longer than the low time - a plain 555 astable can never produce a perfect 50/50 square wave.
Worked example
Worked example
R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF
t_high = 0.693 × 11,000 × 1e-7 ≈ 0.76 ms
t_low = 0.693 × 10,000 × 1e-7 ≈ 0.69 ms
f = 1 / (0.76 ms + 0.69 ms) ≈ 690 Hz
