Try it
One trigger. One pulse.
Press Trigger. The output goes HIGH, then returns LOW and waits.
Calculated pulse: 1.1 s
Illustrative playback: 1.5 seconds on screen. Use the calculated pulse duration for the circuit.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 → R → pins 7 (DISCH) and 6 (THRES) → C → ground. Drive pin 2 (TRIG) from a normally-high external signal; pulse below VCC/3, then return high before the timer expires. Do not leave this input floating.
Logical schematic, not physical package layout. TI LM555 datasheet, figures 11 and 14.
Understand the result
What's going on
In monostable mode, a 555 timer sits quietly until it’s triggered - then it outputs one clean pulse of a fixed length and goes back to waiting. It’s the “one-shot” mode: a short low trigger pulse starts one timed action. Return the trigger high before the timing interval ends; holding it low can extend the output pulse.
The formula
T = 1.1 × R × C
T is the pulse width in seconds, R in ohms, C in farads.
Show the derivation, mnemonic, and worked example
Build it up
The trigger starts a single charge cycle on the capacitor through the resistor. The output stays high for as long as that charge takes to cross a fixed internal threshold - about two-thirds of the supply voltage. Bigger R or bigger C means the capacitor takes longer to reach that threshold, so the pulse lasts longer.
Worked example
Worked example
R = 100 kΩ, C = 10 µF
T = 1.1 × 100,000 × 0.00001
T = 1.1 seconds
