NE555 Monostable One-Shot Timer

Press a button, get a single calibrated pulse — the textbook 555 monostable circuit that times an LED, relay, or any logic input for exactly the duration you set with one R and one C.

Difficulty: Breadboard. Estimated build time: about 25 minutes. Estimated parts cost: about US$3.20. 8-line bill of materials. Compare supplier offers when available. A one-shot timer that fires exactly one output pulse every time you press the trigger button.

Wire the trigger

Connect the NE555 like this: pin 1 (GND) to ground, pin 8 (Vcc) and pin 4 (Reset) to +5 V. Put a 10 kOhm pull-up resistor between pin 2 (Trig) and Vcc so the trigger sits idle HIGH. Wire a momentary push button from pin 2 to GND so pressing it briefly pulls Trig below 1/3 Vcc and starts the pulse.

Set the pulse length

Connect a 100 kOhm resistor between pin 7 (Discharge) and Vcc, then a 10 uF electrolytic capacitor from pin 6/7 (tied together) to GND. Pin 6 (Thresh) and pin 7 (Disch) share a node — the capacitor charges through the resistor while the output is HIGH, and when it reaches 2/3 Vcc the timer ends the pulse and resets. T ≈ 1.1 × 100k × 10u = 1.1 s.

Drive the LED

Connect a 330 Ohm resistor from pin 3 (Out) to the anode of an LED, and the LED's cathode to GND. Add a 10 nF ceramic from pin 5 (Ctrl) to GND — it filters the internal divider node and makes the pulse length much more stable. Power it up: every button press should give one clean LED flash of the calculated duration.