PAGE 078 OF 150 · TIMERS, FLASHERS & DIMMING
555 Monostable One-Shot
One trigger gives one timed pulse.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
A single timed pulse
In monostable mode the 555 gives one pulse of fixed length each time it is triggered. The length is T = 1.1 × R × C.
Example
R = 100 kilohms and C = 10 microfarads gives about 1.1 seconds. Pulling the trigger pin low briefly starts the pulse, and the output then returns low by itself.
Uses
- A light that stays on for a short time after a button press.
- A pulse stretcher for brief signals.
Keep the trigger pulse shorter than the output pulse, and use a pull-up resistor of about 10 kilohms on the trigger pin.
REMEMBER
Switching an LED on and off quickly can look like steady dimmer light.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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