555 monostable (one-shot timer)
A short low pulse on TRIG sets the output high and lets C charge through R. When it reaches ⅔ V_CC the 555 resets and empties C. Every trigger gives one pulse of fixed width t = 1.1·R·C.
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How it works
t = 1.1 · R · C = 1.1 × 22 kΩ × 47 µF = 1.14 s
The 1.1 comes from ln 3: C charges from 0 to ⅔ V_CC, and −ln(1 − ⅔) = 1.0986. The scope’s 1 MΩ input on C stretches it slightly; the table allows for that.
Try it on the bench
- Hold the Trigger button for a moment, then let go: the LED stays on for about 1.1 s.
- Hold the button longer than the pulse: TRIG low keeps the output high until you let go.
- Change R to 100 kΩ for a 5 s timer. The scope’s 1 MΩ input on C now loads it enough to stretch the pulse by about 10 %, which is why real probes are switched to 10×.
Parts on the bench
- Bench power supply
- 555 timer
- 3 × Resistor
- Electrolytic capacitor
- Push button
- Film capacitor
- LED
- Oscilloscope
In the course
Zen’s Electronics Design Mastery, module “Op-Amps & Analog ICs”.