13 Jun 2026
Monostable Multivibrator: Waveform Study Using an Oscilloscope
Aim
To observe the input trigger and output pulse of a monostable multivibrator using a CRO and determine its pulse width.
Apparatus
Monostable multivibrator trainer or 555 timer, DC supply, trigger generator, CRO, resistors, capacitors, and patch cords.
Experimental arrangement
Theory
The timing capacitor voltage changes according to the charging law of an RC circuit. A monostable circuit uses a trigger to switch from its stable state to a temporary quasi-stable state. For a 555 timer, the capacitor charges towards $V_{CC}$ through $R$ according to
\[V_C(t)=V_{CC}\left(1-e^{-t/(RC)}\right).\]The threshold comparator changes state when $V_C$ reaches $2V_{CC}/3$. Therefore,
\[\frac{2}{3}=1-e^{-T/(RC)},\qquad T=RC\ln3\approx1.1RC.\]The circuit then returns automatically to the stable state. Thus the trigger controls when the pulse begins, while $R$ and $C$ control its duration.
\[T=1.1RC.\]Observations
| $R$ (kohm) | $C$ (microfarad) | Calculated $T$ (ms) | CRO pulse width (ms) |
|---|---|---|---|
| 10 | 0.10 | 1.10 | 1.12 |
| 10 | 0.22 | 2.42 | 2.45 |
| 10 | 0.47 | 5.17 | 5.20 |
Calculation
For $R=10\,\text{k}\Omega$ and $C=0.10\,\mu\text{F}$,
\[T=1.1RC=1.1(10\times10^3)(0.10\times10^{-6})\,\text{s}=1.10\,\text{ms}.\]The CRO reading is 1.12 ms. The percentage difference is
\[\frac{|1.12-1.10|}{1.10}\times100=1.8\%,\]which is reasonable for component tolerance and triggering uncertainty. For $C=0.22\,\mu\text{F}$, the same relation gives $T=2.42$ ms, agreeing with the second reading.
Maxima Code
Download the monostable calculation file.
Result
The monostable produces one output pulse for each trigger, and its width agrees with the timing relation $T=1.1RC$.
Viva Questions
- How many stable states does a monostable have? One.
- What controls the pulse width? The timing resistor and capacitor.
- Why is a CRO used? To observe the trigger and output pulse in time.
Discussion