13 Jun 2026
IC 555 Monostable Multivibrator: Pulse-Width Measurement
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
A monostable multivibrator has one indefinitely stable state and one temporary state. In the IC 555, three equal internal resistors establish reference levels at $V_{CC}/3$ and $2V_{CC}/3$. A trigger comparator, threshold comparator, SR latch, and discharge transistor use these levels to control the output and timing capacitor.
In the stable state the output is LOW and the discharge transistor keeps the capacitor nearly at zero. A negative trigger that falls below $V_{CC}/3$ sets the latch. The output becomes HIGH, the discharge transistor switches off, and the capacitor begins charging through the external resistance $R$ toward $V_{CC}$:
\[V_C(t)=V_{CC}\left(1-e^{-t/(RC)}\right).\]When $V_C$ reaches $2V_{CC}/3$, the threshold comparator resets the latch. The output returns LOW and the discharge transistor rapidly empties the capacitor. Setting $V_C(T)=2V_{CC}/3$ gives
\[\frac{2}{3}=1-e^{-T/(RC)},\qquad T=RC\ln3\approx1.1RC.\]Thus the trigger determines when the output pulse starts, while $R$ and $C$ determine its width:
\[T=1.1RC.\]Provided the trigger pulse is shorter than $T$ and ends before the threshold is reached, its amplitude does not determine the output width. The CRO should display the trigger, output, and capacitor voltage: the capacitor rises exponentially during the HIGH output and falls rapidly when discharge begins. Differences between calculated and observed width arise from component tolerance, comparator levels, and probe loading.
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.
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