Astable Multivibrator Using IC 555

Experiment: Astable Multivibrator Using IC 555

1. Aim

To wire the IC 555 timer in astable (free-running) mode, to measure the output frequency and duty cycle, and to verify the theoretical values predicted by the RC component values using SEELab3.

2. Apparatus / Components Required

• SEELab3 or ExpEYES-17 unit
• IC 555 timer
• Resistors: $R_1 = 2.2\text{ k}\Omega$, $R_2 = 1\text{ k}\Omega$
• Capacitor $C = 1\text{ }\mu F$ (timing), $C_{bypass} = 0.01\text{ }\mu F$ (pin 5 to GND, noise suppression)
• DC supply: $5\text{ V}$ (or $V_{CC}$ from SEELab3)
• Breadboard and connecting wires
• PC or Smartphone with SEELab3 / ExpEYES software

3. Theory & Principle

3.1 The 555 in Astable Mode

In astable mode the 555 has no stable state — its output continuously oscillates between HIGH ($\approx V_{CC}$) and LOW ($\approx 0\text{ V}$) without any external trigger. The timing is set entirely by $R_1$, $R_2$, and $C$.

Internally, the 555 contains a voltage divider that sets two thresholds: the upper threshold at $\frac{2}{3}V_{CC}$ and the lower threshold at $\frac{1}{3}V_{CC}$. The capacitor $C$ charges through $R_1 + R_2$ and discharges through $R_2$ alone, cycling between these two thresholds indefinitely.

3.2 Timing Equations

The charge time (output HIGH) while $C$ charges from $\frac{1}{3}V_{CC}$ to $\frac{2}{3}V_{CC}$ through $R_1 + R_2$:

The discharge time (output LOW) while $C$ discharges from $\frac{2}{3}V_{CC}$ to $\frac{1}{3}V_{CC}$ through $R_2$:

The total period and frequency:

The duty cycle (fraction of period the output is HIGH):

Worked example (ExpEYES-17 values): $R_1 = 2.2\text{ k}\Omega$, $R_2 = 1\text{ k}\Omega$, $C = 1\text{ }\mu F$: \(f = \frac{1.443}{(2200 + 2000) \times 10^{-6}} = 343.6\text{ Hz}, \quad D = \frac{3200}{4200} = 76.2\%\)

Because the capacitor charges through $R_1 + R_2$ but discharges through $R_2$ only, $t_{HIGH} > t_{LOW}$ always — the duty cycle is always above 50% in the standard astable configuration.

3.3 Pin Functions (IC 555)

4. Circuit Diagram / Setup

1. Wire the IC 555 on the breadboard following the schematic in §3.
2. Connect $R_1$ between $V_{CC}$ and pin 7 (discharge).
3. Connect $R_2$ between pin 7 and pin 6/2 (threshold/trigger, tied together).
4. Connect $C$ between pin 6/2 and GND.
5. Connect $C_{bypass}$ ($0.01\text{ }\mu F$) between pin 5 and GND.
6. Tie pin 4 (reset) to $V_{CC}$.
7. A1 monitors pin 3 (output square wave). Also connect pin 3 to IN2 for frequency and duty cycle measurement.
8. A2 monitors pin 6 (capacitor voltage — the sawtooth charging waveform).

5. Procedure

1. Open the SEELab3 / ExpEYES app and navigate to the “IC 555 Astable” experiment or use the oscilloscope directly.
2. Power the 555 from $V_{CC}$ ($5\text{ V}$). The output on A1 should immediately begin toggling — no trigger is needed.
3. Observe A1 (square wave) and A2 (capacitor sawtooth) simultaneously. Confirm:
   • A1 switches between $\approx 0\text{ V}$ and $\approx V_{CC}$.
   • A2 oscillates between $\frac{1}{3}V_{CC}$ and $\frac{2}{3}V_{CC}$.
   • A1 goes HIGH when A2 reaches $\frac{1}{3}V_{CC}$, and LOW when A2 reaches $\frac{2}{3}V_{CC}$.
4. Read the frequency and duty cycle from the IN2 measurement display.
5. Compare with the theoretical values calculated from the formulae in §3.2.
6. Component variation: Swap $R_1$, $R_2$, or $C$ for different values and record how frequency and duty cycle change. Use the online 555 calculator to predict values before measuring.

Mobile App

Desktop App

6. Observation Table

6a. Verification with Standard Values ($R_1 = 2.2\text{ k}\Omega$, $R_2 = 1\text{ k}\Omega$, $C = 1\text{ }\mu F$)

6b. Component Variation

7. Results and Discussion

• The 555 produced a continuous square wave without any external trigger, confirming astable operation.
• At $R_1 = 2.2\text{ k}\Omega$, $R_2 = 1\text{ k}\Omega$, $C = 1\text{ }\mu F$, the measured frequency was ____ Hz against a theoretical value of $343.6\text{ Hz}$ — a discrepancy of ____ %, consistent with the $\pm 5\%$ tolerance of the resistors and $\pm 20\%$ of the capacitor.
• The capacitor voltage at A2 oscillated between ____ V and ____ V, in close agreement with $\frac{1}{3}V_{CC}$ and $\frac{2}{3}V_{CC}$.
• The duty cycle was always above 50%, as expected, because $R_1 > 0$ always makes $t_{HIGH} > t_{LOW}$.
• Increasing $C$ by 10× reduced the frequency by approximately 10×, confirming the inverse proportionality $f \propto 1/C$.

8. Precautions

1. Pin 4 must be HIGH: Pin 4 (Reset) is active-LOW. If left floating it may pick up noise and reset the oscillator unpredictably. Always tie it firmly to $V_{CC}$.
2. Pin 5 bypass capacitor: The $0.01\text{ }\mu F$ from pin 5 to GND suppresses power-supply noise on the internal voltage divider. Omitting it can cause jitter in the output frequency, especially at high frequencies or on a noisy supply.
3. Supply voltage: The 555 operates from $5\text{ V}$ to $15\text{ V}$. The output HIGH level is approximately $V_{CC} - 1.5\text{ V}$. At $5\text{ V}$ supply the output swings $0$–$3.5\text{ V}$, which is within the safe input range of A1.
4. Component tolerance: Standard resistors ($\pm 5\%$) and electrolytic capacitors ($\pm 20\%$) will cause measured frequency to differ from theoretical by up to $\sim 25\%$ in the worst case. Use $1\%$ metal-film resistors and a measured capacitor value for closer agreement.
5. Duty cycle cannot reach 50% with the standard astable configuration because $R_1$ is always in the charge path. To achieve 50% duty cycle, place a diode in parallel with $R_2$ so the capacitor charges only through $R_1$.

9. Troubleshooting