EL25 · Sem 5 · Oscillators

Astable multivibrator (LED flasher)

Aim: Build a two-transistor oscillator that blinks an LED.

🧪 Live simulation🧊 Real-time 3D bench⚠ Simulated values, not physical measurements

🔬 Virtual lab

Starting the lab…

📘 Theory

Two cross-coupled transistor stages switch each other on and off through timing capacitors. Each half period is about 0.69·R·C, set by the base resistor and coupling capacitor.

T ≈ 1.38·R·C

🧰 Apparatus (virtual)

9 V
two NPN transistors
47 kΩ base resistors
10 µF capacitors
1 kΩ collectors
LED

📝 Procedure

Run and watch the LED flash on the bench
Measure the period on the scope
Compare with 2 × 0.69 × R × C
Change C and see the rate change

Observations and result (kept on this device)

🎤 Viva questions

Why does the circuit start oscillating by itself

Tiny differences between the two halves get amplified by the positive feedback loop.

How do you make an asymmetric duty cycle

Use different R or C in the two halves.

🛡️ Safety

Virtual lab at safe low voltages. On a real bench: switch off before rewiring, check capacitor polarity, and never touch mains wiring without supervision.