This astable multivibrator uses two NPN transistors (Q1 and Q2) that alternately switch on and off. When Q1 is ON, Q2 is OFF, and vice versa. The switching is controlled by the RC timing circuits formed by the base resistors and coupling capacitors.
Operation:
1. When Q1 turns ON, its collector voltage drops, which is coupled through C2 to Q2's base, turning Q2 OFF.
2. C2 charges through R4 until Q2's base reaches the turn-on voltage.
3. When Q2 turns ON, its collector voltage drops, coupling through C1 to Q1's base, turning Q1 OFF.
4. The cycle repeats continuously, creating oscillation.
Frequency Formula: f ≈ 1/(1.4 × R × C) where R is the base resistance and C is the capacitance.