What starts a free-running multivibrator?

A free-running multivibrator, also known as an astable multivibrator, starts oscillating on its own as soon as it is powered on. Here are the steps:

1. Power Supply: The first step to start a free-running multivibrator is to provide a power supply. This can be done by connecting the circuit to a DC power source.

2. Initial State: In the initial state, one of the transistors is in the 'on' state while the other is in the 'off' state. This is due to the slight imbalance that is always present in practical circuits.

3. Capacitor Charging: The capacitor connected to the 'on' transistor starts charging towards the supply voltage through the resistor.

4. Reaching Threshold: Once the voltage across the capacitor reaches the threshold voltage of the 'off' transistor, the 'off' transistor turns 'on'.

5. Discharging: This causes the capacitor to start discharging and the 'on' transistor to turn 'off'.

6. Oscillation: This process repeats, causing the circuit to oscillate between the two states. This oscillation continues as long as the power supply is connected.

7. Output: The output is taken across the resistor or the capacitor, and it is a square wave with a frequency determined by the RC time constant of the resistor-capacitor network.

So, the free-running multivibrator starts oscillating as soon as it is powered on, without any external trigger.