Experiment 6 - The monostable Multivibrator II

How does a monostable multivibrator work?

Take a look at the visual explanations. All parts connected to the negative terminal of the battery (0 Volt) are shown in blue, all the components on a positive voltage (>0 Volt) are shown in red.

1. Step

After connecting the battery, the LED2 lights up...

• The transistor T2 switches through.
• Through the resistor R2, the base of the transistors T1 is pulled down to 0 Volt and blocks. The LED1 remains off.

Visual explanation of how of a bistable multivibrator works (step 1).

2. Step

You press S1. As a result a positive voltage applies to the basis of the transistors T1...

• The transistor T1 switches through and LED1 lights up.
• Through the capacitor C1 flows a charging current. As a result the base of the transistor T2 is pulled down to 0 Volt and blocks. The LED2 goes out.

Visual explanation of how of a bistable multivibrator works (step 2).

3. Step

Now, the push-button switch S1 is open again and the capacitor C1 is charged through the resistor R1.

Once the capacitor is charged, the voltage at the base of transistor T2 starts to rise.

Visual explanation of how of a bistable multivibrator works (step 3).

Once the voltage at the base exceeds 0.7 Volt, the transistors T2 switches through and the multivibrator returns to the state from first step.

What influence has now the capacity of the capacitor...?