A monostable multivibrator is a type of multivibrator that has a single stable state. MONO means one thus the name itself indicates one state stable and a quasi-stable state. It is an oscillator that uses deep positive feedback to alternately turn on and off two electronic devices through resistance-capacitance coupling, thereby generating a square wave output by self-excitation. The multivibrator has two different electrical states: the output “high” state and the “low” state. It provides a stable or quasi-stable state according to the type of the multivibrator.
Hardware Required
| S.no | Component | Value | Qty |
|---|---|---|---|
| 1. | IC | NE555 Timer | 1 |
| 2. | Capacitor | 0.01uF | 1 |
| 3. | Push Switch | – | 1 |
| 4. | Resistor& Capacitor | R1 & C1 | as per your requirement |
| 5. | Connecting Wires | – | – |
| 6. | Battery | 9V | 1 |
Circuit Diagram
Working Explanation
As shown in the circuit, the main part of the circuit is IC 555. Primarily the internal transistors of IC 555 and external capacitors in the circuit are shorted to the ground. This state is considered the stable state of the monostable 555 multivibrators. As we know, when the voltage at the second pin of 555 IC goes below 1/3 Vcc, the output becomes high. The high state is known as the ‘Quasi-stable state’. As the IC 555 triggers, it causes the transition from a stable state to a quasi-stable state. So the voltage at the 2nd pin becomes less than 1/3 Vcc (Disconnected from Vcc).
Hence the output turns high. Then the internal discharge transistor of the IC is cut off and the capacitor starts charging toward Vcc. Charging of the capacitor is done through the resistor R1 with a time constant R1C1. As the capacitor voltage increases and finally exceeds 2/3 Vcc, it will reset the internal control flip flop, thereby turning off the 555 timer IC (More than 2/3 voltage at the threshold pin (6th pin) causes IC to reset). Thus the output goes back to its stable state from the quasi-stable state. The duration of the stable state or the pulse width is determined by the charging time constant of the RC network. We can transfer the multivibrator from a stable state to a quasi-stable state by a pushbutton trigger switch.
Applications
It can be used for switching as shift registers or temporary memories and as a square wave timing oscillator or clock.
