Monostable Multivibrator using 555 Timer

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The Monostable Multivibrator has one stable state but whenever the trigger signal is applied then at this moment the output goes into the unstable state.

The time duration for which the output goes into the unstable state can be controlled by the external resistor and capacitor.

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555 Modes

Basically 555 timer IC can be used in 3 modes 1. Monostable Mode 2. Bistable Mode 3. Astable Mode.

In this blog we discussed about how 555 Timer can be used as Monostable Multivibrator.

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Monostable Application

Whenever this monostable multivibrator is designed using the 555 timer IC then it can be used in the following applications: 1) Generating Time Delays 2) Frequency Division 3) To generate Pulse Width Modulated (PWM) Output 4) Switching the Relays

555 Timer Pin Diagram

PIN 1 is Ground Pin, Pin 2 is the Trigger Pin, PIN 3 is Output Pin, Pin 4 represents Reset Pin, Pin 5 is the Control Voltage, PIN 6 is Threshold, PIN 7 is the Discharge PIN and PIN 8 represents Power Supply or Vcc.

555 Pin Diagram

555 Timer Internal Architecture

555 Timer is divided into 5 parts. First part is voltage divider circuit, second comparator block, third flip-flop circuit, fourth discharge circuit and last output driver circuit.

555 internal block diagram

To learn briefly on it’s Internal Architecture please read our previous blog on this Topic WHAT IS 555 TIMER?

Monostable Multivibrator Circuit Diagram

A LED is connected to PIN3 of 555 Timer with a resistance of 150 ohm. PIN2 is connected to PIN8 with 1K ohm resistor as PIN8 is the Power Supply or VCC also a Pushbutton is connected with PIN2 & Ground. PIN6 is connected to PIN8 with a 10K ohm resistor and a capacitor is connected with PIN6 & Ground. PIN6 is also connected to PIN7 of 555 Timer.

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How Monostable Multivibrator Works Internally

If power supply is 9V then Comparator2’s Positive Terminal gets 3V but Comparator2’s Negative Terminal gets 9V as PIN2 is connected to VCC with a 1Kohm resistor.

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Capacitor State : Discharge

If power supply is 9V then Comparator1’s Negative Terminal gets 6V but as long as the capacitor has no charge (Connecting the capacitor to Ground through Discharge PIN7) Comparator1’s Positive Terminal gets 0V.

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Both Comparator’s output LOW, not(Q) becomes HIGH and LED is in LOW or OFF state(One Stable State) until PushButton is pressed.


When PushButton is Pressed

When the PushButton is pressed then at the Negative Terminal of the Comparator2 is connected to Ground. So, Positive Terminal of Comparator2 takes priority and Comparator2’s output becomes HIGH.

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Capacitor State : Charging

When PushButton is pressed not(Q) becomes LOW,Discharge Transistor OFF, Capacitor disconnected from Ground and connected with VCC and Capacitor charges up.

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PIN3 output becomes HIGH and LED has been turned ON.


When PushButton is Released

When the PushButton is released then at the Negative Terminal of the Comparator2 is connected to VCC again. So, Negative Terminal of Comparator2 takes priority and Comparator2’s output becomes LOW.

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Capacitor State : Charging

When PushButton is released not(Q) stays at LOW state, Discharge Transistor stays OFF, Capacitor still disconnected from Ground and connected with VCC and Capacitor still charging.

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PIN3 output still HIGH and LED stays ON until capacitor has a higher than 6V.

Capacitor Discharges

When Capacitor has higher voltage than 6V, Comparator1’s Positive Terminal takes priority and Comparator1’s output becomes HIGH, not(Q) becomes HIGH, Discharge Transistor becomes ON, Capacitor has been connected to Ground again.

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Now Capacitor starts Discharging, Comparator1’s Positive Terminal gets 0V, Comparator1’s Negaitive Terminal takes priority and Comparator1’s output becomes LOW but not(Q) is still HIGH, Discharge Transistor is still ON.

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Both Comparator’s output LOW, LED is in LOW or OFF state(One Stable State) until PushButton is pressed again.

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