Monostable circuit using 555 timer ic

 

 Project Title:

Monostable Multivibrator Using 555 Timer IC

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 Objective:

·   To design a monostable multivibrator circuit using the 555 Timer IC.

·   To generate a single output pulse of fixed duration when an external trigger is applied.

·   To understand and demonstrate the working principle of monostable mode in 555 timer IC.

·   To control an output device (like an LED) for a specific time interval.

·  To explore how resistor and capacitor values determine the pulse width.

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Components Required:

S.N	Component	Quantity	Description
1.	NE555 Timer IC	1	Used to configure the timer in monostable mode
2.	Resistor (R)	1	Timing resistor to set output pulse width
3.	Capacitor (C)	1	Timing capacitor to set output pulse width
4.	Push Button Switch	1	To manually trigger the pulse
5.	LED	1	Indicates when the output pulse is active
6.	Power Supply (5V–12V)	1	Powers the 555 timer circuit

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Theory

A monostable multivibrator, also known as a one-shot timer, is a circuit that generates a single output pulse when triggered. The 555 timer IC is commonly used in monostable mode to produce a fixed-width output pulse, making it ideal for applications like delays, timers, and pulse generators.

In monostable mode, the 555 timer has one stable state (LOW) and one temporary state (HIGH). When a trigger pulse (a momentary LOW signal) is applied to pin 2, the output at pin 3 goes HIGH for a predetermined time period. After this time, the output automatically returns to LOW, regardless of the trigger

Block Diagram / Circuit Layout:

             

Monostable Configuration:

• Pin 2: Trigger (connected to push button)
• Pin 3: Output (connected to LED)
• Pin 6 & 7: Connected together with a resistor to VCC
• Pin 6 to GND via capacitor
• Pin 4 & 8: VCC( 4 is due to avoid from reset of IC.
• Pin 1: GND
• Pin 5: Optional, usually connected to ground via 10nF capacitor for stability

Output Pulse Width Formula:
T=1.1×R×C
Where T = pulse duration in seconds, R = resistance in ohms, C = capacitance in farads.

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 Working Principle:

·   When the push button is pressed, a low signal (0V) is applied to the trigger pin (pin 2) of the 555  

     timer.

·   At that moment, the capacitor voltage is less than 1/3 of Vcc, which activates the internal flip-flop.

·   This causes the output (pin 3) to go HIGH, starting the timing cycle.

·   While the output is HIGH, the LED glows, indicating the timer is active.

·   The capacitor (C) starts charging through the resistor (R) toward Vcc.

·   Once the capacitor voltage reaches 2/3 of Vcc, the internal comparator resets the flip-flop, and the

     output goes LOW.

·   The charging time (pulse width) depends on the RC time constant:
       T=1.1×R×C

·   The circuit can only be retriggered after the timing cycle ends (i.e., when the output returns to LOW).

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 Observations:

·         LED turns ON when the button is pressed and stays on for a set duration (e.g., 5 seconds).

       Capacitance=10uf=10,000nf

Resistace=100k ohm

T=1.1×100,000Ω×10×10−6.

• Duration remains the same for every trigger.
• The circuit ignores multiple triggers during the active pulse (non-retriggerable during timing).
• Pulse width changes if resistor or capacitor value is changed.

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 Application Area:

Application	Description
Timer Circuits	Delay-based circuits in appliances or projects
Debouncing Switches	Avoiding switch bounce in digital circuits
One-shot Pulse Generator	Generating a fixed-length pulse for triggering other circuits
Industrial Timers	In machinery needing a timed response

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Conclusion:

The monostable multivibrator using a 555 timer is a simple yet effective way to generate fixed-width pulses.

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Recommendation:

• Use precision components for accurate timing.
• Add a capacitor at pin 5 (10nF) for noise reduction.
• Avoid retriggering during active pulse to maintain timing accuracy.
• Simulate the circuit in software like Tinkercad or Proteus before real-world implementation.

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References:

While exact sources are not listed, the content for this project has been prepared based on knowledge gathered from:

·         555 Timer IC Datasheet (NE555)

• Digital Electronics Book(third semester)
• Proteus for Circuit Simulation
• YouTube video
• Assistance and formatting by ChatGPT