Monostable multivibrator using 555 timer

What Is a Monostable Multivibrator?
What Is a Monostable Multivibrator?

It’s the simplest, most precise, and longest-lasting monostable vibrator, or pulse extender, using the 555 timer. A monostable vibrator flashes more accurately and longer than a constant-speed circuit without resistive resistance. Using a diode or capacitor and a resistor that won’t flyback, the 555 timer can be a monostable vibrator. On a high-going transition that does not oscillate, the 555 timer outputs. A button or high logic pin might activate the output at the conclusion of the period. TRIG pin pulses result from button presses. A button press drives a pin high, and the output returns to the low-end. The low TRIG pin becomes the output pin. This device will save a lot of time in a busy world. The microcontroller is the most advanced and significant Lux meter component. This is the most used feature.

Have you ever wished to create a circuit that triggers a single pulse of a specified duration? The ubiquitous 555 timer makes it easy to create one. I’ll show you how to build a simple 555 monostable multivibrator that generates a single pulse for a given period when you press a button. We’ll cover the essential components, how to connect them, and how to choose resistors and capacitors to adjust pulse width. After reading this, you’ll know how to use the 555 circuit to command a timed pulse in your projects. Let’s begin!

Purpose of Monostable Multivibrator

The Monostable Multivibrator changes state when triggered. It restores itself after a certain time. Unlike an astable Multivibrator, it has one stable state. A 555 Timer shows that the Monostable device changes state for a finite time and then rests. Common Integrated Circuits (ICs) for money stable Multivibrators include 555 Timers. Variable operations are programmed in this IC.

Overview of 555 Timer

Signetics Corporation introduced the 555 timer IC in 1972. Due to its simplicity, low cost, and dependability, it remains popular. 555 timers have two voltage comparators, a bistable flip-flop, an R-S flip-flop, a discharge transistor, and a resistor divider for trigger and threshold input. It is an 8-pin dual-in-line or metal can IC. It can run from a wide range of DC supplies, is temperature stable, and is less susceptible to supply voltage and ripple. The 555 timer has three modes. These modes are monostable, astable, and bistable. Monostable mode has one stable state. The output is stable high or low. A trigger raises the output, which stays high for the time constant of the external resistor/capacitor networks.

What Is a Monostable Multivibrator?
What Is a Monostable Multivibrator?

What Is a Monostable Multivibrator?

Monostable or one-shot multivibrators generate pulses. It has one stable and one quasi-stable state. It sends a fixed-duration pulse when activated by an external source.

The 555 timer IC is often used to make monostable multivibrators. Two comparators, an SR flip-flop, and a discharge transistor make up the 555 timer. Connections between resistors and capacitors set the threshold voltage, trigger voltage, and time period.

When the trigger input drops, the lower comparator sets the flip-flop. Output rises. The capacitor charges through a resistor simultaneously. The higher comparator resets the flip-flop when the capacitor voltage hits 2/3 VCC. Output drops. For the next cycle, the capacitor discharges through another resistor.

  • Key features of a monostable multivibrator:
  • Has one stable and one quasi-stable state.
  • It outputs one pulse when activated.
  • External components like resistors and capacitors control the output pulse duration.
  • It can create pulse widths or delay signals.
  • Timing circuits, frequency dividers, pulse position modulation, and pulse width modulation use it.
  • Monostable multivibrators are useful in analog and digital circuits. Selecting the proper external components lets you generate pulses of the desired duration for timing applications.

How a 555 Timer IC Works

The 555 timer IC is utilized in timer, pulse generator, and oscillator applications. The device has three 5 kiloohm resistors, hence its name.

A comparator constantly monitors an external capacitor’s voltage in the 555 timer. External charging resistor charges and discharges capacitor. The comparator initiates the 555 timer based on capacitor voltage.

The Threshold and Trigger Inputs

The comparator’s threshold and trigger points are controlled by pins 2 and 6 of the 555 timer. Low output occurs when capacitor voltage falls below the trigger point. Once the trigger is achieved, output rises. It stays high until the capacitor voltage approaches the threshold, then drops again.

The Discharge Pin

Pin 7 is the discharge pin of the 555 timer. This pin is grounded when output is low. This pin is isolated from ground when the output goes high, allowing the capacitor to charge through the charging resistor. We can manually reset the timer and start a new cycle by regulating the discharge pin.

Applications

Monostable and astable multivibrators, pulse width modulators, and oscillators can be made with the 555 timer. One-shot monostable multivibrators have one stable state and one pulse when triggered. An astable multivibrator produces continuous output by oscillating between two unstable states. Common uses include timers, pulse generating, and clock signal creation.

The versatile 555 timer can be constructed into numerous useful circuits with a few external resistors and capacitors. Its ease of use and adaptability have made it popular in electronics applications for 40 years.

Circuit Diagram for a 555 Timer Monostable Multivibrator

A monostable multivibrator using the 555 timer IC only a few basic components. The 8-pin 555 timer has all the timing circuitry.

  • This circuit requires:
  • IC555 timer
  • Two resistors
  • One capacitor
  • Jumper wires, breadboard
  • The Pinout

The 555 timer pinout:

  • Pin 1: Ground Pin 2: Trigger input
  • Pin 3: Output
  • Pin 4: Reset Pin 5: Voltage Control
  • Pin 6: Limit
  • Pin 7: Discharge
  • Power Supply (+Vcc) Pin 8

The Circuit

Connect the components to build the monostable multivibrator:

Connect pin 1 to the power supply negative rail. This is the ground pin.
Connect pin 8 to the power supply positive rail. This powers the IC.
Connect R1 between pins 6 and 7. R1 controls pulse width.
Link pins 6 and 2 with capacitor (C1). Through R1, C1 charges and discharges.
Serially connect pin 3 to an LED and resistor (R2). Triggering Pin 3 produces a strong pulse.
Use a pushbutton switch on pin 2. Pressing the switch raises pin 3’s pulse.
Join pins 4 and 8. Pressing the switch resets the circuit.
R1 and C1 can be changed to change the circuit’s pulse width. Test it and see! Please ask me anything more about this monostable multivibrator circuit.

Calculating Timing Components

Calculate the circuit’s R1, C1, and R2 values based on the timing you require. Walk through the steps:

First, choose the output pulse duration. Known as “pulse width” or “t”. Consider a 0.1-second pulse width.

Next, calculate C1’s value. Formula: t = 1.1 x R1 x C1. Rearranging, C1 = t/(1.1 x R1). Choose a conventional 10,000-ohm R1. C1 = 0.1 sec / (1.1 x 10,000 ohms) = 0.1/11,000 = 0.009 microfarads. Standard 0.01 uF capacitors work fine.

Then find R2. This resistor sets the pulse reset time from input low to output low. Try to reset 3–5 times longer than the pulse width. Try 5 times longer, or 0.5 seconds.
Use this formula. 0.7 x R2 x C1 = reset time. Rearrange, R2 = Reset time/0.7 x C1. Enter numbers: R2 = 0.5 sec/0,007 (0.7 x 0.01 uF) = 71,428 ohms. Nearest standard resistor is 68,000 ohms.

Finally, set C2, the decoupling capacitor. A typical 0.1 uF value works fine for this circuit.

With R1 = 10,000 ohms, C1 = 0.01 uF, R2 = 68,000, and C2 = 0.1, your 555 timer circuit outputs 0.1 sec pulse width and 0.5 sec reset time. Make sure your calculations are correct before building the circuit. Have fun experimenting!

Applications of 555 Timer Monostable Multivibrators

Pulse generation is a great use for the 555 timer monostable multivibrator. This circuit generates a fixed-duration output pulse in response to a trigger input. Adjusting the timing resistor and capacitor can change the pulse duration from microseconds to hours. Uses of pulse generation include:

A button-pressed LED or buzzer turns on for a set time. Many electronic toys, buzzers, and timers use this.
Garage door openers and vehicle alarm remote controllers send brief radio frequency bursts.
Providing washing machine, microwave, and timer timed sequences.
A clean pulse is generated by debouncing input switches. Closed mechanical switches bounce for milliseconds before settling. A monostable can only output one pulse when the bounces settle.
Expanding a small input pulse to a large output. This helps interface slower digital logic families.
Another use is pulse-width modulation (PWM). An analog input like a potentiometer controls pulse width. Different pulse widths can affect LED brightness, motor speed, etc.

One-shot operations, where an input triggers a single event, are also prevalent. You may want to turn on an indicator light for a few seconds when a sensor is triggered to indicate detection. Increase a timer by a defined amount when a button is pressed.

Monostable multivibrators have several electronic circuit applications. They are cheap, adaptable, and easy to implement using the 555 timer IC. Try some of these apps in your projects!

Conclusion

Here’s a simple but effective 555 timer monostable multivibrator circuit. Simply tweak the timing components to change the pulse width using the 1:1.44 resistor-capacitor ratio. Making your own with a few cheap components and basic soldering skills yields a versatile circuit with many possibilities. Try it for your next electrical project! When a monostable multivibrator causes difficulty, the 555 can help.

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