555 Timer Monostable Mode with LED

Introduction

In monostable mode, the 555 timer operates as a one-shot pulse generator. When triggered, it produces a single output pulse of a set duration. This setup is commonly used for time delays or to control other circuits for a specified period, indicated here by an LED that lights up for the pulse duration.

Objective

The aim of this experiment is to configure the 555 timer in monostable mode and observe the output pulse duration using an LED, with timing defined by resistor and capacitor values.

Components Needed

1 x 555 Timer IC
2 x Resistors (R1, typically 10kΩ; R2, optional for variable control)
1 x Capacitor (C1, typically 100µF)
1 x Push Button (Trigger)
1 x Pull-up Resistor (10kΩ for the trigger pin)
1 x Breadboard
Connecting Wires
Power Supply (5V)
1 x LED (for visual output)
1 x Current-limiting Resistor for LED (220Ω)
1 x Oscilloscope (Optional for waveform observation)

Circuit Diagram

Procedure

Connect pin 1 (GND) of the 555 timer to the ground of the power supply.
Connect pin 8 (VCC) to the positive voltage supply (5V).
Connect resistors R1 and R2 in series between pin 7 and pin 8 (suggested R1 value: 10kΩ; R2 is optional for fine-tuning).
Connect a capacitor (C1) between pin 6 and ground (suggested C1 value: 100µF).
Place a push button between pin 2 (trigger) and ground, and connect pin 2 to VCC via a 10kΩ pull-up resistor.
Connect pin 3 (output) to an LED in series with a 220Ω current-limiting resistor.
Power on the circuit, press the button to trigger the timer, and observe the LED lighting up for the duration of the output pulse.

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Results and Calculations

The pulse width (T) of the 555 timer in monostable mode is determined by the formula:

T = 1.1 * (R1 + R2) * C1

Press the trigger button and observe the LED lighting up for a duration matching the calculated value based on your chosen resistor (R1 + R2) and capacitor (C1) values.

Conclusion

In this experiment, we configured the 555 timer in monostable mode to produce a single pulse when triggered, visualized by an LED. The LED stayed lit for the duration calculated, demonstrating how the selection of resistor and capacitor values directly controls the pulse duration.