555 Timer Flashing Led

This circuit uses the 555 timer IC in an astable operating mode which generates a continuous output via Pin 3 in the form of a square wave. This turns the LED (D1) on and off. The speed at which the LED (D1) is turned on and off is set by the values of R1 and R2.

 PART LIST
R1: 1K
R2: 470K
R3: 1K
D1: LED
C1: 1 uF
and 555 Timer


Basics

The final outcome of this project is to build a circuit which results with an LED that blinks on and off. This project is centered around the 555 timer chip, a short description of how the 555 timer chip works will be helpful.

The 555 Timer

The 555 timer is an integrated circuit (a circuit built on a piece of semi conductor material that performs a defined function) which can be used in many applications which require oscillator, pulse generation, or timer controlled devices. The 555 timer has 3 operating modes; monostable, astable, and bistable. This utilizes the 555 in astable mode, thus we will focus on the basics of astable operation.

Astable operation - in astable mode, the 555 outputs a constant stream of rectangular pulses. The rectangular pulses will be outputted at a specific frequency that is defined by the components that are placed in between the pins of the 555 timer. Lets start by looking at the Pin connections and functions of the 555 IC.

PINS

     Pin 1 (GROUND) -  The ground (or common) pin is the most-negative supply potential of the device, which is normally connected to circuit common when operated from positive supply voltages.

     Pin 2 (Trigger) - This pin is the input which causes the output to go high and begin the timing cycle. Triggering occurs when the trigger input moves from a voltage above 2/3 of the supply voltage to a voltage below 1/3 of the supply. For example using a 12 volt supply, the trigger input voltage must start from above 8 volts and move down to a voltage below 4 volts to begin the timing cycle. The action is level sensitive and the trigger voltage may move very slowly. To avoid retriggering, the trigger voltage must return to a voltage above 1/3 of the supply before the end of the timing cycle in the monostable mode. Trigger input current is about 0.5 microamps.

     Pin 3 (Output) - The output pin of the 555 moves to a high level of 1.7 volts less than the supply voltage when the timing cycle begins. The output returns to a low level near 0 at the end of the cycle. Maximum current from the output at either low or high levels is approximately 200 mA.

     Pin 4 (Reset) - A low logic level on this pin resets the timer and returns the ouput to a low state. It is normally connected to the + supply line if not used.

     Pin 5 (Control) - This pin allows changing the triggering and threshold voltages by applying an external voltage. When the timer is operating in the astable or oscillating mode, this input could be used to alter or frequency modulate the output. If not in use, it is recommended installing a small capacitor from pin 5 to ground to avoid possible false or erratic triggering from noise effects.

     Pin 6 (Threshold) - Pin 6 is used to reset the latch and cause the output to go low. Reset occurs when the voltage on this pin moves from a voltage  below 1/3 of the supply to a voltage above 2/3 of the supply. The action is level sensitive and can move slowly similar to the trigger voltage.

     Pin 7 (Discharge) - This pin is an open collector output which is in phase with the main output on pin 3 and has similar current sinking capability.

     Pin 8 (V +) - This is the positive supply voltage terminal of the 555 timer IC. Supply-voltage operating range  is +4.5 volts (minimum) to +16 volts (maximum).

 CIRCUIT DIAGRAM