# 555 IC Intro: Flashing LED

Introduction

This Instructable is to bridge the gap between circuit theory you would find in your books and the application to a real working circuit. This circuit uses some of the most basic and common components. Everything used in this project can be purchased at a local Radioshack or electronic parts store.

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).

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PrakharM13 months ago

can I use 10k potentiometer instead of 100 k??

i dont have enough material to make this circuit so I wanna buy some things..

will i have enough material to make this circuit?

is there any difference between capacitor (not electrolytic, just capacitor) and ceramic capacitor??

can i use less resistance resistor instead of 4.7k ohm??

thanx a lot..

3 months ago

nixz4u6 months ago

If I am using 12V LED instead of 3V, should there be any change in the circuits. Do I need to change the values of any resistors? I am to use 12V power source also.

steven8478 months ago

Hello, I am new to electronics and I am trying this circuit out. I had to use a different type of potentiometer (100k ohm Cermet Square 22-Turn Preset Potentiometer). The circuit did not work. I am also using a breadboard without the positive and negative horizontal pin spaces and so I had to improvise the wiring of the board. Could you tell me whether the circuit has gone wrong because I have used the wrong potentiometer or could it be the breadboard or both. I know this maybe a silly question! Cheers

BenRyan (author)  steven8478 months ago

Make sure that when you are wiring anything that goes to the 9V source or ground, that you run a jumper out to a location on the breadboard that is not on the same vertical column as the 555 IC or any other component. I am sure the potentiometer is fine, just double check you indeed have it wire correctly. I would bet your issue is with the wiring of the power pins so look there first.

8 months ago

Thank you! Managed to get it working :)

BenRyan (author) 10 months ago

This is the schematic that I use for fading LEDs using the 555 timer. Changing the values of the capacitors will alter the delay time etc.

schel10 months ago

Nice n' easy to follow! Thanks BR.

where could one throw a couple of capacitors into this circuit for FADING on n' off? yeah there is a couple of 'ibles on this but, they focus more on making the pcb from scratch and very little on the actual circuit. Whaddaya think?

doorag1 year ago
where to putt photo diode for trigering the sistem?
BenRyan (author)  doorag1 year ago
I don't use a photo diode in this particular tutorial.
Maybe you could place one in series with the positive or negative lead of the battery.
iqbal1231 year ago
thanks for providing total tutorial...
be nice
jukarr1 year ago
thank you very much. A well explained tutorial step by step. Important for someone starting out like me.