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).

Step 1: Gather Materials

First you are going to want to gather all the materials necessary to build this circuit. If this is the first or one of the first circuit you will be building, it may be a good idea to purchase either a kit or some starter packs that include an assortment of parts. FutureLEC provides very reasonably priced packs of different components. Everything used in this Instructable is available in one of the value packs from FutureLEC. Any local Radioshack should carry everything required in this Instructable as well.

FUTURLEC Value packs

Parts Required

NE555N IC
4.7k Ohm Resistor
100k Trim Potentiometer
330 Ohm Resistor
10 Microfarad Electrolytic Capacitor
.1 Microfarad Ceramic Capacitor
3V LED
9V Battery
Battery Clip
Jumper Wires
Bread Board

Reading Resistor Values
Many times, you will receive resistors in a large pack that contains resistors of varying level of resistance. Resistor values are written on the resistor by means of color bands. Being able to read the values from these color bands is essential for any circuit. The color band code works as follows.

1st Band
The first band represents the first digit in the value of the resistor (e.g. If the first band color is orange, the first figure is a 3).

2nd Band
The second band represents the second digit in the value of the resistor (e.g. If the second band color is orange, the second figure is a 3)

3rd Band
The third band represents the multiplier for the value of the resistor (e.g. If the third band color is red, then the multiplier is 100.

4th Band
The fourth band represents the tolerance (how close the actual resistance of the resistor is to the claimed resistance given by the color bands) of the resistor (e.g. if the 4th band is silver, then the tolerance is + or - 10%)

Example
Resistor with the following color bands:

1st Band - Orange
2nd Band - Orange
3rd Band - Red
4th Band - Silver

=33 * (100) = 3300 ohms + or - 10% (Actual value could be anywhere from 3000 to 3660 ohms).

Step 2: Breadboard

Breadboard
Breadboards are a very important and useful tool in building and prototyping circuits. It is very important to be familiar with how a breadboard is designed and how to use one. Breadboards allow components to be connected without the use of solder; therefor parts can be easily swapped and replaced. A good breadboard should last forever. The photograph below shows an example of a typical beginner sized breadboard.

Design
In this particular breadboard, the center columns are labeled 0 through 60 and the center rows are label A through J. The breadboard is designed as so; each column is connected via a strip of metal. In this particular model, the columns connected via the strip of metal run vertically from rows A to E and a separate metal strip connects rows vertically F to J. The Top Rows labeled + and – are connected similarly, but rather than the metal strips running vertically from – to plus, the strips run horizontally from across each row.

Step 3: Assembly: Battery and Battery Clip

Now that you have all of the components required for assembly and a little bit of background knowledge of how they work, we can begin constructing the circuit. From now on, the first picture of each step will represent the state of the circuit at the previous step and the seceond picture will show the added component.

attach the battery clip to the 9 volt battery as shown in the picture.

Step 4: Assembly: 555 Timer IC

Add the 555 Timer IC
Place the center of the 555 timer IC over the break line of the breadboard. This is the location that all Integrated circuits will need to be placed on your breadboard to avoid any problems. Push the pins of the 555 timer IC into the breadboard, a little force may be required to get the chip seated correctly.

Step 5: Assembly: Connect the Battery Leads

Push the positive lead from the battery clip (red) to the positive row on the breadboard (red +). push the negative lead from the battery clup (black) to the negative row on the breadboard (blue -).

The run a jumper wire joining the top and bottom negative buses on the breadboard.

Step 6: Assembly: the Electrolytic Capacitor

Electrolytic Capacitor
Electrolytic capacitors are polarized, meaning the positive lead from the capacitor must be connected to the positive (high) voltage and the negative lead of the capacitor must be connected to the negative (low) voltage. The negative lead of the electrolytic capacitor will be marked with a band of differing color and a "-" sign. Many times the longer lead of the electrolytic capacitor will be the positive one and the shorter lead will be negative. The schematic drawing of an electrolytic capacitor will be the same as a non polarized capacitor, but the polarity will be marked with a (+) sign.

Attach the capacitor so that the positive lead is on the same column as pin2 of the 555 timer and the negative lead is on the same column as pin1 of the 555 timer.

Attach a short jumper cable from pin1 of the 555 timer to the negative (ground) bus on the bread board.

Step 7: Assebly: LED and Resistor

Place the LED in an arbitrary location on the breadboard (not in any columns that are connected to the 555 timer pins). Make sure that the negative lead of the LED (shorter lead) is connected to the negative bus on the breadboard and the positive LED is on a column of its own.

Next, attach the 330 ohm (Orange-Orange-Brown-Tolerance) resistor from pin3 of the 555 timer to the positive lead of the LED. Resistors have no polarity, so no need to worry about the orientation of your resistor.

Step 8: Assembly: Positive Voltage and the Ceramic Capacitor

Attach a jumper from pin4 of the 555 timer to the positive bus of the breadboard. This pin is not being used in our circuit, so we connect it to the positive voltage lead.

Ceramic Capacitor
Unlike electrolytic capacitors, ceramic capacitors are unpolarized, meaning they do not have a positive and negative lead. Thus, we won't have to worry about which lead goes to the positive side and which lead goes to the negative side.

Attach one lead of the ceramic capacitor to pin5 of the 555 timer (top right pin) and the other lead to the negative bus on the breadboard.

Step 9: Assembly: Pin6 and the Trim Pot

Attach a jumper cable, one end at pin 6 of the 555 timer, the other end at pin2 of the 555 timer.

Place the 100k potentiometer in an arbitrary spot on the breadboard (Place it on any columns of pins that aren't being used by another component, a corner is a good spot).

Step 10: Assembly: Wiring the Trim Pot

Run a jumper wire from pin6 of the 555 timer to the center of the trim pin of the trim pot.

Run another jumper wire from pin7 to the right most pin on the trim pot.

Step 11: Assembly: Final Resistor and the Power

Attach one lead of the 4.7k ohm resistor (yellow-purple-red-tolerance) to pin7 of the 555 timer and the other lead to pin 8 of the 555 timer.

Attach one end of a jumper cable to pin8 of the 555 timer and the other end of the jumper cable to the positive power bus on the bread board.

At this point, as long as everything is wired correctly, you should see the LED turn on. Don't be worried if your LED is not flashing at this point, there is probably a good reason if it isn't.

Step 12: Test

Now that you have completed the circuit, it is time to test it to make sure that it is working properly.

Use a trim pot adjustment tool or a straight edge screw driver to turn the trim potentiometer adjuster all the way counterclockwise. This should cause the LED to blink at its slowest rate. When the trim potentiometer is turned all the way counterclockwise, the resistance between pins 2 and 3 are at a maximum (100k ohms). As turn the trim pot clockwise, you should see the frequency of the blinking LED increase until the LED appears to be constantly on (the resistance between pins 2 and 3 is being reduced from 100k ohms to approximately 0 ohms).

Trim potentiometer
A basic trim potentiometer diagram is shown in the picture. Pins 1 and 3 represent ends of the variable resistor and pin 2 represents the wiper. To achieve a basic variable resistor, you attach the two leads of interest to either pin 1 and 2 or pin 2 and 3. The only difference between using pins 1 and 2 and using pins 2 and 3 is the direction that you turn the trim pot to give you 100k ohms or 0 ohms. When the leads are connected to pins 1 and 2 of the trim pot, the resistance between pins 1 and 2 will be 0 ohms when the adjustment knob is turned all the way counterclockwise. When the leads are connected to pins 2 and 3 of the trim pot, the resistance between pins 2 and 3 will be 100k ohms when the adjustment knob is turned all the way counterclockwise.

Adjustment

you can probably already tell that as we increase the resistance of our trim pot (which connected between pins 6 and 7; thresh hold and discharge), the frequency of the blinking LED is reduced. We can find a numerical representation of the on-off frequency of the LED given by the following expression.

Frequency = 1.44 / ((R1+2*R2)*C)
On Interval (in milliseconds) = .693*(R1+R2)*C
Off Interval (in milliseconds) = .693*R2*C