555 timer Flashing LED

In this experiment we are going to use a 555 integrated circuit to create a flashing/blinking LED. The 555 integrated circuit is commonly referred to as a 555 timer and can be used in astable, bistable or monostable mode.

When you see a flashing LED in a circuit it’s almost always the work of a 555 timer, although the 555 does have a lot of other application uses.

In this circuit we are going to be using the 555 timer in astable mode. When the 555 timer is astable it produces a square wave output and alternates between VCC and 0v on a continual basis. This type of circuit is called astable because it is not stable in any state; the output is continually changing between low and high.

The values of R1, R2 and C1 we can determine the period/frequency and the duty cycle. The period is the length of time it takes for the on/off cycle to repeat itself and the duty cycle is the percentage of time the output high.

Increasing C1 will increase the period time (reduce the frequency)

Increasing R1 will increase the time the output is high but will leave the output low unaffected

Increasing R2 will increase the time the output is high and increase the time the output is low.

The 555 timer in astable mode gives us an output on pin 3 as high or low, we refer to this as a pulse.

• The frequency is the number of pulses per second
• The time period is the time covered for one pulse, output high + output low
• The duty cycle is the percentage of time the output is high
• Time high is the time the output is high
• Time low is the time the output is low

Note: these units are only acceptable when the value of R1 and R2 are in k ohms and the capacitance is in micro Farads.

For example if R1 = 1k ohms and R2 = 10k ohms and C1 was 10uf to calculate the time period we would use.

Time period = 0.693 x (1+2x10) x 10 which would give us a time period of 145.53 milliseconds.

Integrated circuits need to be inserted the correct way around or you will damage them. You will always need to check the pin outs for the integrated circuit before you start. Using the image at the bottom of the circuit diagram you can see that there is a semi circle at the top. This is notched in and pin 1 is to the top left. Not all integrated circuits have this some will have the little circle in the top left corner and some will have both markings.

Place the breadboard in front of you and you will see a groove down the centre from top to bottom, this groove isolates the left hand side of the bread board from the right hand side and allows us to place integrated circuits in the board using columns e and f.

• Insert your 555 timer into the breadboard so that pin 1 of the timer is at pin hole e15, pin 4 is at pin hole e18, pin 8 is at pin hole f15 and pin 5 is at pin hole f18.

Next we are going to add the 10uf capacitor. Remember that electrolytic capacitors are polarity sensitive so make sure you insert them the correct way round.

Using the right hand side power strip:-

• Insert the negative leg of the capacitor in the top pin hole of the right hand side negative power strip and the positive leg into the pin hole j1

Now we can add the rest of the components for our circuit.

• Insert the 1k ohm resistor between pin holes a5 and a10
• Insert the 10k ohm resistor between pin holes f1 and d1
• Insert the 470 ohm resistor between pin hole i22 and e22
• Insert the positive leg of the LED (longest leg) into pin hole b22 and the negative leg to pin hole b23

Now that we have the components in we just need to add the jumper wires to connect it all together.

• Insert one jumper wire from the right hand side positive power strip to pin hole d18
• Insert one jumper wire from the right hand side positive power strip to pin hole g15
• Insert one jumper power wire between pin hole d17 and j22
• Insert one jumper wire between pin hole c23 and connect this to a pin hole on the right hand side negative power strip
• Insert one jumper wire in to a pin hole on the right hand side negative power strip and then to pin hole d15
• Insert one jumper wire between pin hole b1 and g16
• Insert one jumper wire between pin hole g17 and h1
• Insert one jumper wire between pin hole d16 and g1
• Insert one jumper wire between pin hole a1 and b5
• Insert one jumper wire between pin hole b10 and then to a free hole on the positive power strip

Connect your battery and battery clip to the breadboards right hand side power strips. You will now see you led flash rapidly. This is now flashing at a rate of 76.23 milliseconds.

Discount the battery and change R2 which is the 1k ohm resistor between pin holes a5 and a10 for a 10k ohm resistor. Reconnect the battery and you will see that the flash rate has changed.

We can calculate this by using the formula Time high = 0.693 × (R1+R2) × C1which would give us a time period of 138.6 milliseconds

Discount the battery and change the capacitor for a 100uf. When you recent the battery you will see that the flash rate is much slower. In fact the time High is longer, the time low is longer and the time period for a full cycle is now longer.

Time high = 0.693 × (R1+R2) × C1 now gives us a flash rate of 1386.0 milliseconds or 1.39 seconds (round to 2 decimal places)