555 timer Dual Flashing LED

Building on from experiment 5 where we use a 555 timer to create a flashing LED we can also use that circuit to create a dual alternating flashing LED.

In experiment 5 555 timer flashing LED we use the output from pin 3 of the 555 timer to flash the LED when pin 3 is high, the LED is connected to the negative power and then to pin 3 through the resistor. We can also use the output of pin 3 when the output is low to create an alternating pair of flashing LEDs.

When the output of pin 3 is high the LED and resistor are connected to the negative power, if we want to use the output when pin 3 is low we need to connect a resistor and LED to the positive power.

We are still using the 555 timer in astable mode and we can use the calculations below to alter the resistors and capacitor to control the timing of the high, low and time period.

• 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.

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 above 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 one of your 470 ohm resistor between pin hole f22 and e22
• Insert the positive leg of the red LED (longest leg) into pin hole b22 and the negative leg to pin hole b23
• Insert the positive leg of your green LED (longest leg) into pin hole h23 and the negative leg to pin hole h22
• Insert your second 470 ohm resistor between pin hole i23 and the other end into a free hole on the right hand side positive power strip.

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 g22
• 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 leds flash rapidly. The red LED flashing at a rate of 76.23 milliseconds, this is the output in high state and your green LED flash at 69.3 milliseconds; your output in low state.

By changing the capacitor C1 for a 100uf capacitor the alternating flash rate slows down. Remember to always discount the battery before changing any components.