We will now use the 555 timer to build an astable multivibrator circuit which alternately flashes two LEDs.
This is what the schematic for this circuit looks like. We have the usual power pins, the output section and the RC network. The output consists of 2 LEDs that are connected with the opposite polarity. This means that when the timer output is HIGH, LED 1 will switch ON, and when the timer output is LOW, LED 2 will switch ON. The capacitor voltage is fed into pins 2 and 6, and we know that the voltages at these pins control the output state. The RC network does not charge from the supply voltage, but it charges from the output pin instead.
When we first power ON the circuit, the output will be LOW and this means that C1 will remain in the discharged state. Since the voltage at pin 2 is lower than 1/3rd of the supply voltage, it will cause the output to go HIGH. Since the output is connected to the RC network, it will cause C1 to start charging and the voltage at its terminals will start to increase. As this voltage rises above 2/3rd of the supply voltage, it will reach the threshold voltage and it will cause the internal comparator to switch states, causing the output to go LOW.
The RC network now has a discharge path which will force the capacitor voltage to reduce and when it reduces below 1/3rd of the supply voltage, it will cause the 555 timer to change states causing the output to go HIGH. This cycle will keep repeating and the LEDs will alternate. The timing can be changed by changing the values of the RC network.
Here’s what the assembled PCB looks like. Please ensure that you place the capacitor, IC base, IC and LEDs with the correct polarity. Since the circuit has no single stable state but keeps alternating between two states, it is called an astable multivibrator. These kinds of circuits are used to generate waveforms, particularly square waves. We can also distort the output wave using resistors, capacitors, diodes to give us the different waveforms that are commonly used. We can use the information we have learnt thus far to create a few application circuits. Let’s move on to the next project.