We’re now going to learn how to build a simple delay circuit. The 555 IC is a very popular and versatile timer IC that can be used to design interesting circuits. In this project, we will be using it to generate a short delay.
The 555 timer has 8 pins and here’s a glimpse at what it is made up of. We will dive into the details a bit a later but first, let’s take a look at the schematic and understand what’s happening.
Pins 1 & 8 are the power supply pins and we supply 5V to the IC. Pin 4 is the reset pin and it is active low. This means that it will force the 555 timer to stay in the reset state if it is pulled LOW. We apply 5V to this pin to keep the IC in the active state. Pin 3 is the output pin and we connect an LED with a current limiting resistor so that we can visualize the output.
Pins 2 & 6 are the most important pins for this application. C1 and R1 form an RC timing circuit. When the circuit is first powered ON, the capacitor is uncharged and the voltage across R1 is 5V. As the capacitor starts charging, the voltage across C1 starts building up while that across R1 drops. The voltage applied to pins 2 & 6 determine the state of the output.
As long as the voltage at the threshold pin, that is pin 6, is above 2/3rd of the supply voltage, the output will be LOW.
When the voltage at the trigger pin, that is pin 2, is less than 1/3rd of the supply voltage, the output will be HIGH.
In our circuit, the output is initially held LOW as the capacitor begins to charge and the output goes HIGH once it charges to a voltage greater than 2/3rd of the supply voltage. It takes about 5 seconds for the capacitor to charge and this time can be increased by changing the values of R1 & C1. The relationship between the component values and the time delay is given by the following equation:
T = 1.1*R1*C1
The 555 timer contains two comparators and two NOR gates which together create a flip-flop. In order to understand how it functions, we need to look at them first.
This is the symbol for a comparator. It has two inputs and one output. The comparator output will be a 1 or HIGH, as long as the voltage at its positive terminal is greater than that at its negative terminal. The comparator output will be 0 or LOW, in all other cases.
A flip flop is a device that has two inputs and two outputs. There are different types of flip-flops but the one used in the 555 timer is called an SR flip flop or a Set-Reset Flip-flop. The two inputs are marked as shown in the image above. One is the S or set input, while the other is the R or reset input. Q is the output while Q’ is the inverted output. What this means is that when the output Q takes a value of 1, the value of Q’ will be 0 and vice versa. When we apply a logic 1 to the Set input, it will cause the Q output to switch to the HIGH state and it will remain this way even after we remove the input signal. The Q’ output will be in the LOW state.
The Q output will only go LOW when we apply a logic 1 to the Reset input. The output will continue to stay LOW until we apply a logic 1 to the set input again. When the Q output goes LOW, the Q’ output will be HIGH. Let’s use this information and look at the block diagram for the 555 timer.
Notice that we also have a voltage divider circuit comprising of three 5K resistors, this means that the voltages will be equally divided among the three resistors. The voltage at the top point is 2/3rd the supply voltage, while the voltage at the lower point will be 1/3rd the supply voltage. These two voltages are also fed to the inputs of the two comparators.
When the circuit is first switched ON, the capacitor is discharged and this causes the voltage across R1 to be 5V. This causes the comparator output to be 0 which is also fed to the flip-flop.
Once the capacitor charges, the voltage across R1 will drop. Once this voltage drops below 1/3rd of Vcc, it will cause the comparators to switch states. The output of comparator A will now be 1 while the output of comparator B will be 0. This will place a logic 1 signal at the Set pin of the flip-flop and it will cause the output to go HIGH.
This should give you a basic understanding of how the 555 timer works. Please ensure that you mount the IC base and the capacitor with the correct polarity. Gently insert the 555 timer and use the 5V output from the power supply board to power the circuit. The LED will light up after about 5 seconds. You can tweak the values or R1 and C1 to change the delay. Let’s move on to the next project.