Simple Time Delay Circuit using 555 Timer

In this project we are going to design a Simple Time Delay Circuit Using 555 Timer IC. This circuit consists of 2 switches one for start the delay time and other for reset. It also has a potentiometer to adjust the time delay, where you can increase of decrease the time delay by just rotating the potentiometer.

Here we have used 9 V battery and 5V optional relay for switching the AC load. A 5v voltage regulator is used for giving 5v regular supply to the circuit. Also check our 1 minute timer circuit using 555.

Required Components:

1. 555 timer IC
2. Resistor- 1k (3)
3. Resistor- 10k
4. Variable Resistor – 1000k
5. Capacitor – 200uF, 0.01uF
6. LED- Red and green
7. Push Buttons- 2

555 Timer IC:

Before going into detail of Time Delay Circuit, first we need to learn about 555 Timer IC first. Below you can find the pin diagram of 555 timer IC along with the details of each pin.

Pin 1. Ground: This pin should be connected to ground.

Pin 2. TRIGGER:  Trigger pin is dragged from the negative input of comparator two. The comparator two output is connected to SET pin of flip-flop. With the comparator two output high we get high voltage at the timer output. If this pin is connected to ground (or less than Vcc/3), the output will be always high.

Pin 3. OUTPUT: This pin also has no special function. This is output pin where Load is connected.

Pin 4. Reset: There is a flip-flop in the timer chip. Reset pin is directly connected to MR (Master Reset) of the flip-flop. This pin is connected to VCC for the flip-flop to stop from hard resetting.

Pin 5. Control Pin:  The control pin is connected from the negative input pin of comparator one. Normally this pin is pulled down with a capacitor (0.01uF), to avoid unwanted noise interference with the working.

Pin 6. THRESHOLD: Threshold pin voltage determines when to reset the flip-flop in the timer. The threshold pin is drawn from positive input of comparator1. If the control pin is open. Then a voltage equal to or greater than VCC*(2/3) (i.e.6V for a 9V supply) will reset the flip-flop. So the output goes low.

Pin 7. DISCHARGE: This pin is drawn from the open collector of transistor. Since the transistor (on which discharge pin got taken, Q1) got its base connected to Qbar. Whenever the output goes low or the flip-flop gets reset, the discharge pin is pulled to ground.

Pin 8. Power or VCC:  It is connected to positive voltage (+3.6v to +15v).

 

If you want to learn the 555 IC in detail, check out our detailed 555 Timer IC article.

 

Monostable Mode of 555 Timer IC:

555 Timer IC is configured in the Monostable mode for this Time Delay circuit. So here we are explaining the Monostable mode of 555 Timer IC.

Below is the Internal structure of 555 Timer IC:

Operation is simple, initially 555 is in stable state i.e. OUPUT at PIN 3 is low. We know that Non-inverting end of Lower Comparator is at 1/3Vcc, so when we apply negative (< 1/3Vcc) voltage to the Trigger PIN 2 by connecting it to Ground (through a PUSH button switch), two things happens:

1. First is, Lower comparator becomes HIGH and Flip flop gets Set and we get HIGH OUTPUT at PIN 3.
2. And second thing is, Transistor Q1 becomes OFF, and Timing capacitor C1 get disconnected from the Ground and start charging though the Resistor R1.

 

This state is called the quasi stable state and remains for some time (T). Now when capacitor starts charging and reaches to the voltage slightly greater than 2/3 Vcc, voltage at Threshold PIN 6 becomes greater than the voltage at inverting end (2/3Vcc) of Upper comparator, again two things happens:

1. Firstly, Upper comparator becomes HIGH and Flip flop gets Resets and the OUTPUT of the chip at PIN 3 becomes LOW.
2. And secondly, Transistor Q2 becomes ON, and capacitor starts discharging to the ground, through the Discharge PIN 7.

 

So 555 IC automatically fall back to the stable state (LOW) after the time determined by the RC network. This duration of quasi stable state can be calculated using this 555 Monostable Calculator or can be calculated with the formulae given below:

T= 1.1*R1*C1 Seconds where R1 is in OHM and C1 is in Farads.

So now we can see that MONOSTABLE mode has only one stable state and require a negative pulse at PIN 2, for the transition to Quasi stable state. Quasi stable state only remains for 1.1*R1*C1 seconds and then it automatically switch back to stable state. Remember one thing, while designing this circuit, that Trigger pulse at PIN 2 must be shorter enough to the OUPUT pulse, so that the capacitor gets enough time to charge and discharge.

 

Circuit Diagram:

Below is the Circuit Diagram for Simple Delay Circuit using 555 IC:

 

Working of Time Delay Circuit:

The entire circuit is powered by 5V using 7805 voltage regulator. Initially when no button is pressed the output of the 555 IC remains LOW and the circuit remains in this state, until you press the START button and the capacitor C1 remains in discharged condition.

As we explained above, the time delay for the quasi stable state (unstable) is depend upon the value Timing capacitor and resistor. As you change the value of these, the time delay for quasi stable state will also be changed. Here the Blue LED glows in quasi stable state for the particular time and Red LED glows in stable state. So here we have replaced this timing Resistor with the Variable Resistor, so that we can adjust the time delay just by rotating the knob of potentiometer on the board itself. Here we have also connected a optional Relay for trigering the AC appliance after a the time delay. Learn here to interface Relay for triggering AC loads.

When you press the Start button, the countdown timer starts and Blue LED turns on and after the particular time (defined by the formula T= 1.1*R1*C1) the 555 timer goes in stable state, where Red LED turns ON and blue LED turns Off. You can increase and decrease the time delay by using the potentiometer as demonstrated in the Video Below.