Project Basics


The relay is simply an electrically operated switch i.e. relay has two states and state of the relay depends upon the presence of electrical signal. The relay is usually cuboid shaped and has 5 pins.

WHERE TO USE: The relay is used as a switch between two independent circuits. I.e. one circuit can be used to control the working of another circuit. . We generally use a small current circuit to switch on large current circuits via relay. For example we can use a small battery to switch on an AC 220V supply and this battery circuit is isolated from the AC supply. So we can use relay to switch on any circuit.

HOW TO USE: We first discuss different pins of relay;

The front view of the relay is as follow:

Pin1 and Pin 2: The activating electrical signal is supplied across these pins.

Pin 3: This pin represents the stable state of relay. We can also call this a Normally Closed position i.e. this is the state of the relay when there is no signal.

Pin 4: This pin represents the excited state of the relay. We can also call this a Normally Open position i.e. this is the state of the relay when there is an electrical signal present.

Pin 5: is the Common Pin. This actually is the moving part of the switch.

Suppose we have to use a relay to switch on a bulb on 220V AC using the signal from a small voltage signal. So we connect the different pins of relay as:


When there is no voltage difference applied between the pin 1& 2, then the bulb doesn’t glow.

When we apply the voltage difference across the pins 1& 2, the current starts flowing in the coil and the magnetic field is created in the coil. Due to this magnetic field, the moving common part is attracted towards the pin 5. Hence the common pin, pin 5 is disconnected from pin 3 and gets connected to pin 5. Hence 220 V circuit gets completed as one wire is directly connected to the bulb and the other wire is connected through relay. Also note that it doesn’t matter how we connect the voltage signal to the pins 1 & 2 i.e. we only need to create the voltage difference between pins 1 & 2.


When the input signal is weak, it can’t trigger the relay as we require minimum value of current to switch the state of relay. So to amplify the current we use the current buffer IC as shown below:

When the input signal is low, there is no voltage difference across pin1 and 2. Hence bulb doesn’t glow. How ever when the input signal is HIGH, voltage difference is supplied at pin 1 & 2 with amplified current and relay gets switched on, the bulb glows.

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