Making an Automatic Street Light using just two Transistors and a Relay

So now let us say you are new, that means you are just starting and you are searching for some very simple kind of automatic street light circuit, right? Then maybe this design we have here is going to work fine for your need.

This one, is the easiest, most basic automatic street light system we can think of. Why? Because it uses only two BC547 transistors and relay and some resistors, and a complete newcomer that means a beginner can quickly put this together and install it for getting the required outcome.

How It Works

Now this system is designed using a light-activated method, so that means this thing will automatically switch ON a street lamp or many lamps when it gets dark and then switch OFF the lamp when daylight comes, right? It works purely by sensing the surrounding light levels.

Once we build this electrical circuit then it will work by itself. When the sun rises, then it will switch OFF the lamp, and when it gets dark, then it will switch it back ON.

Understanding the Circuit

So we can call this circuit an automatic day-night light controller or simply a light-activated switch, whatever suits best. But let us understand how this circuit is working, right? And how we can make it so simple.

When we look at the circuit diagram, then we see that this thing is super simple. It just has two transistors, a relay, and some resistors. That is it! These form the heart of the system.

But we cannot forget the LDR, right? Because this is the most important part, the main sensing component.

The two transistors are connected in such a way that they work opposite to each other. That means when the left-side transistor is conducting, then the right-side transistor will be OFF and when the left-side one is OFF then the right-side one will be ON, simple, right?

Now the left transistor which is T1 is set up as a voltage comparator. It has a resistor network where the upper side resistor is actually the LDR and the lower side resistor is a preset. This preset helps in adjusting the light level at which the circuit should switch ON or OFF.

The second transistor T2 is working as an inverter. That means whatever it gets from T1, then it does the opposite, right?

How the LDR Works

Now let us see how this LDR is behaving in different light levels.

First let us assume there is less light meaning it is dark, right? So what happens? The LDR has high resistance in the dark, and this stops current from flowing to the base of T1.

Because of this, the collector voltage of T1 is high and that activates T2. Since T2 gets activated then the relay also turns ON, and that means the lamp will be ON in the dark, right?

But now when the light level increases, like in the morning then the LDR’s resistance drops, which means it allows more current to pass through it, right? This current reaches T1’s base and makes it conduct.

How the Transistor Responds to LDR

So now when T1 starts conducting then its collector voltage drops to ground, meaning it pulls the voltage down.

Because of this, T2 stops conducting and that means the relay switches OFF. And when the relay switches OFF then the lamp also turns OFF, simple!

Power Supply Details

Now for powering this whole circuit, we need a normal power supply. This is just the usual transformer, bridge rectifier, and capacitor setup, right? This gives us a smooth DC voltage that runs the circuit.

We can build the whole thing on a small piece of vero board. Then, we can put the circuit and the power supply inside a nice, strong plastic box, right?

How to Position the LDR

Now one important thing! The LDR must be placed outside this box, right? That means its front face should be open to the surrounding light because it has to sense the light levels properly.

But careful! We must make sure that the light from the street lamp itself does not fall directly on the LDR. If that happens then the circuit will get confused and keep switching ON and OFF unnecessarily causing problems, right?

Parts List

PCB Design

How to Build

How to Make This Automatic Street Light Circuit Step by Step Right Here.

Alright. Now we will see how we can make this street light system right. We will do it step by step. So no confusion, no problem, all clear, right. Even if someone is totally new, they can do it, okay.

Step 1: Get All the Parts First. No Parts, No Circuit, Right.

First we need all the parts. No parts, no circuit. So first collect everything.

• Transformer TR1 – This brings down the AC voltage, right.
• Bridge Rectifier D1–D4 1N4007 diodes – This turns AC into DC so we can use it.
• Capacitor C1 100uF 25V – This smooths the DC, so no problem with supply.
• LDR – This is the main sensor. This detects light.
• Resistors R1, R2, R3 – These adjust how things work. Small but important.
• Variable Resistor VR1 – This sets how much light we need before switching happens.
• Transistors T1, T3 BC547 – These do the switching work.
• Capacitor C2 – This helps in stability. No random switching.
• Diode D5 1N4007 – This protects the relay. Otherwise, it may damage things.
• Relay 12V 400 Ohms – This turns the bulb ON and OFF.
• Lamp, Any 230V AC bulb – This is what we are controlling. Main thing.

Step 2: First Make the Power Supply. No Power, No Work, Right.

Now first we have to make the power supply. Because without power, what will work. Nothing. So first do this.

Take the transformer TR1 and connect its primary side to AC mains. This is the high voltage side. Be careful.

The secondary side will give us low voltage AC. But we need DC, so we fix that, right.

Connect four diodes D1 to D4 as bridge rectifier. This will change AC into DC.

But DC will not be smooth. So now connect C1 100uF 25V across the rectifier output. This will clean the DC. No ripples. Smooth power.

Step 3: Fix the LDR and Resistors. This is the Heart of the System.

Now the main part is the LDR, right. This thing decides when light is there or not. So we must connect it properly.

One side of the LDR goes to positive supply.

The other side goes to R1 and VR1.

The other side of VR1 goes to ground.

Now, when light is more, then LDR resistance is low. When it is dark, then LDR resistance is high. So it works like a switch, right.

Step 4: Put the Transistors. These Guys Do the Main Work.

Now we add the transistors T1 BC547 and T3 BC547. These will act like electronic switches.

The base of T1 is connected to the junction of LDR and VR1.

The collector of T1 is connected to the base of T3 through R3.

The emitters of T1 and T3 go to ground.

Now, when LDR detects dark, then T1 stops working, which makes T3 work. And the relay gets activated, and the bulb switches ON, right.

Step 5: Put the Relay and Connect the Bulb.

Now we put the relay because this is the thing that will actually turn the bulb ON and OFF.

The collector of T3 is connected to one side of the relay coil.

The other side of the relay coil is connected to positive supply.

The relay has a normally open NO contact. And this is where we connect the bulb.

One side of the bulb goes to AC mains, and the other side connects to the NO contact of the relay.

Now, when the relay turns ON, then it connects the AC mains to the bulb, and the bulb switches ON, right.

Step 6: Do Not Forget the Protection Diode. Otherwise, Problem Will Come.

Now the diode D5 1N4007 is very important.

This diode is connected across the relay coil, but in reverse direction.

When the relay switches OFF, then it makes a sudden voltage spike. And this diode stops that spike from damaging T3.

If we do not put this diode, maybe after some time, the transistor T3 will burn out, and the circuit will stop working, right.

Step 7: Testing. See If It Works or Not.

Now we have done everything. So now we test, right.

Adjust VR1 to set how much light is needed to turn the lamp ON or OFF.

When it is bright, then the LDR has low resistance. T1 turns ON. T3 turns OFF. So relay is OFF, and the lamp is OFF.

When it is dark, then the LDR has high resistance. T1 turns OFF. T3 turns ON. So relay activates, and the lamp turns ON.

If this is happening, then our circuit is working. If not, then check connections. Maybe some mistake. Fix it and try again.

Step 8: Final Fixing. Make It Look Nice and Work Properly.

Now the circuit is working. But we cannot just leave it like this, right. We must assemble it properly.

Solder everything on a vero board or PCB, so it does not break.

Put everything inside a plastic box, so no dust or water can spoil it.

The LDR must be placed outside, where it can sense sunlight properly.

But be careful. The lamp light must not fall on the LDR. Otherwise, it will switch ON and OFF again and again, which is wrong.

Done. Now Automatic Street Light is Ready. No Need to Touch Anything.

Now everything is done, right. This street light will work automatically.

Daytime, then it will switch OFF the lamp.

Nighttime, then it will switch ON the lamp.

No need to press any switch. No need to do anything. It will work by itself, right. So now we are done, and the circuit is ready to use.