Learn LDR automatic LED flashlight

How to quickly learn transistor and LDR circuits?
In this post, I am going to show you how easy learning.

My son wants to build a simple automatic flasher. It is special. Why?

It will be light in the dark. This circuit uses 5 parts only and cheap.

He likes to play LEGO very much. So builds the Lighthouse tower with LEGO block.

It is not important for you. Even more importantly.

I hope this helps you improve understanding it works. For example, the voltage divider circuit, resistor works, and more.

Ready to get started?

How circuit works

automatic LED flashlight

As Figure 1, this circuit uses the simple transistor driver for flashing LED.

He uses the Multicolor Flashing LED for LED1

How to build

He assembles them on a small breadboard as Figure 2. When applying the 2.4V power supply to the circuit. The small current flow through to R-10K to the base lead of transistor Q1- 2N3904, makes large current flow through the collector to emitter lead, and LED1 will glow up. As Figure 3.

Assemble the circuit on breadboard

Figure 2 assemble the circuit on a breadboard

LED glow up

Figure 3 LED glow up

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Then, he put LDR into the circuit as Figure 4. When the light to LDR, its resistance is low. So the R1’s current flow through to LDR instead. It makes no current to the base of transistor so it not work. LED will go out.

And after, he turns off the lamp. The resistance of LDR is high, so the R1’s current can flow through the base of the transistor again. It makes large current from collector flow to the emitter of the transistor so LED glow up or flashing light.


Figure 4 Lego automatic LED flasher light circuit

As video below he builds and make them!

Not only that we will learn more by making this project below.

Simple dream baby automatic sensor night light

My daughter is about 1 year old. It is normal to wake request milk several times each night. If turn on lighting up. It causes inconvenience and uneconomical.

We are thinking to buy a baby night light instead of a normal lamp. It is lower light to reduce visual distractions. But it is enough to see the bottle and able to breastfeed.

Simple baby automatic night light

This method makes it more power-efficient than turning on the lamp for the night. Because I turn on use power of 20 watts, but this system consumes just 1 watt only.

I design simple dream baby automatic sensor night light as Figure 1. Since it is cheap and easy to makes.

This ideas circuit

I see in amazon look at many baby night light and I like auto light ideas because it is convenient. It does not need to have switch on-off. The LEDs glow brightly just when in night or dark only.

How does it work

Simple baby automatic night light

Figure 1 is the circuit diagram that I use 4 LED super bright as nigh light so saving. And use an old small DC adapter 9volts is a power supply that has high performance than a transformerless power supply.

I use a photoresistor(LDR1) as a light sensor he resistance of it will change with the light.

The current flow through to VR1-potentiometer and LDR1 is set circuit in divider form.

Learn how voltage divider work

Normally the lighter to strike the photoresistor. It causes a voltage drop across it is low voltage. So not have a current flow to the base of Q1.
Therefore Q1, Q2 not works. So the LED goes out.

Then, not light to LDR, cause voltage across them is high. It causes have a current flow to R1-120K to the base of Q1, Q2. So, they have connected as Darlington compound so high gain ratio.

How transistor works on Darlington

Thus the current through to LED1, LED2 to collector and emitter of Q2 to ground. The LED1, LED2 is glowing now.

How to build its

This is a very simple project. I assemble parts and wiring on the perforated board or Universal PCB board. And install them into the universal box As Figure 2.

Read Also:

Next, I test it in a bedroom. We can adjust the sensitivity of the circuit with VR1.

assemble parts and wiring on perforated board
Assemble parts and wiring on perforated board
Prototype of Simple baby automatic night light
A Prototype of Simple baby automatic night light

Lastly, I take it to really use it as the video below.

1. This circuit does not use R-limiting current because we use LED super bright that uses voltage 3-4V in fully light.
And This supply voltage is 8.4 volts approximately.
2. If you use an AC adapter that not a good filter, I always insert a 100uF 25V electrolytic capacitor across +V and Ground to smooth noise or ac ripple voltage. But it does not need it. If you use a normal DC power supply regulator.

Check out these related circuits, too:

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I always try to make Electronics Learning Easy.


  1. Reiner

    no resistor in line with each led?
    a no go!

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