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Light Activated Day Night Switch – Science Fair Project

The simple light activated day night switch circuits explained here can all be used for controlling a load, normally a 220V lamp, in response to the varying levels of the surrounding ambient light.

The circuit can be used as a commercial automatic street light control system, as a domestic porch light or corridor light controller or simply can be used by any school kid for displaying the feature in his school fair exhibition.The following content describes four simple ways of making a light activated switch using different methods.

Light Activated Day Night Switch using Transistors

The first diagram shows how the circuit can be configured using transistors, the second and the third circuits demonstare the principle by using CMOS ICs while the last circuit explaines the same concept being implemented using the ubiquitous IC 555.

Let’s evaluate the circuits one by one with the following points:

The first figure shows the use of a couple of transistors in association with a few other components lk resistors for the construction of proposed design.

dawn dusk triggered automatic switching light circuit

The transistors are rigged as inverters, meaning when T1 switches, T2 is switched OFF and vice versa.

The transistors T1 is wired as a comparator and consists of an LDR across its base and the positive supply via a preset.

The LDR is used for sensing the ambient light conditions and is used for triggering T1 when the light level crosses a particular set threshold. This threshold is set by the preset P1.

The use of two transistors particularly helps to reduce the hysteresis of the circuit which would have otherwise affected the circuit if only a single transistor would have been incorporated.

When T1 conducts, T2 is switched OFF ans so is the relay and the connected load or the light.

The opposite happens when the light over the LDR falls or when darkness sets in.

Light Activated Day Dark Switch using CMOS NAND gates and NOT Gates

The second and the third figure incorporates CMOS ICs for executing the above functions and the concept remains rather similar. The first circuit out of the two utilizes the IC 4093 which is quad two-input NAND gate IC.

darkness sensor circuit using IC 4093 CMOS gates Nand

Each of the gates are formed into inverters by shorting its both the inputs together, so that the input logic level of the gates now get effectively reversed at thie outputs.

Though a single NAND gate would be enough for implementing the actions, three gates have been engaged as buffers for getting better results and in a view of utilizing all of them as in any case three of them would be left idle.

The gate which is responsible for the sensing can be seen accompanied with the light sensing device LDR wired across its input and the positive via a variable resistor.

This variable resistor is used for setting the triggering point of the gate when the light falling over the LDR reaches the desired specified intensity.

As this happens, the gate input goes high, the output consequently becomes low making the outputs of the buffer gates high. The result is the triggering of the transistor and the relay assembly. The connected load over the relay now flips into the intended actions.

light operated sensitive switch using LDR and NOT gates 4049 IC

The above actions are exactly replicated using the IC 4049 which is also wired with similar configuration and is quite explanatory.

Light Activated Relay Switch using IC 555

The last figure illustrates how the IC 555 may be configured for executing the above responses.

IC 555 based day night triggered relay switch circuit

Video Clip demonstrating the practical operation of the above IC555 based day night automatic lamp circuit

Parts List

R1 = 100k

R3 = 2m2

C1 = 0.1uF

Rl1 = 12V, SPDT,

D1 = 1N4007,

N1----N6 = IC 4049

N1----N4 = IC 4093 IC1 = 555

Need Help? Please leave a comment, I'll get back soon with a reply!


  1. hi can you elaborate please cant understand your reply..for LDR

  2. make the last 555 circuit given in the above article, interchange R1 and the LDR positions, remove the relay and use the positive supply coming out from its pin3 to power the flasher circuit meaning the flasher circuit's positive line will get powered from the above circuits pin3 and NOT from any external 12V source. This configuration will shut off the LEDs during day and vice versa.

  3. @Swagatam Majumdar what is the value of resistor and the variable (potentiometer) in figure no.1 please help my for my project to be submit in monday please

  4. Hello swagatam
    I know what is LDR function but I don't know how to buy it and what is its name in the market.
    Can you please sho me its name in the market.
    Thank you

  5. Please check the following link for the details:

  6. Hello Sasa,

    It's called LDR everywhere, even the shopkeeper will know this and identify this component as LDR only, so you can tell him this name.

  7. can you help me how to make LDR opposite ? in a way that when the loght come through the LDR the electricity cant flow or turn off and when the LDR sense dark the electricity can flow or turn on...?

  8. in the above shown diagrams, you just need to change the position of the lDR from the upper arm of the preset to the lower arm of the preset or vice versa for getting the desired results

  9. Hello,

    Can you tell for the first diagram what are R1, R2, R3 and VR1 values?

    Thank you


  10. you can refer to this article for more info:

  11. lease the pin 7 and 5 connected with a resistor are not taken to ground and the pin 1 again can i use preset in place of one connected to pin 2 thanks

  12. pin7 and 5 should be exactly as shown, they shouldn't be grounded.

    Pin#1 line is the negative supply line.

    1M preset can be used with pin#2

  13. Hi Swag,

    A bad manip.... Here again my question:

    Can I change R3 by a variable resistor or potentiometer if I want to use different light ambient intensity to adjust the threshold that will activate the relay. What would be the range value of this component to obtain this.

    Thank's and regards

  14. Hi Papa Ciela, thanks for posting your question again! If you are referring to the IC 555 circuit, you can replace R1 with a 1M pot for getting the required adjustments.

    R3 and C1 are placed for implementing a slight delay in the operation, so that the relay does not vibrate during twilight transitions.

  15. Hi Swag,
    You answer at the light speed !!!

    Thank you so much !!

  16. How can we connect more bulbs to this circuit say 4 bulbs do we need to make 4 circuits

  17. You can add more bulbs to the same circuit, by connecting the bulbs in parallel. The shown relay will be able to handle upto 300 watts, if your are above this level then you may have to replace the relay with a higher rated one.

  18. Thanks swag for the reply


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