How to Make a Light Activated Day Night Switch Circuit – Science Fair Project

The circuits explained here can all be used for controlling a load, normally a 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. 

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 lke resistors for the construction of proposed design.

 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.

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.

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.

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

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

Parts List

R1 = 1M

R3 = 2m2

C1 = 0.1uF

Rl1 = 12V, SPDT,

D1 = 1N4007,

N1----N6 = IC 4049

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

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