Contents
Making a simple Automatic 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 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.
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.
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.
Light Activated Relay Switch using IC 555
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
Faith Jumbo says
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
Swagatam says
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
fasat2112 says
Hello,
Can you tell for the first diagram what are R1, R2, R3 and VR1 values?
Thank you
BR
Swagatam says
you can refer to this article for more info:
https://homemade-circuits.com/2011/12/super-simple-light-activated-switch.html
tristan pagdanganan says
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…?
Swagatam says
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
Sasa Hadia says
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
Swagatam says
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.
Keenmer Jon Serrano says
@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
Swagatam says
Please check the following link for the details:
https://homemade-circuits.com/2011/12/super-simple-light-activated-switch.html
Mukesh says
hi can you elaborate please cant understand your reply..for LDR
Swagatam says
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.
Anonymous says
could the last circut on this page be used with the power supply-battery backup circuit by removing d-1 ,t-1 and the relay to power a nightlight(3leds) via a wall plug directly ?
Swagatam says
you mean you want to power the circuit from a battery DC? Yes it can be done as you have suggested.
Anonymous says
any other website for comparator as a night switch
Swagatam says
Google it, you'll find many
Anonymous says
no need a regulator for preventing the voltage variation in the base of first transistor ??
Swagatam says
the given capacitor does the job….
Hunain khan says
swagatam plz told me that R2 and transistor are connected to which pin of the 555 timer
Swagatam says
pin#3
Hunain khan says
thank you Swagatam for replying me
in potentiometer there are 3 points but in the schemetic in which 555 timmer is used there are only 2 terminal plz point th third one.
Swagatam says
The symbol has three points, the upper, the lower and the center arrow, all these correspond to the three points of the pot.
In 555 also the center one is the arrow which is shorted with the lower pin. It is not shown by mistake.
Or do in this way:
Connect the center to ground and any one of the outer pin to pin#2 of the IC
Hunain khan says
hello swagatam
i dont understand that what is p1 is this a resistor or potentiometer plz clear it as soon as possible because we mostly use p for preset.
Swagatam says
Hello Hunain,
You may use a preset or a pot, depends on the user preference, both will work.