The 4 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.
1) 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.
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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 VR1.
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.
- R1, R2, R3 = 4k7 1/4 watt
- VR1 = 10k preset
- LDR = any small LDR with around 10k to 50k resistance in day light (under shade)
- C1 = 470uF/25V
- C2 = 10uF/25V
- All diodes = 1N4007
- T1, T2 = BC547
- Relay = 12V, 400 ohms, 5 amp
- Transformer = 0-12V/500mA or 1 amp
2) 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.
- R1 = Any LDR with resistance of around 10k to 50k in day light (under shade)
- P1 = 1M preset
- C1 = 0.1uF ceramic disc
- R2 = 10k 1/4 watt
- T1 = BC547
- D1 = 1N4007
- Relay = 12V, 400 ohm 5 amp
- ICs = IC 4093 as in the first example or IC 4049 as in the second example
3) Light Activated Relay Switch using IC 555
The last figure illustrates how the IC 555 may be configured for executing the above responses.
Video Clip demonstrating the practical operation of the above IC555 based day night automatic lamp circuit
- R1 = 100k
- R3 = 2m2
- C1 = 0.1uF
- Rl1 = 12V, SPDT,
- D1 = 1N4007,
- N1----N6 = IC 4049
- N1----N4 = IC 4093 IC1 = 555
4) Automatic Night Operated LED Lamp Circuit
This fourth circuit is not only simple but very interesting and very easy to build. You might have seen the new flashlights manufactured with new high bright high efficiency LEDs.
The idea is to achieve something similar but with an added feature.
To make our circuit operative after dark, a phototransistor is employed, so that when the daylight is void, the LED gets switched ON.
To make the circuit extermely compact one button battery type is preferred here, quite akin to those used in calculators, watches, etc.
Understanding the diagram:
As long as ambient light illuminates the phototransistor, the voltage at its emitter lead is sufficiently high for the base of PNP transistor Q1 to keep it shut off.
However when darkness sets in, the phototransistor starts losing conduction and the voltage at its emitter diminishes causing the phototransistor to slowly switch OFF.
This prompts Q1 to begin getting the biasing via its base/ground resistor R and it starts to illuminate brightly as darkness gets deeper.
In order to control the level of the ambient light for which the LED may be desired to be switched ON, he resistor R values may be varied until the desired level is satisfied. Putting a potentiometer may not be recommneded, just to ensure a compact and sleek dimension of the unit.
The circuit may consume approximately 13 mA when the LED is illuminated and just a few hundred uA when its switched OFF.
Bill of material for the discussed automatic night operated LED lamp.
- 1 PNP BC557A
- One compatible phototransistor
- 1 super bright white LED
- 1 battery 3V coin
- One 1K resistor
You can find lots of applications in which photoelectric detection is employed to turn a circuit on/off. This below shown straightforward circuit is configured like a bistable multivibrator.
The Q1 base resistor is actually a photo-resistor with the number ORP12. In the absence of light, the resistance of the photo-resistor is high, this causes Q1 to conduct and Q2 is remains shut off. As the incident light on the photo-resistor OPR12 increases, its resistance falls to a point, until Q1 switches OFF and Q2 switches ON hard, activating the relay coil.In order to reset the circuit we can use the given push-button.
The freewheeling diode connected across the relay coil is to protect the transistor from relay coil reverse EMF spikes, and this diode could be any silicon diode such as the 1N4148 or 1N41007.
Automatic Light Sensitive Switch with Adjustable Dawn or Dusk Switching
This automatic ON/OFF light switch features a selector switch which facilitates the lamp (load) either to switch ON during night and switch OFF during day, or the opposite, that is switch ON during day and switch off at night or in darkness.
In other words the circuit can be used like a day activated automatic switch or a darkness activated automatic switch, depending upon the user preference or the specific application need.
The selection can be implemented simply with a flick of a DPDT switch.
WARNING: The circuit is not isolated from the AC mains supply and will be floating at the mains level, which can be fatal for anybody who touches the circuit in powered ON condition, without an insulated enclosure.
- Parts List
- All resistors are 1/4 watt 5% CFR, unless, otherwise specified.
- R1 = 100K 1 watt
- R2, R3 = 100K
- R4 = 4.7K
- R5 = 220K
- R6 = 470K
- R7 = 68K
- R8 = 33K
- R9 = Any standard LDR
- P1 = 100K preset or trimpot
- C1 = 100uF/25V Electrolytic
- D1 ----- D4 = 1N5408
- D5 = 10V 400 milliwatt zener diode
- T1 = BC547 Transistor
- Th1 = SCR C106
- IC1 = IC 741
Referring to the schematic above, the working of this dual function light activated switch can be understood with the following points:
The op amp 741 forms the heart of the circuit and is wired as a comparator.
Its non-inverting input pin#3 is clamped with a fix reference derived from the junction of the resistive divider formed by R2/R3.
R2, R3 being equal in value, the reference voltage is set at the 50% of the zener voltage D5 which is used for stabilizing the rectified 310 VDC to 10 V DC.
The input DC power is supplied directly from the AC mains via a bridge rectifier set up, while the rectified DC high current is dropped through R1 to suit the attached electronic circuitry.
Now, the non-inverting pin of the op amp being fixed at around 5 V reference, the inverting input pin#2 is used for the detection of the light level via another resistive network formed by R1/P1 and the LDR.
Using as Light Activated Switch
Since the pin#3 is fixed at 5 V, means, as long as the pin#2 remains below this reference level, the op amp output remains high, enabling the T1 to remain switched ON, and the SCR/load switched OFF.
This situation takes place when the R4 end is connected with the positive line, and the LDR is connected at point B which is the ground line, and illuminated by day light.
This is because, during day time the LDR resistance drops drastically causing the pin#2 potential to drop significantly and below pin#3 potential.
So with the selector switch contacts connected across points E and B, the light sensitive switch works like an automatic light activated switch.
Using as Night or Darkness Activated Switch
In order to flip the response, and enable the light sensitive switch to work like a darkness or night activated switch, we just have to toggle the selector switch such that the relevant contacts connect the points D with the positive line, and point C with the negative line.
Once this is implemented, the LDR now gets associated with the positive line, and the R4 end gets connected with the negative line.
In this situation, if the LDR is sufficiently illuminated, causes its resistance to drop, which in turn causes the pin#2 potential to rise over the pin#3 reference level. This instantly causes the op amp output pin#6 to go logic zero, and switch OFF the BJT driver.
With the BJT turned OFF, the SCR and the load are also turned OFF in the presence of day light on the LDR.
Next, when darkness sets in, the LDR resistance increases sufficiently, causing the pin#2 potential to drop below the pin#3 potential, switching ON the BJT, the SCR, and the load, during the night time.
The circuit is now transformed into a darkness activated switch for the load or the connected lamp.
Therefore, just by flipping the selector switch connections across the B-C and D-E points, the light sensitive switch can be quickly pushed into the desired modes, either as an automatic light activated or darkness activated switch.
Resistor R5 introduces some level of hysteresis to the op amp response so that the output of the op amp does behave erratically during the twilight or the transition periods where the light level on the LDR is at the threshold points.
The R5 ensures that the op amp output switches ON or OFF firmly, only once the light level has convincingly crossed the switching threshold.
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