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    There are a wide range of applications for light sensitive switches: lighting, entrance door, automatic staircase, automatic opening of doors to the action of a ray of light, alarm systems, etc… Many of us are familiar with photosensitive switches made with a single transistor that is controlled by a photoresistor placed between the base and the recharger, depending on what is desired: normal functioning “closed” or normal “open” the switch.

    This configuration allows easy installation more complex, involving the use of operational amplifiers, especially those of type 741, which are very cheap. Another way to optodetect, less known, uses a bridge assembly, which operates on the principle that when the current is zero diagonal axle, means that the bridge is balanced.

    This last principle is used in the sensitive switch installation you will be presented next. Photocell is placed in a bridge circuit and a comparator is used as detector bridge balanced. The comparator’s output control thyristor through a transistor. In this circuit, protective measures must be taken because it is isolated from the network.

    Light Switch Circuit Diagram

    light sensitive switch circuit diagram

    Power supply circuit is taken from the bridge rectifier D1…D4 is filtered and stabilized by R1, C1, and D5. The bridge circuit is difficult to identify in the given diagram, but it is made of R2 ….. R4, P1 and photoresist (LDR). IC1 is used in the comparator configuration and power level will be about 1.8 V when potential entry vice versa (negative) exceed that of the entry non vice versa. Resistance R5 conducts a hysteresis of about 1V to prevent thyristor to oscillate at T1 and threshold lighting.

    Switching point of the light sensitive switch may be adjusted from P1. With the potentiometer set the minimum (minimal resistance) lamp will light a crepuscular light. If you want more flexibility, replace P1 with one that has a value of 1 MΩ. If desired operation reverse position in the scheme of the photoresistor can be changed with the group P1/R4. LA1 lamp will be extinguished, in this case, after dark.

    Some practical advice: if you want to use higher power lamps then diodes D1 …. D4 need to be replaced with other type 1N5404 and thyristor TH1 will be equipped with a heat sink. With these changes, the circuit can control currents up to 3A.

    Maximum gate for TH1 is 250 µA, which means that you must use a very sensitive thyristor. It can use any type of photoresistor. Remember precautions to be taken due to lack of isolation mounting over the network. It is important that the entire circuit to be protected in a plastic box, of any shape, provided with a hole in the upper side, in order to allow to the photoresistance “to see”. Assure that the entry cable and the exit cable are fixed. This precautions will assure you from accidents.

    Light Sensitive Switch PCB Layout

    light sensitive switch pcb layout

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    16 Responses to "Light Sensitive Switch Circuit"

    1. Can anyone please modify the circuit by activating a relay instead of a thyristor? I believe it will increase power handling and reduce heating.

    2. omole michael says: on May 21, 2010 at 5:32 pm

      hi dis is michael,i need help(s) on my project dt am doing now.the project based on exit/entry counter using PIC 16f877 with LDR to count people coming in and out of a particular place ranges from (1 to 1000).pls i need a write up on it .i would be very grateful if can see somebody helped out with dis short period of time.thanks a lot.

    3. hey what should be connected to as output .. plzz help

    4. what is abcde terminals denotes pls explain

    5. there is too many terminal to the ldr whats that

    6. hello friends…., i am doing ECE 2nd yr…., i want 2 do my mini project in my core…., please send me some mini project titles related my core 2 my mail id vino2sucess@yahoo.com…,

    7. prasanna awatade says: on December 8, 2011 at 7:09 pm

      can you tell me what is that A,B,C,D,E
      pleas help!!!!!!!!!!!!

    8. how the 230v can be directly given to the diodes(bridge rectifier) What are the terminals A,B,C,D,E

    9. Can uplzgive me the internal circuit of the comparator & IC

      • Jim Keith says: on November 12, 2012 at 3:21 am

        For the internal schematic check out the LM741 datasheet:

    10. Without even checking the circuit for errors, I have cast this to one side. Not only are the components left unmarked but the English is so bad that it must be a nightmare for students to try to understand.

    11. The circuit diagram is incorrect. According to the PCBs, the lamp is not connected directly to the mains, but to the output of the rectifier bridge before R1 (100K 1W).

      Another thing: A, B, C, D, E are the PCB connection points to connect the LDR photoresistor and to configure bridge resistors to the circuit to function normally (turn on lamp by darkness) or in reverse mode (turn on lamp by light).

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