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  • This automatic light dimmer circuit makes it possible to control a lighting system so that it turns on or off slowly. The circuit works this way: when switch S1 is closed, the capacitor C1 is slowly charged. Once the voltage at C1 reaches 0.6, transistor T1 begins to conduct and the LED also begins to light. If the capacitor voltage increases further, then transistor T1 conducts more current and in return the LED lights brighter.

    If the LED lights up, the LDR resistance decreases causing the SCR to conduct periodically earlier. This tehnique causes the lighting system to turn on slowly.
    On the other hand if switch S1 is turned off, meaning the switch is opened, the LED does not immediately turn off since the capacitor voltage at the base of T1 discharges slowly. The LED slowly dims until finally turns off. This causes the lighting dim out before it finally turns off. Potentiometer P2 must be set so that the anode voltage of D1 is about 0.7 volts. If this is done, the capacitor voltage will be around 0.5 volts during standby, meaning lights off.

    Lamp dimmer circuit diagram

    light dimmer circuit schematic

    Never use replacement diode types for diode D1. Be sure to use an originally marked 1N4148 signal switching diode.

    Caution! Danger of electrocution!
    You are working with a line voltage of 220 Volts AC. Extreme shock hazard.

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    25 Responses to "Automatic Lamp Dimmer Circuit"

    1. interested says: on April 25, 2010 at 9:17 am

      how long does the LDR resistance take to decrease all the way allowing the light to reach its maximum output? also when its turning off.
      is it possible to control the time it takes to reach its maximum output from zero and vise versa ?!?!

      i’m really interested!! thanks :D

    2. Anybody have a suggestion for the value of R1 …. ?
      And how many Volt should the part with the switch be supplied with ?

      Anybody build it ? Does it work ? Should the be any changes made regarding reliability ?

      Any comments / answers to the other questions from the other guy ?


      • Unless I’m not a good reader, I saw no reference to the Input potential of the dimming circuit.
        R1 value depends on the “input” voltage.
        I presume the input voltage to the opto device is that of a DC potential, 12 VDC?
        I have seen a 220 ohm resistor used for 2 opto LED’s in series at 5 VDC!
        One must be careful not to exceed the max current thru the LED! Research the opto device specs.

      • DaShrek50 DaShrek50 says: on July 23, 2015 at 10:16 am

        R1 should be between 220 Ohms and 330 Ohms.
        Use a variable voltage regulator and usually
        between 6V DC and 12V DC, should do the job.
        Also measure the voltage on the 2SC3112, it should
        be in few volts range (4-10)volts.

    3. can i use LED to replace the lamp?
      since i need to build my prototype for my project

    4. Howcan the above circuit be modified to just reduce the intensity of the output light with out turning it off,to use it in automatic light dim system.

    5. Petiebird says: on July 29, 2013 at 11:52 am

      What modifications to this circuit would be required to be powered by household 110V instead of 220V? I would like a gradual lamp dimming to off triggered from an external 110V timer. Would other modifications be necessary to employ a 600V triac?
      (also, R1 value?)

    6. Petiebird says: on July 30, 2013 at 10:26 am

      Circuit simulation? This novice would require more details but, may try. Thanks much for the prompt response.

    7. George Small says: on August 30, 2013 at 12:34 am

      I work for an installation artist who needs an automatic dimmer for an LED display such that the LEDs will turn on AND off slowly when the circuit is activated by a motion detector. The artist would be willing to pay someone to build an automatic dimmer as described. Is anyone interested? This is for an installation in Amsterdam. Thanks for any assistance y’all can provide. If you are interested, let me know and I will get in touch with you regarding the specifics of what we need.
      George Small

    8. Hallo,

      Try and look at this product – its ready to install. There is also an other one called EUD61NPS

      Just install it and it does just the job of above curcuit.



      • George Small says: on August 30, 2013 at 6:27 pm

        Thanks Julian! I think one of the switches from the company you suggested will work for us. I really appreciate your information. Regards, George s

      • C.Mitra C.Mitra says: on April 3, 2014 at 2:07 pm

        Dear George Small,

        Happy to know that you may have found your solution. But in case you still need help for your project, you may contact me. I shall be happy to help you out by designing the automatic dimmer.


        Chinmoy Mitra

    9. Petiebird says: on August 31, 2013 at 10:25 am

      Perhaps works for me also. Looking for prchase info. Thanks,Pete S.

    10. If the circuit is going to be powered from 12v so R1must be in the region of 560 to 680 ohms remember resistance equals voltage divided by current so 12v divided by 0.02A (LED current) approx

      • Petiebird says: on April 5, 2014 at 11:04 am

        Lamp dimmer circuit not yet built by this novice. Still unsure of R1 value for 110V power supply.

    11. I need to have a lamp dimmer that can “cascade” from one lamp to the next (up to 4 lamps). Basically, when you close a switch, the first lamp begins to light from dim to 100%. Once lamp #1 reaches 100%, then that triggers dimmer circuit #2 and lamp #2 begins to go from dim to bright. This is repeated up to four lights. How to cascade dimmer circuits?

      Obviously, when the switch is opened, dimmer #4 goes to zero, then dimmer #3 ramps down to zero, then #2, and finally #1 circuit……help please.

    12. R. Broderick says: on February 21, 2014 at 4:41 am

      What type of linear Optocoupler is being used in this circuit? I do not recognize the symbol. Also, what changes if any to the AC side would be required to run this on 129VAC? Thank you. I’m looking forward to prototyping this circuit.

    13. Hello all ,

      please note that half of the circuit ie before the optocoupler works on 12v dc.The optocoupler is just an ultra bright white led infront of a cds cell/light dependent resistor.This is made by insert this sensor [ldr] into a piece of heat shrink and on the other side of the tube insert the white led than heat them up to shrink the tube.The other half of the circuit ie the triac side can be powered from 110 v to 230 v ac [ be sure to use the appropriate bulb voltage depending on the input voltage ].Now the trickiest part is the value of R1.In theory the value of R1 is approx 470 to 560 ohms , remember ohms [resistance] is voltage in this case 12v divided by led current which is approx 0.02A or 20 mA.Now if R1 is approx this theoretically value ie approx 470 to 560 ohms the circuit does not works good.When S1 is closed the lamp does not goes on gradually but on nearly instantly, which is not good. When S1 is open yes the lamp goes off gradually as we needed.So to solve this problem the R1 560 ohms must be in series with at least 5k pot or preset so the led can we make it to go on very gradually for example the led is in series with a 2k resistor.
      Example suppose that a person has a 10nm of force [this represents the voltage] and has to push a pushchair [this represents the resistance] to 5mph.Maybe after a second this is achieved because the weight is so small.Now consider the same force but now you have to push a car [on flat of course] to the same speed .You can move the car as well but to reach the 5mph speed it sure take more than a second maybe a few seconds.So by making a big value in series with the led the led will goes on very gradually so the lamp goes on very gradually.So now you know the value of R1.


    14. Hello, i have a question. I am looking to realize this project, but i need some calculations about the elements, how can i calculate the value of R1 if i have +12V input voltage, and how to calculate all other Resistors and Capasitors? I would also like to ask how should i remake the schematic so i can control the lamp brightness somehow ? Can anyone skilled in this type of electonichs to help me out. I am still a beginer and do not understand this schematic very well, thanks in advance.

    15. Hello,
      This circuit looks like I would like to do but isn’t it possible to make it more simple, without the 12V portion and the optocoupler ?

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