light sensitive switch with ldr

DIY LDR Switch Circuits

Here are some DIY LDR switch circuits available on our website and the web. LDR switches are used to switch a relay or other device when light are present or absent. LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits.

Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.

LDR switch circuit diagrams

  • This is a light switch or light activated relay circuit. The relay on when LDR uncoverd and relay off when LDR covered. Adjust VR1 for light sensitive. LED will turn on at the same time with relay.
  • light activated relay circuit

  • This is the circuit diagram of a light activated switch based on National Semiconductors comparator IC LM 311 and a LDR.
  • light activated switch circuit diagram

  • The circuit below shows a simple application of ldr operated switch. This circuit senses the ambience light and depending on the presence/absense of sufficient ambience light, it turnes the bulb On/Off automatically. The bulb gets on when there is insufficient ambience light (e.g evening or night time) and switches off automatically when enough ambience light is present. (e.g In day time, when the sun light is present additional light may not be needed.) This bulb can also be your street lamp that needs to be switched on every evening, and switched off every morning. The circuit does this job automatically without any manual interference.
  • ldr operated switch

  • Light sensitive switch with LDR, actually it has a buzzer which will sound an alarm…
  • light sensitive switch with ldr

If you other LDR switch circuits related links, then please write about it on the comments section.


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  • prasad

    light sensitive ckt which switch is used

  • Andrew

    Could you send me a link to this component if that’s possible? Thanks.

    • Jim Keith

      Paste the link I provided in your browser address field, then change “~” to “.”

      I do this because this web page requires webmaster moderation before links are published.

  • Andrew

    Hi, I am a student studying Electronics and i was wondering if this idea of using an LDR as a switch could be used by firing a laser at it? Would the intensity/(irradience) of the of the laser light switch this circuit on if you made it in a cerain way that the LDR only reacted to high intensity light?

    • Jim Keith

      You could set the threshold very high so that ambient light would have no effect. My only question is that the laser spot may be much smaller than the LDR element (6mm typical) and thus not turn it on. Some optics may be required to spread the beam.

    • Andrew

      The purpose of this circuit is to be able to shine a laser onto the LDR from a distance of about fifty yards. (The purpose of this is a long story) Would the beam, at this distance, most likely have spread out enough to register on the LDR?

    • Jim Keith

      I do not have experience with this stuff, but one inexpensive device (US$ 4.07) has about a 3° angle which makes it easily large enough at 50yd.

  • Agus

    hi, may i know when the 9v drop to 8v or other. do we need adjust the potensio everytime > tx.

  • shahid gul

    the circuit is good

  • shohel

    myneed circuit dyagram for ldr day night

  • vhutshilo

    hi!!, am a student at tswane university of technology studying electrical engineering (power). i found my self doing lot of light current circuits that i get in internet just to learn to build them. i have a dream of having my campany when time goes on. my question is (is it allowed to use circuits from internet for persona business or not?) because up to now i dont know how to build a specific circuit from scrach, if not would you advive a better way like if i have to register with a company to use its work(circuits). would you please send me an email to answer me.

    from sadiki ve