Dark sensor using two transistors


This works same as the other dark detector circuit. When light is falling on LDR, its low resistance drives transistor Q1-BC547 into conduction. This keeps transistor Q2 cut-off due to low base bias. The D1-LED does not get power as long as ambient light falls on LDR. When the resistance of LDR becomes high in darkness, transistor Q1 stops conducting and transistor Q2 starts conducting to turn on the D1-LED.

Datasheet of BC547: http://www.datasheetcatalog.org/datasheet/fairchild/BC547.pdf

Click the video below to see the output.

 Components for this experiment are available at buildcircuit.net.

components 2

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  1. Hi. This is work.
    But I still confused why when we use only
    one transistor with the same ldr and r3 position,
    and we add led near r1,
    the circuit be light sensor not dark sensor.
    I mean reverse.


  2. thanks for this circuit! 🙂 this works but the other one which you only use one BC547 didnt work. will this circuit can last and a reliable one? because i will replace the LED with a buzzer in short im doing an alarm, also I did parallel connection of 1000micro farad capacitor with the buzzer/led to last longer, and having an input of 9V battery.

  3. i used LDR ….for sun light detection…but ldr only sense the tourch light….what can i use in place of ldr….i used 555 timer for switching purpose…reply

  4. Hi Sagar, Thanks for the dark detector circuit its great work. Q. I would like increase the voltage from 6V to 24V, how would this effect the circuit resistors R1,R2, and R3. which size should I select ? Many thanks Richard.

  5. hi like it. Works but have to be very close with light to turn off led is this normal .got a neat idear for this got a windmill in garden with lots of small windows and a solar light .

  6. Dear sir, i want to use this circuit to create a “smart” toy car, so when the light is off (like when passing your hand over the lrd) it starts running, is this powerful enough to ignite a dc motor like those in the toy cars?
    Or it better to use the other circuit with just 1 transistor?

  7. i already tried use the same circuit but i didnt use BC547. I change it to BDX33C, the result is the same but the light is not totally off when i expose LDR to the bright light. and LED keep lighted eventhough i test it on my desk with light open. it need to get exposed to a very bright light but it’s not totally off. is there any problem when i use BDX33C instead of BC547??? please help