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This is a Dark Detecting LED light schematic I found here

LED Light that turns on at night(dark)

Few Questions(As I'm new to electronics),

  1. Can I use this circuit with a 6v input ?
  2. When there is light on the LDR(Photo-resistor) curent will flow through R2 and PH1 (LDR) right ? So when there is light power will be wasted ? If so, how many Ma(Mili-amperes)?
  3. What to change if I want to use a 5v led(70ma @ 5v while input is 6v as stated in question No.1. It's 4Leds removed from a torchlight) ? (The resistor values)

Edit : I edited the questions after "Peter Bennett" answered.

Edit: I (Peter Bennett) edited my answer after the OP's edit.

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2 Answers 2

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  1. I expect that the supply voltage is not very critical - 6 volts should be fine.

  2. You can determine using Ohm's Law, that the maximum current through R2 will be about .05 mA.

  3. An LED advertised as "5 volt" will be a bare LED with a suitable resistor or current limiting circuit so that it can be operated from 5 volts with no additional current limiting components. (I believe the highest voltage bare LEDs are blue (or white, which is just blue with a white phosphor) which have a forward voltage of 3.0 - 3.5 volts)

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  • \$\begingroup\$ As the new edit goes on Question No.3 Can I use this to power a 5v 70ma led ? Or Do i have to change the R1 value? \$\endgroup\$
    – IshanFdo1
    Commented Sep 3, 2015 at 3:00
  • \$\begingroup\$ If you have an LED (really an LED/current limiter assembly) advertised to operate from 5 volts, you don't need R1. \$\endgroup\$
    – Peter Bennett
    Commented Sep 3, 2015 at 3:02
  • \$\begingroup\$ Thanks for the answer. My LED runs fine at 5v. I use it on my small solar panel. I used it for about 30days everynight (12hrs+ each day) its working well in 5v. Your answer solved my problems. Thank you very much :) \$\endgroup\$
    – IshanFdo1
    Commented Sep 3, 2015 at 3:03
  • \$\begingroup\$ Most small LEDs run fine at 5V for an hour or so, and then they die. if your led isn't specified as "5V" , use the resistor. \$\endgroup\$
    – Jasen Слава Україні
    Commented Sep 3, 2015 at 9:25
  • \$\begingroup\$ Another question, In the transistor data sheet it says max voltage is 60v and max current is 200ma, That means i can run 60v 200ma through this transistor ? and can i run 12v 1a through this transistor ? fairchildsemi.com/datasheets/2N/2N3904.pdf \$\endgroup\$
    – IshanFdo1
    Commented Sep 4, 2015 at 3:20
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  1. Yes, you can use it for 6V input. It already has current limiting resistors in place.

  2. Current when light is shining on LDR : 5 / (100K + LDR resistance in light), if we assume LDR resistance to be zero (just worst case, it will never be!), current flowing from that arrangement will approximately equal to ~ 5/100K = 5 uA.

  3. LEDs advertised as anything above 3V will have current limiting resistor inside them. How to ensure this ? Destructive method: Connect your, more than 3V led with suspected internal current limit, to 5-to-6V hight current supply (e.g.: Rechargeable batteries, Lead acid batteries, etc.) and wait for a minute. If your LED doesn't fry and turns off, it means it has internal current limiter resistor.

Note: Given solar panel will have limited current output, so if your solar panel is small enough you can still connect regular LED without any current limiter to them. If your solar panel is large (>0.25W or so) it will destroy the unprotected LED.

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  • \$\begingroup\$ This is not a single led. Its 4 leds. In series or parallel I dont know, But when I connect the LEds to 6v. It doesnt burn out or get hot. My solar panel is so small. it only gives 6v 300ma at full sunlight. So that wont burn my leds :) As it doesnt fry, can i Remove the R1 as "Peter Bennett" stated above ? \$\endgroup\$
    – IshanFdo1
    Commented Sep 3, 2015 at 3:53
  • \$\begingroup\$ Yes. You are correct, you can remove R1 if this is the case. Btw... 300mA is good enough to burn a small unprotected LED, but it will not burn a mammoth high wattage LED. \$\endgroup\$
    – tEddy
    Commented Sep 3, 2015 at 4:01
  • \$\begingroup\$ Yea to a single 3.2v led will burn from 300ma. but this 4led set doesnt burn :D thanks for the answer anyway. Now I have to buy the transistorm LDR and resistor to make this :) \$\endgroup\$
    – IshanFdo1
    Commented Sep 3, 2015 at 4:33
  • \$\begingroup\$ Another question, In the transistor data sheet it says max voltage is 60v and max current is 200ma, That means i can run 60v 200ma through this transistor ? and can i run 12v 1a through this transistor ? fairchildsemi.com/datasheets/2N/2N3904.pdf \$\endgroup\$
    – IshanFdo1
    Commented Sep 4, 2015 at 3:43

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