LDR working the opposite way

Discussion in 'General Electronics Chat' started by zazas321, Dec 8, 2015.

  1. zazas321

    Thread Starter Member

    Nov 29, 2015
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    So im just testing LDR. I came up with the simplest circuit i could find. I tryed to make it but LED seems to be working the opposite way.. It lights when there are some light in my room, but when i turn off the light, the led also goes off
     
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  2. dl324

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    Mar 30, 2015
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    Swap the position of the LDR and pot.

    Don't bother posting pictures of your breadboard; they're essentially useless because there isn't enough detail. Assuming someone would be inclined to trace it out.

    Post your schematic right side up to make it easier to read, and draw the transistor symbol correctly.

    Don't bother posting videos of how LDRs work or how to use them. I don't watch those videos because most are a complete waste of time.
     
  3. ISB123

    Well-Known Member

    May 21, 2014
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  4. zazas321

    Thread Starter Member

    Nov 29, 2015
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    I can draw it that way
     
  5. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Oh thanks ISB. I will try and figure out how and why both of theese circuits works :eek:
     
  6. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Yooo.. i have built my dark sensor but strange thing happens when i wire some leds in parallel. One LED worked fine to dark sensor but when i add few more LEDs to the circuit some of them lights up , some of them wont light up at all and others light less brighter than other ones.. Why such a mess happens? ;/
     
  7. ISB123

    Well-Known Member

    May 21, 2014
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    Your battery is too weak and not all diodes have the same characteristics.
     
    Last edited: Dec 8, 2015
  8. dl324

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    Mar 30, 2015
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    Post a schematic.

    You shouldn't operate LEDs in parallel unless each has it's own current limiting resistor. If you actually placed several LEDs in parallel using the same current limit resistor, then some are hogging current.
     
  9. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Hm... I have just tryed to put for example 5 same green LEDS in parallel and they light up all nicely, but when i change one green light to a red one and i have 1 red and 4 green, the one red glows brightly and the other 4 green LEDs glows very very bad:eek: Is this because their voltage differs?
     
  10. dl324

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    I meant something like this:
    upload_2015-12-8_8-34-47.png

    Not this:
    upload_2015-12-8_8-40-22.png
     
    Last edited: Dec 8, 2015
  11. ISB123

    Well-Known Member

    May 21, 2014
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    Yes,they need separate resistor.
     
  12. dl324

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    Mar 30, 2015
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    Even with the same color LED, you can get different brightnesses due to current hogging. LEDs of different colors will likely have different forward voltages, which will also result in current hogging.

    The correct way to do this is to have each LED, or chain of LEDs, have a current limiting resistor that can also be used to match brightness.
     
  13. dl324

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    This is the correct way to connect them:
    upload_2015-12-8_8-43-46.png
     
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  14. AnalogKid

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    Aug 1, 2013
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    Yes. In very round numbers, small red LEDs have a forward voltage (Vf) of around 1.8 V, while similar green LEDs come in at around 2.1 V. When connected directly in parallel, the RED LED comes on and holds the voltage across the green LEDs at a level that is too low for full brightness.

    ak
     
  15. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Okayy! sooo to sum up everything! If i understood good , for example if i add 50 LEDs to the parralel they will all have 4,5V and to light up brightly i will need to calculate the right current for them, in this case 20mA * 50 will be 1A. So i have AC to DC adapter 4,5V which can give 1A so thats exactly what i need? :) And if i want to add different colours LEDs in parralel circuit i will need to add resistors to all of them? :eek:
     
  16. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Hmmm i thought in this case the current matters:eek: I mean if i power my circuit with 4,5V battery so every LED in parallel will get 4,5Volts and with this voltage is more than enough to light any colour LED brightly.
     
  17. dl324

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    Voltage and current matter:
    hiEffRedIV.jpg
     
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  18. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Hmm i didint quite get it:eek: This chart means that if i have for example 2.1 forward voltage LED it will need about 30mA to light brightly?and if i have 1,7V LED it iwll need like a 3mA?
     
  19. dl324

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    LEDs are typically operated at 20mA or less unless they're multiplexed.

    If you assume a 2V drop for the LED, 680 ohms and 4.5V would give you about 4mA (3.7mA from I=2.5V/680 ohms). To get the actual current, measure the voltage and use Ohm's Law.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    The chart shows the voltage versus current for that particular LED.
    Normally you pick a current and then use the LED voltage at that current to calculate the proper series resistor needed to get a resistor voltage drop for that current with the supply voltage you have.

    You apparently do not fully understand series and parallel circuits.

    Two diodes in parallel will share the same voltage, so the LED with the smaller voltage will not allow the LED with the higher voltage to glow brightly since it has only the smaller voltage across it.

    Two LEDs in series will share the same current, but will have a voltage drop across each of them, as shown from their data sheets at that current (which will be different for different LEDs).
    The total voltage drop is then the sum of those two LED voltages.
     
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