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Basic dark activated circuit with LDR help.

Discussion in 'General Electronics Discussion' started by kyjosh, Aug 5, 2012.

  1. kyjosh

    kyjosh

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    Dec 15, 2009
    I'm trying to turn on an LED with a dark activated circuit, and I've got a basic circuit working with an LDR, but the LED is barely lighting up. I'm only getting about 3V out in complete darkness.

    I have a resistor and the LDR forming a voltage divider on the base of a transistor. I assume my problem is because my voltage divider is too far out of balance; but I'm not totally certain. I do know I'm getting similar results with a 2N2222, a 2N3904, and a TIP120. Right now I have a 100K resistor in with the LDR, and I've tried other values but none seem to really help this problem. I can get the circuit to switch on in more and less light, but it always seems to only put out about 3V.

    Anyone else have experience with circuits like this?
     
  2. CocaCola

    CocaCola

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    Apr 7, 2012
    You need to provided your circuit, and/or as many details as you can before we can help... For example saying you are only getting 3V out is useless information without other details because if you are using a 3V supply that is perfect, but if you are using a 12V supply it's pretty disappointing...
     
    Last edited: Aug 5, 2012
  3. kyjosh

    kyjosh

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    Dec 15, 2009
  4. CocaCola

    CocaCola

    3,635
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    Apr 7, 2012
    Where are you measuring 3V? Before the LED? After the LED? At the base of the transistor? What is the value of R1? What kind of LED, specs? What other values have you tried for R2? What is the LDR value? What is the LDR value in full light? What is the LDR value in dark?

    As I stated previous you need to give details (don't stop at what I asked, give as much as you can) we only know what you tell us, and that has been very little thus far certainly not enough to even begin to solve the problem...
     
    Last edited: Aug 5, 2012
  5. donkey

    donkey

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    Feb 26, 2011
    you also state its a 9volt supply. is that a power supply (like a walltransformer) or a 9 volt battery? either way have you checked it is still outputting 9 volts?
    as coca cola has stated the more info the better, if you send a pic of your setup that might help too.
    giving us details on all components will be a big help.
    remember we can guess, but the more info you give us the better our guesses get
     
  6. kyjosh

    kyjosh

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    Dec 15, 2009
    I tinkered with it some more today and got it. It was the fixed resistor in the voltage divider. I could have swore I played around with this in the past and settled on 100K as the perfect value for the sensitivity I wanted; but I guess not.

    I added a parallel 100K to drop the resistance to 50K and now it works perfectly. I've got it soldered together and in a box.

    What it is now that its completed is a control box I'm going to use with an LED lamp I'm making. I've got it wired through a two position switch so that I can turn it to "auto mode" or constant mode. Its running off a battery now, but I'm going to add a wall transformer to it. I've got one, but I'm using it as my bench supply right now. lol

    Thanks for trying to help though! I guess I just should have tinkered more before asking for help.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Did anyone look at the second version of that circuit located just below the one kyjosh has built? It's labeled Fig.Automatic dark sensor with variable resistor

    Any of our junior engineers care to opine on what's wrong with that circuit? Hint1:..The LDR part number is not relevant to the question. Hint2: There's a missing component.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I am deeply suspicious of any circuit that has a potentiometer connected between a supply rail and the base where the emitter is connected to the other supply rail.

    It is a recipe for destruction of the transistor
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Because it's a common emitter BJT amplifier (with no emitter resistor) it's technically not a "voltage divider". It's more accurately described as a current divider.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Steve, you're disqualified from answering that question. Junior Engineers! :p

    Chris
     
  11. kyjosh

    kyjosh

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    Dec 15, 2009
    Well I'm not a junior engineer, just a lowly tech, but I noticed that there isn't a fixed resistor between the positive supply rail and the base. At least 1K is the universal choice there right? Though I have to be honest I can't say I haven't been guilty of doing something similar when just toying with a setup on a breadboard. I did get quite the burn from a TO-92 package 2N2222 because of it once though; I hooked it up just to see if my circuit would work, got distracted by something, and by the time I got back and touched the package it was hot enough to leave a little mark on my finger. It didn't burn as bad as a shorted voltage regulator I grabbed at work one day though... that one actually left the imprint of the nut that holds it to the heatsink in my finger. lol

    I did not know. Learn something knew every day.
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  13. kyjosh

    kyjosh

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    Dec 15, 2009
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I think you need to learn a better technique for handling potentially hot items :D

    A TO-92 device that has managed to burn you is probably one you should discard (that's not just as a form of revenge either).
     
  15. kyjosh

    kyjosh

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    Dec 15, 2009
    Well that voltage regulator was a tricky one, it wasn't a dead short so it appeared to be working just fine. It wasn't even drawing a huge current load. I programmed the unit, ran through some functions, and thought everything was fine... then I grabbed it by the heatsink to pull the test cables free and yowza! Turned out to be a very tiny solder bridge only visible under a scope.
     
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