Light Dependent Resistor

Discussion in 'General Electronics Chat' started by aussiemike, Jul 19, 2011.

  1. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    Hi Guys,
    I am relatively new to electronic circuitry and at 60 years old am having a little trouble getting my head around some stuff.
    I started this hobby with help from a guy in Ireland called Ron jay.
    I built a few of his circuits and he has helped me enormously. I feel though I am imposing on his time and decided to come here to learn as much as I can.
    One circuit I built was a custom circuit I asked him to design for a special need.
    It can be seen here
    dianibeach.com/ldr.htm
    The idea is to get the light from the screen of a mobile phone ( Inside a dark box )to activate the LDR. Turns out the light from the phone is not strong enough to trigger it.
    Any tweeks our added circuitry I might be able to add to make this functional? In simple English please as not only am I getting old I am also an Aussie.
    Regards from Kenya
    Aussiemike
     
  2. monster_catfish

    Active Member

    Mar 17, 2011
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    This is of no help at all Mike, but I haven't heard an Aussie joke in my entire life, though I did chuckle at the "confession" in last line of your post.
     
  3. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    Glad I brightened your day up Mr. Catfish.
    No offence. I am a well balanced Aussie.
    I got a chip on both shoulders.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Did you connect the pins 7 and 14 of the 4001?
    Pin 7 should de to the - of the battery.
    Pin 14 should be to the + of the battery.

    [​IMG]

    Bertus
     
  5. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    Yes I have. The arrows at top and bottom of the diagram indicate that.
    The circuit works fine. If you short the LDR the siren goes off.
    Just LDR is not working well enough to trigger the system with low light level.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What value are you using for R1?
    If the value is to low the LDR will need more light to trigger.

    Bertus
     
  7. aussiemike

    Thread Starter New Member

    Jul 19, 2011
    13
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    It is 1 M.
    Do I need more?
    Sorry M8, new to this stuff.
     
  8. Metallica_Boy

    Member

    Nov 8, 2006
    10
    1
    Using a transistor switch with sensors

    The top circuit diagram shows an LDR (light sensor) connected so that the LED lights when the LDR is in darkness. The variable resistor adjusts the brightness at which the transistor switches on and off. Any general purpose low power transistor can be used in this circuit.
    The 10k fixed resistor protects the transistor from excessive base current (which will destroy it) when the variable resistor is reduced to zero. To make this circuit switch at a suitable brightness you may need to experiment with different values for the fixed resistor, but it must not be less than 1k.

    If the transistor is switching a load with a coil, such as a motor or relay, remember to add a protection diode across the load.

    The switching action can be inverted, so the LED lights when the LDR is brightly lit, by swapping the LDR and variable resistor. In this case the fixed resistor can be omitted because the LDR resistance cannot be reduced to zero.

    Note that the switching action of this circuit is not particularly good because there will be an intermediate brightness when the transistor will be partly on (not saturated). In this state the transistor is in danger of overheating unless it is switching a small current. There is no problem with the small LED current, but the larger current for a lamp, motor or relay is likely to cause overheating.

    Other sensors, such as a thermistor, can be used with this circuit, but they may require a different variable resistor. You can calculate an approximate value for the variable resistor (Rv) by using a multimeter to find the minimum and maximum values of the sensor's resistance (Rmin and Rmax):

    Variable resistor, Rv = square root of (Rmin × Rmax)

    For example an LDR: Rmin = 100, Rmax = 1M, so Rv = square root of (100 × 1M) = 10k.

    You can make a much better switching circuit with sensors connected to a suitable IC (chip). The switching action will be much sharper with no partly on state.
    [​IMG]
    LED lights when the LDR is dark


    [​IMG]

    LED lights when the LDR is bright
     
    Last edited: Jul 19, 2011
    Sparky49 likes this.
  9. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    I really thank you for the time you have taken to reply to my questions.
    What I am really looking for is a circuit that is activated by the light coming in from the screen of a mobile phone. The circuit then activates a relay that sounds a siren for maybe 5 seconds.

    Why do I want this?
    I live in Kenya and security is sometimes a problem.
    We all have Askaris ( Night Watchmen )
    What I need to do is make a phone call from my car before I get to my house late at night. This phone call will activate the light on the phone I call and that in turn will sound a siren for 5 seconds to alert the Askari that I am on the way and he should be at the gate to let me in.
    You might say why don't I buy the Askari a phone and just call him.
    I did.
    He lost the first one. Broke the second one, and his brother stole the third one.
    A siren in a locked box at the top of a tree all encased in razor wire is really the only way to go.
    Anyone with a circuit out there that matches these requirements?

    Regards from Kenya
    Aussiemike
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    While that would work, there are simpler ways. Bet I can find a simple transmitter/reciever combo with google with no fuss. Think remote control. No phone bills, much simpler over all. Even a wireless intercom would work well in this application, or a cheap walkie talk (like FMS). You can buy FMS base stations, as well as incredibly cheap hand held walkie talkies.
     
  11. aussiemike

    Thread Starter New Member

    Jul 19, 2011
    13
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    A mobile phone is the ultimate remote control and it is free because no one answers the phone when I call the circuit. I also think you may have missed the bit about lost, broken, stolen. I also use a mobile to turn my hot water system on when I want a shower. A "GSM Auto" But that is inside my house and safe.
    The ultimate, but that system cost $200
     
  12. Sparky49

    Active Member

    Jul 16, 2011
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    Bolt a walkie talkie to desk for him.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Or the base station, some are meant to be fastened down.

    A phone with a plan is over $100. The walkie talkies are less than $30, and can be found much cheaper. The remote can sidestep the guard all the way, and is more secure as far as the channel goes.

    Obviously you were intending to fasten the phone down, so what is the difference (other than the phone costing continuously for the plan and being basically more expensive).

    No big deal though, it was just a suggestion.
     
  14. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you want to try to get your LDR arrangement working, there are some things you could try, if you have access to a meter. The first would be to measure the resistance of the LDR alone, both in the dark and when lit by the phone. Unless the resistance changes noticeably, you would need to get a more sensitive device.

    The next thing to check is the load resistance. This needs to be quite big, perhaps just a few times the LDR dark resistance, so thar the input voltage to your chip will fall through the switching threshold when the phone lights up. Are you sure the pot is really providing 1MΩ resistance?(Perhaps this is not enough:as the gate is CMOS higher resistance could be tried, but I'm not sure if that would be good in an African climate.)
     
  15. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    Cunning plan but flawed.
    You can get phones here for $10. Calls are only 1 cent a minute.
    On top of that I have 25 acres on the sea. Not many exposed electrics survive the salt. Plus the guard has to patrol. The idea with the siren is that he can hear it from almost anywhere. And if I wrap the whole lot up in a silicone sealed old video cassette box covered with an Aussie sized condom even the salt don't get it.
    Walkie Talkies run on batteries. Expensive items here.
    Appreciate the imput though!
    Thanks
    Mike
     
  16. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You might also want to think about whether your money might be better invested in trying to hire more trustworthy staff.
     
  17. aussiemike

    Thread Starter New Member

    Jul 19, 2011
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    You have obviously never lived in Africa.
    Lovely people but thieving is a sport cum religion down here.
    Starting right from the very top of the political elite.
     
  18. Sparky49

    Active Member

    Jul 16, 2011
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    Perhaps you should be asking for help for a battery charging circuit instead.;)
     
  19. monster_catfish

    Active Member

    Mar 17, 2011
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    Mike, I'd be way out of my depth trying to analyse the tech issue you have described, but if you have not already done so, may I suggest that you invest in a PACK of large German Shepherds for your property.

    I reside in Africa for much of the year, and home invasion robberies are a major concern. African bandits have far greater fear of dogs than they do of armed human guards, and would think twice about laying siege on any property that erupts with a blood-curdling cacophony of baying and howling at the slightest twig break during the night.

    The element of surprise would be lost for any intending invader confronted by such a herd watching over your spread, AND, as those banditos flee the scene never to return, they will assume those rampaging hounds are trained to kill, even if that is not the case. Naturally, a 12 guage pump-action "staff of office", with enough buckshot for an old-fashioned Mexican standoff, would be a good supplement to any security setup you have.

    Apologies for the slight diversion from the electronics theme of this forum.

    This bud's for you.
     
    Last edited: Jul 19, 2011
  20. aussiemike

    Thread Starter New Member

    Jul 19, 2011
    13
    0
    Got a German Shepherd. Her name is Sabine and she is 5'8" and blonde. Put her to work 15 years ago. :)
    Problem with dogs is we have so much wildlife on the plot that the dogs would scare them all away.
    Back to electronics I guess.
    Where in Africa?
     
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