Using LDR to switch off 555

Discussion in 'The Projects Forum' started by JohnRo, Nov 16, 2015.

  1. JohnRo

    Thread Starter New Member

    Nov 16, 2015
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    My project is to build a solar powered beacon. Basic target functionality is a beacon using a 1W LED, blinking with a 25% duty cycle, 1 sec on and 3 sec off. It should be on during night and off during day. This should be powered by 4 AA rechargeable batteries and the batteries should be charged by a solar panel during daytime. All this will be placed where I will not be able to reach it easily so it should operate autonomously without maintenance. And it needs to be low power since the charging during daytime will have to cover the operations during night time.

    I have started with building the circuit on a breadboard to check if everything works. Since I only have basic circuit building skills I have searched the Internet for examples to start with and current setup is based on instructions from amandaghassaei on instructables.com (555 timer article) for the "blink" circuit and dan also on instructables.com (High Power LED Driver Circuits article) for the circuitry needed for driving the 1W LED.

    Then I have added a LDR in series with a 1M resistor to draw the RESET input to GND when the sun is up during daytime. The solar pannel and the circuitry for charging the batteries is not yet included.

    Circuit.jpg

    This works fine during nighttime and daytime. The LED blinks with the frequency I want and it is shut down nicely in dark.

    But the problem is in dusk and twilight. When the light goes gradually from dark to light the resistance of the LDR will decrease which results in a decrease of the voltage over the LDR and the RESET input. When the voltage is above 0.75V the 555 operates as expected and the LED blinks. When the voltage is below 0,55V the 555 is shut down by the RESET input and the output (pin 3) is kept low which results in LED turned off.

    But when the voltage is between that, (0,56-0,74V) the result is unexpected to me. The output from the 555 is then kept constantly high wish results is that the LED is turned on constantly. Apparently my simple solution with the 1M resistor in series with the LDR is not good enough. I need something that moves more quickly through the interval 0,55-0,75 V.

    So my questions is, can anyone suggest an alternative solution that will bring the voltage down at the RESET input directly at a specific light level? I assume that I should keep the setup with the LDR in series with the resistor but between that and the RESET input I probably need some kind of amplifying circuitry that I am not skilled enough to figure out. And even better would be if there were some hysteresis built into this so I don't get the problem of frequent turing on and off the 555 then the light is exactly on the threshold.

    Hope this questions are not to simple for your guys. :)
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Whats probably happening is the battery voltage is fluctuating at the trigger level,
    so I would add a second 555 timer as a dusk/ dawn sensor like this...
    set the 2 Meg preset for dark on this will give you a snap action trigger


    555.png
     
    Last edited: Nov 16, 2015
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    How long are AA batteries going to power a 1W LED, even at a 25% duty cycle?
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    A 1W LED generally takes 350 mA but run at 250 or less. So, for 1500 mAh battery, about 12 -15 hours.
     
    Last edited: Nov 16, 2015
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    25% duty-cycle seems high for a beacon. Can you use a lower values to extend battery life?
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    I'd go for a comparator with hysteresis and feed that to the reset pin.

    Cadmium sulphide LDRs are getting harder to come by because cadmium is a hazardous material. I have a hand trace of an outdoor lighting unit that turns off in daylight, it uses a common IR photo diode - an LM358 is used as a high input impedance voltage follower and the Schmitt trigger (comparator with hysteresis).

    When I post the hand trace, it gets taken down because it includes a transformerless supply - I've downloaded a schematic editor to re draw it, but its a pain to use, so its a "when I can be bothered".
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Just expand the real met area (with the power supply off-screen) and do a screen capture (ALT+screen print), then paste into the post.
     
  8. JohnRo

    Thread Starter New Member

    Nov 16, 2015
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    Thanks Dodgydave!
    I have a extra 555. I will try your proposal.
     
  9. JohnRo

    Thread Starter New Member

    Nov 16, 2015
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    As MikeML said, maximum current for the 1W LED is 350 mA. But I am using less than that and still getting good enough light. According to my measurements the circuit is drawing 2,2 mA in off state and 124 mA in on state, so the average is 32 mA in active state. With my 2300 mAh batteries this would mean 72 hours operation. At least in theory.
     
  10. JohnRo

    Thread Starter New Member

    Nov 16, 2015
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    I have tried to search the net to learn typical duty cycles for naval beacons but I have not found anything. Do you know? 25% duty cycle (1 + 3 sec) "feels" right to me but I might be mistaking. According to my calculations a fully charged battery would keep it blinking up to 70 hours so that would do for my application. More important is if the solar cell will be able to charge it enough during daytime. Where I live in Sweden daytime is 6-7 hour right now and we don't have clear sky and sun that often.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    Its that elusive "round tuit".

    Probably easy to open it in Paint and block over the offending bits.
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Lets see what happens with this:-
     
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    That version looks like you may have worked for "the agency" at some point in your past.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Its an evil plot to hide wattless droppers from the masses.
     
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