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LED FLASH/ON

Discussion in 'LEDs and Optoelectronics' started by Biazo, Oct 8, 2015.

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  1. Biazo

    Biazo

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    Oct 8, 2015
    Hiya, was wondering if anyone would be able to help, or even suggest the type of IC to use for a Light dependent resistor circuit, in which a led stays on when the resistance is low, and begins to flash when resistance is high

    thankyou
     
  2. duke37

    duke37

    5,271
    730
    Jan 9, 2011
    Welcome to the forum, what country are you in, it is nice to see the country flag and gives information about mains supplies and local suppliers.

    I suggest making an oscillator with a 555 to flash the LED, then to use a comparator to switch an extra current to the LED when the LDR resistance is low. You may need a couple of diodes to stop one output interfering with the other.
     
  3. AnalogKid

    AnalogKid

    2,329
    653
    Jun 10, 2015
    Or use a 555 as the flasher, and connect the LDR and a fixed resistor as a voltage divider with the center connected to the 555 reset. Since the 555 can drive an LED while in thee reset state, this should meet both operating criteria.

    ak
     
    duke37 likes this.
  4. Arouse1973

    Arouse1973 Adam

    5,164
    1,078
    Dec 18, 2013
    The 555 timers reset pin is not recommended to be used as an input pin. This is because of the very low and variable threshold. Another quirk of the 555 timer that has caught people out before. But that's only my opinion.
    Adam
     
  5. AnalogKid

    AnalogKid

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    Jun 10, 2015
    True, and I don't know how it will respond to a slowly-changing input. But some combination of LDR value, series resistor value, and light level will cause a state change. With so little to go on from the OP, it's seems like a less complicated starting point.

    ak
     
  6. GPG

    GPG

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    Sep 18, 2015
    OP: resistance value for change? Flash rate? abrupt change or gradual?
     
  7. Alec_t

    Alec_t

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    Jul 7, 2015
    Here's one way, using a 555.
    LDRcontrolledLEDpulser.PNG
    The blue trace represents the LDR resistance (for kV read kΩ).
     
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