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Dusk to Dawn Flashing LED

Discussion in 'Electronic Basics' started by George Dewar, Jan 5, 2005.

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  1. George Dewar

    George Dewar Guest

    I would like to have somebody knowledgeable in electronics construct a device
    that would turn on at dusk and flash until dawn. I want to power it with AA
    batteries so I assume the best type of light would be a red LED.

    Possibly such a device might be commercially available.
     
  2. Bob Myers

    Bob Myers Guest

    What's the application? How far away (and in what direction, or
    rather in how many directions) does it need to be visible? How long
    do you need it to run unattended (i.e., before you could change the
    batteries)? What environment? Is there no possibility of AC power
    at all?

    Bob M.
     
  3. JeffM

    JeffM Guest

    a device that would turn on at dusk and flash until dawn.
    Another weenie with a half-formed idea.

    |How far away (and in what direction, or rather in how many directions)
    |does it need to be visible?
    |How long do you need it to run...before you...change the batteries ?
    |What's the application?
    | Bob Myers
    |
    ....and what is the duration of the "flash"? How often will it flash?
    Duty cycle will be important for battery life.
     
  4. 4.5V (3x AA battery)
    ---------o----o----------o-------------------o--------.
    | | | | |
    | | | .-. V 1N4148
    | | | 470k | | -
    .-. | | | | |
    220k | |<--' | '-' |
    pot | | | | V 1N4148
    '-' | | -
    | | >| |
    | | 2N3906 |------o
    | CMOS | /| |
    | 555 | | |
    | __ | | |
    .------|--------1| |o-' RED LED | |
    | | .----o| |o-------------|<------o |
    | | | -o| |o---------------------o |
    | o--------o|__|o------. | |
    | | | | | |
    | | | | | |
    | | '----------------|--------------o |
    | | | | |
    | .-----. | | |
    | | LDR | | | .-.
    | | | --- 1uF --- 1uF | | 100k
    | '-----' --- --- | |
    | | | | '-'
    | | | | |
    -----o------o--------------------o--------------o--------'
    Ground

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    Buy an LDR (a light sensitive resistor) from radio shack or wherever,
    and measure it at the darkness you want the thing to turn on at. Then,
    buy a pot that is about twice that value of resistance. Build the
    circuit, and adjust the pot so the 555 turns on when it gets dark.

    The larger the resistance of the LDR, the better, from a battery
    perspective. You can also use a 9V battery for this circuit, which might
    be more convienient, but won't last as long.

    It will flash every second or so. You can adjust the flash rate by
    changing the 470k resistor. Using a 9V battery may increase the time
    between flashes.

    YMMV. I haven't constructed it.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. George Dewar

    George Dewar Guest

    Have a look at <www.geocaching.com>. The LED will mark a cache meant to be found at night probaly
    in a wooded area. The light should be visible for 180 degrees at about 50 feet and run for a
    reasonable length of time on battery power.

    Thanks to those who have responded.
     
  6. There are a couple of changes to my circuit for those requirements.
    First, the use of reset is problematic, because it allows leakage when
    it's light, decreasing battery life. Here is a circuit that will supply
    9V when it's dark:

    P-MOSFET like
    IRF7104
    9V battery
    ---o-----o--------+^+----- To 555 circuit Vcc
    | | |||
    | | ===
    | | |
    | | |
    .-. .-. |
    100k | | | |100k |
    | | | | |
    '-' '-' |
    | | |
    | '------- o
    | |
    | |/
    o------------|
    | |>
    | |
    | |
    .-----. |
    | LDR | |
    | 10k | |
    | Max | |
    '-----' |
    | |
    | |
    | |
    | |
    ---o--------------'
    GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    When the LDR changes from 1k during the day to 10k at night, the NPN
    transistor will be biased into operation, which will turn on the
    P-MOSFET pass transistor. The leakage during the day is probably less
    than 100uA.

    As to the 50' radius thing, you are going to have to blast away with an
    LED to get this kind of distance. Thus, the little scheme I suggested
    before won't work, because there won't be enough current to power the
    LED at a high enough output. However, by simply replacing the LED in the
    prior diagram with a 10k variable resistor, you can use the output to
    switch on a PNP transistor at the 9V rail, which will give you a switch
    at the 9V rail. Put some of those bright white LEDs between the
    collector and ground, and you can flash pretty brightly. You may need to
    limit the current, which can be done by varying the resistance between
    the output and the base of the PNP transistor using P1. You can also
    limit it using a resistor in series with the LEDs.

    P2 controls how long the flash lasts. This circuit will fry most LEDs if
    the current through them is set too high by P1.


    From power cutoff circuit above
    -o-------|----------------o-------------------o--------.
    | | | | |
    |---------o----. | 470k | | -
    /| | | | | | | |
    | | | | | '-' |
    | | .-. | | | V 1N4148
    | | | |<--' | | -
    | | | | P1 47k | >| |
    V-> | '-' | 2N3906 |------o
    - | | CMOS | /| |
    | | | 555 | P2 .---. | |
    | | | __ | 10k | | | |
    | | .-|-----1| |o-' _V_ | | |
    | | | | .---o| |o----------|___|-o----o |
    | | | '-----o| |o---------------------o |
    | |---------o|__|o------. | |
    V-> | | | | |
    - | | | | |
    | | '---------------|--------------o |
    | | | | |
    | | | | |
    | | | | .-.
    | | --- 1uF --- 1uF | | 100k
    | | --- --- | |
    | | | | '-'
    | | | | |
    -o---------o-------------------o--------------o--------' GND

    P1 controls brightness
    P2 controls length of flash
    Use 2 bright white LEDs, that can handle at least 200mA for a bit

    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  7. JeB

    JeB Guest

    there's a chip (LM3909)that will flash an LED for a long time using a
    single 1.5v cell. the 180 degree issue might mean using several of them
    depending on the coverage of the LED you pick. There are inexpensive
    ready made flashers for cyclists/hikers but they don't have the on at
    dark/off at dawn option as far as i know.
     
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