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Bright flashing light for fake alarm box?

Discussion in 'Electronic Basics' started by [email protected], Dec 7, 2004.

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  1. Guest


    I bought a fake/empty alarm box for my house, but thought I'd make it
    look more realistic by putting some kind of periodic flashing bulb in
    the unit.

    I made up an astable on stripboard with uneven RC sides, so the thing
    flashes very briefly every 1-2 seconds. This works fine, but I used an
    LED, which isn't bright enough to be seen through the blue transparent
    panel of the alarm box, from any distance at all.

    The cct is driven by a 9v battery. Can anybody suggest something I can
    use for a bright periodic flash?

  2. There are several approaches I'd try:

    1) The obvious one - substitute a 10mm 'superbright' LED with your
    existing circuit

    2) Ditto, but with improved circuit using higher voltage, either from
    2 x 9V batteries, or mains-derived supply.

    3) Ditto, but use substantial capacitor and new circuit to discharge
    it quickly through LED

    4) Ditto, but experiment with normal filament bulb (e.g, car headlamp

    5) Use your existing or slightly modified circuit to activate a relay,
    and connect suitable mains bulb via relay contacts to domestic supply.

    6) Buy a 12V strobe unit as used in non-fake alarm boxes

    7) Make your own strobe unit from a Xenon flash tube (if you know what
    you are doing and are comfortable with potentially lethal voltages).
  3. Guest

    Thanks for the suggestions Terry.

    I didn't know you could get super bright led's. I'll try them first.
    Can you get white/clear ones? I can only see red and green at 3000mcd
    at the moment?

  4. JeB

    JeB Guest

    You might move the led to the surface of the box.

    and I think you can find a led flasher chip that runs a long time off a
    single 1.5v battery.
  5. They make blue ones that bright and white ones even brighter. Try
    Hosfelt Electronics for "hobbyist" quantities. You can get prime grade
    ones from Nichia in fairly small quantities also - see their website

    - Don Klipstein (,
  6. Bill Bowden

    Bill Bowden Guest

    lsdiodes has 12,000 mcd white LEDs for 90 cents each at:
    The price is right, but they are temporally out of stock.

  7. Rich Grise

    Rich Grise Guest

    I can't imagine how the OP thinks that anything but a jellybean red LED in
    a grommet through the skin of the box would look like any kind of "real

    Unless you're a casino, with those black domes in the ceiling, security
    cams have a red LED, even the real ones.

    Good Luck!
  8. peterken

    peterken Guest

    there are several types and colors of led having the multivib incorporated
    beware of the front panel, since it might act as a filter thus actually
    "filtering" the red light out
    I'd consider bringing the led out of the box by mounting it through the
  9. Guest

    I thougth about the filtering of the blue front panel, so I decided to
    find blue LED's I bought some clear/blue 10mm 14,000mcd bombs that I'm
    hoping to try out this weekend.
  10. Guest


    I made up the cct with blue led, and fitted the box, all running from a
    PP3 9v battery.

    Worked like a charm....for 3 days, then I guess the battery ran out!

    Any megal-long running 9v batteries? or is it back to the drawing

  11. Guest

    How long do you need it to run between replacements?

    Two 9V batteries in parallel will last twice as long (using 1/2 the
    current per battery).

    N batteries in parallel will last N times longer.

    A 9V DC converter will last as long as there is power in your
  12. Rich Grise

    Rich Grise Guest

    Use 6V worth of alkalines or Ni-Cds or NiMH, cut the duty cycle way down,
    and run the LED with as little current as you can and still see it.

    Good Luck!
  13. How about this circuit? It should keep you going for a while:

    | | | | |
    | | V LED | .-.
    | | - 3.4V | | | Rt
    | | | | | |
    | | | __ | Rd '-'
    Vb --- | | Ro .------o| |o-' ___ |
    - | | ___ | .----o|U1|o----|___|--o
    | | '--|___|--|-|--- o| |o-----------o
    | '------------|-|----o|__|o--. |
    | | | | |
    | | '------------|--------o
    | | | |
    | | | |
    | | Cc --- --- Ct
    | | --- ---
    | | | |

    U1 TS555CN (Make sure you use a CMOS part like this)

    Vb 9 volts
    Rt 470k ohms
    Rd 1k ohms
    Ro 1k ohms
    Cc 0.1uF
    Ct 10uF

    Time between flashes = 3.26s
    Quiescent current @ 9V = 200uA
    Current required for charging Ct = 12uA
    Current required for flash = 9uA (Averaged over period)
    Total power budget = 221uA * 9V = 2mW

    For a typical 9V, you could have 1000 or more service hours.

    (created by AACircuit v1.28 beta 10/06/04

    You can adjust the length of time of the flash by adjusting the value of
    Rd. You can adjust the frequency of flashes by changing Rt or Ct.

    For a brighter flash, use a smaller value resistor for Ro. That will
    marginally affect the life. Use a high-brightness LED for best results.

    Using a low-leakage capacitor for Ct is probably important for the
    timing, but it'll be hard to get a 10uF cap that is low leakage. Avoid
    electrolytic caps if possible.

    When I built this and measured it, it was using more like 150uA
    quiescent. The datasheet said 200uA, though. The timing (using an
    electrolytic 10uF, 16V) was pretty accurate.

    I used the datasheet from STMicroelectroncs for the TS555CN for this
    design. I ignored discharge pin leakage, which is claimed to be under 100nA.

    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.
  14. Make sure the battery is alkaline...

    Reduce the duty cycle if possible.

    If possible, increase the value of any resistor that controls the LED
    current. Most blue LEDs are nonliear and have efficiency increasing as
    current is changed towards just a few milliamps - most blue LEDs are
    noticeably much more than 20% as bright at 4 mA as at 20 mA.

    See if the flashing circuit draws much current - I know how to make a
    flashing circuit that draws a fraction of a milliamp.

    Use an LMC555 and a section of a 4049 inverter to drive an LED with 4 mA
    for a low duty cycle (like .2 second on, .8 second off). Total current
    draw should be about 1.5 milliamps, which a 9V alkaline battery should be
    able to provide for about 400 hours or about 2 weeks. Other circuits,
    such as ones using a "low power" op amp as an oscillator, can improve upon
    this somewhat. You may get adequate light with just 2 mA or even less of
    LED current - it may be possible to get the average current draw down to
    about 1/4 of a milliamp and then battery life of about 2-3 months is

    - Don Klipstein ()
  15. I think you left out the pin numbers so I put Pin One in.

    10 uF Electrolytics work okay in this circuit, and so do 10 uF
    tantalums. Using a 10 uF plasti cap is going to be really expensive and
    it will be huge.

    I've built simpler flasher circuits using two transistors and they draw
    low current if you set them for low duty cycle, like yours below. See
  16. Better yet, use a two or three AA cell holder, and run it off 3 or
    4.5VDC. The AA cells should last 6 months or more. Here's a good
    Those are 2N2222 and 2N2904 transistors. Blue LED won't work with 3V,
    you have to have 4.5V. This should last more than a year with two AA
    cells. I built one of these and I used a pair of AA cells from a
    flashlight that were 'dead', and it ran for over 6 months.
  17. Guest

    Wow, there's a lot of responses here! Thanks. I'll have to take the
    time to go through all of them and give them due consideration.

    I realised today that when tuning the cct on a breadboard, I varied the
    capacitor values to arrive at what I wanted. I really should have made
    the capacitor values as small as possible, and varied the R values.

    Using an array of 2400MaH NiMh rechargables may have helped too.
  18. Remember that rechargeable cells lose a certain percentage of their
    charge just sitting there, without any load. So you have to recharge
    them every few months just to keep them from going flat.
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