Fake Car Alarm Flashing LED - Help Needed

Discussion in 'The Projects Forum' started by evan.f_nz, Apr 5, 2009.

  1. evan.f_nz

    evan.f_nz Thread Starter Member

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    Hi everyone, I'm new here and pretty new to electronics. I know the basics but thats about it so bear with me.

    I recently installed a flashing LED into the dash of my girlfriends car because we cannot afford a proper alarm system. I went out and got some flashing leds and some resistors and put one in. I hooked the positive lead to constant 12v and the negative to the acc so when the car switches on the led switches off and vice versa. While it worked really well the led flashed way too fast to look remotely like an alarm led.

    So I dug around on the net and found a scematic for a simple flashing led circuit. I bought the parts and put it all together, tested it on an old pc power supply, it worked. I then went and installed it in the car the same way I did the other one. This time it didn't work, it was feeding the acc 12v when the car was off and the led didn't flash.

    Here is a diagram of the circuit i used, only difference being the value of the resistors and capacitor:
    [​IMG]

    Bottom line is, is there anyway I can hook this up so it turns off when the car is switched to acc or on, without powering up the acc line (obviously useless)? I thought maybe a diode or transistor or the like might work but i've no idea on how to wire it up.

    Any help would be much appreciated.
    Last edited: Apr 5, 2009
  2. awdman

    awdman New Member

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    Try and use a relay that breaks the ground when the acc switch is on, (energized) and when the power is turned of it de-energizes the relay closing the contact for ground and activating your 555 timer.
  3. SgtWookie

    SgtWookie Expert

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    Better post the actual schematic you used for the flasher.

    If you used low-value resistors, you may be draining the battery rather quickly.

    Like awdman suggested, a relay could be used to disconnect power or ground from your circuit when the ACC power came on, but you should use a diode across the relay's coil to suppress the reverse-EMF spike when the power is turned off.

    Another option might be to use a PNP transistor like a 2N2907 or BC327 to source power to the 555 and LED, with a weak pull-down (10k Ohms) to the ACC line.
  4. thatoneguy

    thatoneguy AAC Fanatic!

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    This started as a 555 oscillator with a 95% duty cycle at 2Hz. The output of the 555 is high except for a 10 ms every 2 seconds.

    Throw in a transistor to invert the output and give an option of > 200mA drive (not needed here), and the LED gives a quick blink every 2 seconds, which looks like a car alarm. It will automatically run like a blinking LED when power is applied.

    The 200 Ohm resistor limits base current, and R4 limits LED current, Change R4 to match your LED.
    [​IMG]

    Attached Files:

  5. SgtWookie

    SgtWookie Expert

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    Thatoneguy;
    Not a good design, as the current drain when Q1 is conducting would be 50mA or more.

    [eta]
    Here's an improved design:
    [​IMG]

    Approximately 2 flashes per second, pulses about 20mS long.
    Peak current is around 21mA when LED is lit, about 5.44mA when not lit.
    Current draw would decrease if a CMOS 555 were used instead.

    When the ACC switch is off, accessories provide a path to ground for the base of Q1 via R4, a 10k resistor. A nominal 1.2mA current flows through R4 when the ACC circuit is off. This turns on Q1. When the ACC circuit becomes energized, the base of Q1 is pulled high, turning it off, thus disconnecting power to the flasher circuit.

    C3 is a minimal-size bypass capacitor; 10uF to 100uF would be better.
    R1, R2 and C2 were sized to get an approximate 0.5Hz frequency output with around 20mS low; or about 1/25 (4%) duty cycle. Low current consumption in these kinds of things are a necessity.

    Attached Files:

    Last edited: Apr 5, 2009
  6. Mike2545

    Mike2545 Active Member

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  7. thatoneguy

    thatoneguy AAC Fanatic!

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    Good point. That's what I get for trying to make it in a hurry...

    Haven't done much with 555, unsure how to get a 5% duty cycle.

    --ETA: This one might work, changing the value of the resistor to charge the LED to slow down the blink rate:

    http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html
    Last edited: Apr 5, 2009
  8. Mike2545

    Mike2545 Active Member

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    If you use the blinking led like I suggested, it will be cheaper, so when you do get your car stolen, you won't be out as much cash.
  9. SgtWookie

    SgtWookie Expert

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    I edited my prior post to add a schematic and notes about an improved design.

    The Radio Shack blinking LED provides no information about it's current use. I would want to test it for average current consumption before installing it.

    It would remain flashing when the car was running unless a scheme like Q1/R4 in my 1st reply were implemented.

    [eta] I don't know for certain, but I have doubts about whether Radio Shack has stores in NZ.
    Last edited: Apr 5, 2009
  10. Mike2545

    Mike2545 Active Member

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    What makes you think he is from NZ? His name is 'evan.f_nz'

    However they do have a whopping 55Ma draw. I have one here in the blister-pack, albeit some 10 years old...
  11. thatoneguy

    thatoneguy AAC Fanatic!

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    The problem I had with the anode to Vcc and cathode to out, was that the LED stayed on continually due to the V_out of "OUT" not reaching the rail, with input of 12V, and Vf of 1.6V.
  12. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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  13. SgtWookie

    SgtWookie Expert

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    He may not be. He may actually be a "she". However, unless Evan's last name is actually "Fonz", I believe it's likely that they intended the trailing "_nz" to be interpreted that they are from NZ.

    Doesn't surprise me one bit. A continual draw of 55mA would probably leave the battery dead within a couple of weeks. At the least, it would cause the battery to have a shorter life due to being discharged.
  14. Mike2545

    Mike2545 Active Member

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    I never said continual draw. You assume too much. The 55mA is ON-State.
  15. SgtWookie

    SgtWookie Expert

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    That's why I qualified the statement with "continual".
    Do you know what the off-state draw is, or the average current draw over time?
  16. Mike2545

    Mike2545 Active Member

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    The package does not specify but I could run some tests, if the OP was interested in using one for his/her project, and lives in an are where the Shack is located.
  17. evan.f_nz

    evan.f_nz Thread Starter Member

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    thanks for the input guys, wasn't expecting this many replies so quick. Yes I am a "he" from NZ lol and no we don't have radio shack here but there are places like it.

    The scematic I first posted is what I used but with a 470uf capacitor, a 470ohm resistor replacing the 110 and a 2.4k replacing one of the 1k resistors (I forget which one now). It flashes almost identically to the led in my mates car that was part of an alarm system.

    I wouldn't think it would be pulling enough current to drain the battery even with a duty cycle of near 50%, when its on its only pulling 20ma.

    So all I need is a transistor and some rewiring to sort it?
  18. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Did you look at the design I showed? No transistor needed, and minimum parts. I figure the battery would last for years off it.
  19. Oxbo Rene

    Oxbo Rene Active Member

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    I have one of those in my truck that I bought in Walgreens years ago.
    It is a mfg'd plastic device designed to fit into cigarette lighter, and a little red LED on the end just flashes away at 1 sec.
    It does stay on when engine is off, but that's the whole deal about it, it sits there throughout the night blinking away, blinks constantly all the time.
    Has never ran my battery down, of course I drive the vehicle pretty much every day, etc.......
  20. SgtWookie

    SgtWookie Expert

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    OK. In that case, you're running at about 1.5Hz, duty cycle around 22%, your peak current draw is around 32mA and minimum current draw avg is about 7.2mA, about 12.66mA average over time.

    It's actually pulling less than that, so it's not too bad. The thing about the circuit I made up is that you can use poly, metal film, or ceramic capacitors, that will be more reliable and have less leakage current than the electrolytic capacitor that you're forced to use. Also, the capacitance being of much smaller value will enable you to use a physically smaller capacitor - but if size isn't an issue, that's up to you. Switching to a CMOS 555 will reduce the average current use by quite a bit. Remember, any power taken out of the battery will eventually need to be put back in, and over time that could amount to a good bit of change spent on gasoline. If you can reduce the power used in a circuit that will be running 100% of the time that the vehicle isn't in use, you'll come out ahead in petrol saved and battery life extended.

    A PNP transistor with a suggested 300mA Ic capability minimum, a 1uF bypass capacitor, and a 10k Ohm resistor connected from the base of the transistor to the ACC circuit.
    Last edited: Apr 5, 2009
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