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  #1  
Old 04-05-2009, 09:05 AM
evan.f_nz evan.f_nz is offline
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Question Fake Car Alarm Flashing LED - Help Needed

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:


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 by evan.f_nz; 04-05-2009 at 09:10 AM. Reason: scematic added
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Old 04-05-2009, 02:24 PM
awdman awdman is offline
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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.
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Old 04-05-2009, 02:34 PM
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SgtWookie SgtWookie is offline
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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.
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Old 04-05-2009, 04:20 PM
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thatoneguy thatoneguy is offline
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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.
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Old 04-05-2009, 04:28 PM
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SgtWookie SgtWookie is offline
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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:


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.
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Last edited by SgtWookie; 04-05-2009 at 05:14 PM.
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Old 04-05-2009, 04:33 PM
Mike2545 Mike2545 is offline
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Get one of these http://www.radioshack.com/product/in...entPage=family

And put a resistor in series.
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Old 04-05-2009, 04:57 PM
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Quote:
Originally Posted by SgtWookie View Post
Thatoneguy;
Not a good design, as the current drain when Q1 is conducting would be 50mA or more.
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/simples...r_Circuit.html

Last edited by thatoneguy; 04-05-2009 at 05:05 PM.
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Old 04-05-2009, 05:12 PM
Mike2545 Mike2545 is offline
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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.
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Old 04-05-2009, 05:15 PM
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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.
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Last edited by SgtWookie; 04-05-2009 at 05:27 PM.
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Old 04-05-2009, 05:33 PM
Mike2545 Mike2545 is offline
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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...
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