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Author Topic: Traffic Adviser (Lightbar) for Firefighers  (Read 397 times)
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Hello Everybody

I am looking to create a traffic adviser using the arduino board for fun. It's one of the lightbars you see inside cop cars. I am a volunteer firefighter and electronics engineering student (first year). Doing some research told me that companies are selling lightbars with 1 watt generation 3 LEDS.

Using ohms law and the power formula I came up with (I guessed the 3volts):
1 watt LED = 3v, 9ohms and 333mA

The power source would be a 12v from the car battery and the Arduino needs a power supply of ? and outputs of the pins is ?.

I would need to run all the bulbs in parallel since voltage is common in parallel and I would need to use the voltage divider rule to get from 12v to 3v? I can solve for the amount of current being used by the bulbs and pick the appropriate size resisters for the voltage divider since current is common in series.

I haven't covered transistors yet so I know nothing at this point. My guess is that I connect the LED power supply through a transistor that is being controlled by the Arduino. That way I can supply as much power as I want to each led and the arduino just needs to control the switch. A transistor has Vin, Vout and a 3rd connector. Does the power coming from the 3rd controller pin contribute to the power or does it just control open or closed?

Am I completely lost or what and do you guys have any suggestions?
Thanks
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Using ohms law and the power formula I came up with (I guessed the 3volts):
1 watt LED = 3v, 9ohms and 333mA

Ohms Law doesn't com into it until you start calculating series resistors. P = V * I, so if a 3W LED has a voltage drop of 3V, it must be consuming 1A. There is some data on 3W LEDs at http://www.rapidonline.com/electronic-components/3w-power-led-74700/.

The power source would be a 12v from the car battery and the Arduino needs a power supply of ? and outputs of the pins is ?.

The Arduino can run from the 12V supply, provided you are not drawing much current from the Arduino's 5V supply. The Arduino's outputs are 5V, maximum 40mA.

I would need to run all the bulbs in parallel since voltage is common in parallel

I'm not sure what you mean here. Do you need to control all the LEDs independently? If you don't, then it would be better to connect about 3 LEDs in series so as to reach a little less than the 12V you have available.

and I would need to use the voltage divider rule to get from 12v to 3v?

No, voltage dividers are only sensible with light loads. You can use a series resistor to drop unwanted voltage when feeding a LED, but dropping 12V to feed 3V @ 1A to a LED would be very inefficient, because the resistor would dissipate 9W. That's a lot of hear to get rid of. High power LEDs are best fed fro a constant current supply (especially as the nominal 12V car battery voltage varies a lit, depending on whether it is on charge or not). However, dropping 12V to 3V in this way will still waste about 9W of power.

The best way to drive high power LEDs from an Arduino is to use a switching current regulator IC such as http://www.ti.com/lit/ds/symlink/lm3404hv.pdf or http://www.diodes.com/datasheets/AP8802.pdf or http://www.farnell.com/datasheets/1636609.pdf for each LED. You can feed the DIM or CTRL input from the Arduino to turn the LED on/off or to dim it using PWM.
« Last Edit: October 21, 2012, 12:58:21 pm by dc42 » Logged

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