I'm trying to put in a small indicator LED in my car that turns on whenever the brake lights come on. I want to wire it to the same power line that the brake lights are attached to, so that whenever the switch is pressed and the brakes come on, that same power turns on the indicator LED.
Now, I'm aware I could just get a 12 volt LED, but I really don't want to do this. It's not as much about functionality as it is about my personal learning.
Now, I was just going to use a 1k ohm resistor, but then I learned about power. 20 amps times 12 volts is 240 watts. It's hard to find a 240 watt, 1k ohm resistor.
So then I thought about somehow decreasing the voltage. But then, I realized that even at .5 volts, .5 times 20 is 10 watts and it's still difficult to find a resistor even higher than 1 watt.
I'm so confused, I know this has to be possible, 12 volt LEDs didn't always exist and people have had to put LEDs in their cars before they did.
7805 voltage regulator
Get yourself a 7805 from radio shack. or anywhere with diy electronics. This regulator brings the voltage from 12v to 5v (4-6, really). Then on the regulated output of the 7805, get a resistor of about 220 or 330 ohms to the led. A heatsink for the 7805 is recommended if you’re using anything beyond an LED.
Thanks!
Thanks so much, but still, wouldn’t the resistor have to handle a lot of watts?
wattage vs voltage vs amperage
the only current that will go through the 7805 is what the LED is pulling. It helps to think of voltage as “pressure” PUSHING the electricity, and current/amps as “how much” is being PULLED to ground by your components. In this case, an LED with a resistor will pull hardly anything at all. Calculon doesn’t have the numbers in front of him, but let’s assume an LED pulls .1 amp. At 5 volts, that’s 1/5 watt. no problem as long as you have a resistor, that is. Because LEDs have no internal resistance at all, you’ll make a short circuit without a resistor of some kind.
The way to calculate a resistor for a LED is:
Supply Voltage - Forward Voltage / LED current = Current Limiting resistor
Forward Voltage for a red LED is about 2v and current is about .02A or 20mA
(12V - 2V) / .02 = 500 ohm resistor
I ‘believe’ the wattage should be for the 10v that are dropped across the resistor at the current .02A or .2W or 1/4W resistor.
No regulator needed.
no, the resistor for the LED
no, the resistor for the LED only has to reduce 12v to 2v for the amount of current going to the LED. LEDs always draw 20ma, so 10v drop @ .020a is .2 watts. a 510 - 560 ohm resistor is good for 12v source. a little higher would be ok since car dc is usually higher than 12v - say 13.8v.
duh, I should have seen and
duh, I should have seen and agreed with Birdmun first. 
actually its worth pointing
actually its worth pointing out that it can be done without the 7805, i was wondering that yesterday…
cool
Calculon has learned something new today.