LED Taillights, Diy

[Matt21lutz]

Originally Posted by ray_mcavoy

From the looks of it, I think the LM2940 regulator IC (integrated circuit) you found in the 2nd link is what they're using on that pre-built circuit board in the 1st link. Specifically, it appears to be the LM2940CT-12 or LM2940T-12 version in a TO220 package.

I took a quick look through the LM2940 datasheet from your 2nd link and it appears as though it would be a good choice for your application. In fact, the description on the first page says it's designed for vehicular applications and has built-in protection against reverse polarity, transient voltage spikes, etc. so that's a plus.

The low 0.5V dropout or "overhead" is also good for your application. That means the regulator only needs a 0.5V difference between the input & output to maintain regulation. In other words, you'll have the full 12V output you're expecting as long as the input stays above 12.5V ... that's much better than the NTE1970's 14.8V min requirement.

As long as your truck's charging system is working properly, you shouldn't have to worry about input voltages lower than that. But as you can see from the "low voltage behavior" (figure 21 on the LM2940 datasheet), it's a fairly linear drop-off once the input goes below about 12V. So a low battery situation would allow the LEDs to dim, but the regulator isn't going to suddenly "switch off" under those conditions.

The "typical application" circuit (shown near the bottom of the first page of the LM2940 datasheet) is all you really need. Notice how the capacitors are connected ... that's what I was trying to describe when I said they were connected incorrectly in your original diagram.

So basically, take that "typical application" circuit for the LM2940 and substitute it into your original diagram in place of the NTE1970 & the 2 capacitors and you should be all set. But if that doesn't make any sense, just let me know and I can draw up a diagram for you.

Finally, your original circuit shows 56Ω resistors in series with each LED string. That's a good value to use for a 12V input, but like Dead Parrot pointed out earlier, don't forget about the voltage drop from the diodes being used to isolate the tail from the brake/turn circuits. With silicon diodes (0.7V drop) you'll only have 11.3V available to the LEDs. So you'll need smaller current limiting resistors in order to maintain the full rated 20mA through the LEDs. I'm calculating 15Ω resistors for five 2.2V, 20mA LEDs in series with an 11.3V supply.

awesome, I did see that it said for vehicular use on that just wanted to make sure. and as far as the diodes actually taking away some voltage.. i would have never thought about that unfortunately when i ordered all my resistors i got 56 ohm as the smallest ones, i could always buy more but then im paying triple just for shipping a few dollars worth of resistors, but i might. The only thing would that would be different is the 56 ohm would make the led's dimmer , just not sure how much actually... we'll find out! I'll probably make a few tester first then make the final product.

But i would like to get your opinion on something else that i thought about, what do you feel is the best way to dim the led's? with a 1/2 watt resistor or to use a voltage regulator with a lower output voltage? for instance a 10v output.

but thank you for all the help so far as well! i greatly appreciate it!