# question on how to build a 12v LED circuit

#### stringer1.1

Joined Oct 11, 2018
3
My Q. is about can anyone draw me a solution to this?
I'm looking for solutions in a pending creation but what job I have is to wire a CIRCUIT TO A 12v-13.8v car battery(fitted to a car) Consisting of 10 LED's in each.
I know it's 2 rows of 5 LED's. But where these parts go and how to do it has baffled me.

I've got 10 x Crystal Clear Case Style: 5mm Round Forward Voltage: 1.5v - 1.6v Forward Current: 60mA Wavelength: 850nm Viewing Angle: eg: (IR) LED 5mm - 850nm.
I have 5x (needs less I should think) LM317 Positive Voltage Regulator 1.25v to 37v 1.5A IC 3 pin TO-220.
4x (needs less) 20 ohm, 20R, 3W metal film resistor, HIGH Q UALITY – ref:818a.
&1 Led Dot Light 12V Car Auto Boat Round Rocker ON/OFF Toggle Switch.

Anyone who could draw me a solution will
be in my good books forever. thank you

#### dl324

Joined Mar 30, 2015
14,699
Welcome to AAC!
1. What current do you want to operate the LEDs at?
2. Why do you want to use 20 ohm 3W resistors?

#### rsjsouza

Joined Apr 21, 2014
356
By reading the problem proposal I can't figure out what exactly you want to do, but if you are going to simply turn on the LEDs using the 12V battery via the toggle switch and only using what you have, there are many ways to connect all this.

#### k7elp60

Joined Nov 4, 2008
562
Here is a schematic that should solve the problem

#### Attachments

• 119.6 KB Views: 18

#### Tonyr1084

Joined Sep 24, 2015
6,890
I agree, you are baffled. So am I.

What I think I gather is that you have a rocker switch with a built in LED, designed for a 12 volt circuit. If so, it's likely you don't need to worry about that LED at all. It also sounds to me like you have a remaining 9 LED's you want to power from a 12 volt circuit.

First, if we're talking automotive 12 volts, when the engine is running your typical voltage should be around 13.6 to 13.8 volts, depending on engine speed. Lower at idle, higher when driving.

At any rate, whatever forward voltage you have for your LED (or LED's) you have to subtract them from the highest voltage you expect to see. Depending on your vehicle (again, assuming we're talking about automotive electrics) your electrical system can very likely see a short period of time at as much as 14.6 volts. So that's where I'd start. 14.6 volts minus the forward voltage of your LED's. CAUTION: How you wire them will make a difference in what the total forward voltage drop will be. You mentioned a forward voltage (Vf) of 1.5 volts. I'm assuming (a dangerous thing to "ASS-U-ME") you mean each LED has a Vf of 1.5 volts. When you string them together, one after the other (series) you add their voltages together for a total Vf. 5 LED's of 1.5 Vf in series has a total Vf of 7.5 volts. Subtract that from your highest anticipated voltage (14.6) and you're left with 7.1 volts to power the circuit. THAT'S NOT A PROBLEM! That's just what you have to work with.

You didn't mention the specs on the LED's you have. I'll assume they're typically rated for a max operating amperage of 15 milliamps (0.015 amps). You divide your working voltage by the desired amperage and that gives you the necessary resistance. In this case, 7.1 ÷ 0.015 = 473.3 ohms. On that formula you'll have 15 mA of current running through a string of 5 LED's, plenty bright. But you said they're infrared. I have no experience with those, but the theory is still the same. Again, assuming your IR LED's are designed to operate on 15 mA.

Regarding resistor selection: you have (in this instance) (typically) a 470Ω resistor running 15 mA. To figure the wattage you calculate the wattage through the resistor by multiplying the voltage across the resistor (7.1 volts) times the current (0.015 amps), giving you 107 milliwatts (0.1065 watts (rounded)). That's less than 1/4 watt. So you don't need 3 watt resistors. And 20 ohms is not enough resistance. You'll burn out your LED's quicker than snot in a storm.

Like I said, your goal is not clear. Just HOW you decide to wire them up is up to you. The best thing you can do for us is to draw your best idea of how you want to wire it and post that. Then we have a clear starting point and can redirect you to the best solution for your problem (goal). Till then I'm going to refrain from drawing diagrams. But to reiterate, your LED rocker switch - you probably don't have to take any consideration any Vf or current. Likely the manufacturer has already done that and incorporated a current limiting circuit for that. And its limitations will not affect your circuit at all.

#### stringer1.1

Joined Oct 11, 2018
3
Welcome to AAC!
1. What current do you want to operate the LEDs at?
2. Why do you want to use 20 ohm 3W resistors?
I did have a diagram that I had done some months ago on a project that I'm working on and we as in me and my old friend had everything working but our little let's say workshop got vandalized and robbed and we lost everything but as far as i can remember we had it working on 20 ohm 3W resistors. just as a matter of interest for you we are working on a night vision system and obviously will be using more than just 10 LED's. we work in the dark and cant use conventional lighting as a part of the job. Don't panic we aren't criminals but quite the opposite actually. we protect privacy so we cant stand out like a saw thumb.

#### dl324

Joined Mar 30, 2015
14,699
but as far as i can remember we had it working on 20 ohm 3W resistors.
I'd eliminate the voltage regulator and just run off the battery.

7 LEDs @ 1.6V = 11.2V
At 20mA, R = (12.6V-11.2V)/20mA = 70 ohms

With the car running, current increases to about 40mA. Your LEDs will tolerate 60mA. Worst case, you'll get just under 50mA (assuming 1.5V forward voltage).

If you need fixed current to get constant brightness, use current sources.

#### stringer1.1

Joined Oct 11, 2018
3
By reading the problem proposal I can't figure out what exactly you want to do, but if you are going to simply turn on the LEDs using the 12V battery via the toggle switch and only using what you have, there are many ways to connect all this.
View attachment 161354

I quite like your 2nd diagram thanks, I'ts 05:04 sat 13/10/2018 and it looks like I'm pulling out the solder lol thanks

#### Tonyr1084

Joined Sep 24, 2015
6,890
If you're going to run off of a battery then I would definitely advise against using a regulator. They waste a lot of power and your battery life will be extremely limited. Even a big battery can be drained fairly quickly. I'd suggest looking into PWM (Pulse Width Modulation) for controlling the amount of power that goes to the LED's

PWM turns them on full brightness then turns them off again real quickly. Depending on the variance of the width of the pulse you control your brightness. Just be sure to have a final current limiting resistor to protect against accidental over stressing the LED's. PWM also won't drain your batteries nearly as fast as a regulator.

#### eirman

Joined Aug 18, 2017
14
When you rev up a car engine you may get all sorts of spikes from the car's alternator,
when compared to the pure DC from the battery.

#### k7elp60

Joined Nov 4, 2008
562
sounds like the problem solved

#### Audioguru

Joined Dec 20, 2007
11,248
What is the function of having 10 invisible infrared LEDs running at full blast in a car?? Cooking food?

#### Tonyr1084

Joined Sep 24, 2015
6,890
One thought I've toyed around with is having IR LED's on the front of my car flashing at a rate similar to that of a cops flashing light. Approaching a red light, the IR will be seen by the traffic camera and trigger a cycle change, limiting the time I spend at red lights, OR changing them to green before I get there, thus conserving momentum and not wasting as much gasoline. IR because regular flashing lights that can be seen by the human eye would definitely attract the wrong attention. Having something invisible to human sight but visible to traffic sensors - those that work off of light changing as cars enter the field of view - might trigger the cycle. Like I said, I've toyed around with the idea but really don't plan on pursuing that.

In my state the new traffic sensors are some sort of radio wave sensors. Now - If I only knew the frequency.