LED headlights

[Derby Dad]

I'm seeking help creating a simple circuit to light 4 LEDs for a Cub
Scout pinewood derby car.

Here's the situation:

My son has asked me to help him put 2 white headlights and 2 red tail lights onto his Scouts pinewood derby car.

So, I bought some LEDs from Radio Shack (2 white and 2 red) but I
don't know how to provide electricity to them.

I threw away the packaging but my recollection is that the white LEDs
are 3.6 V and 20mA. The red LEDs are 1.9 V and 36 mA.

To power these 4 lights, I bought a case to hold 2 AAA batteries. With
the two leads from this case, I can light either a single white LED or
a single red LED. But, not two of any color (let alone 4).

So, I tried using a 9 Volt battery. But, that just fried the LED
(fried one red one and one white one) when I tried to light each.

Can anybody help me figure out how to get these 4 LEDs to light?

Can anybody help?

Thanks very much!

[terryep]

With a 9 V battery make two parallel circuits each consisting of one white LED, one red LED and one 180 ohm resistor connected in series. The resistor is needed to limit the current to 20mA.
Terry

[Darin McGrew]

I'm seeking help creating a simple circuit to light 4 LEDs for a Cub Scout pinewood derby car.

First, make sure there isn't a blanket ban against electronics in your derby's rules. My derby would allow such decorative electronics; in fact, we had a dad-built police car with working lights and siren a few years ago. But your derby might not allow it.

Anyway, a simple circuit for an LED uses a resistor in series with the LED: the battery terminal connects to a resistor that connects to the LED that connects to the other battery terminal.

The battery provides a certain voltage. The LED is rated for a certain voltage, lower than the battery's voltage. The voltage across the resistor makes up the difference. If the current starts increasing for some reason, then the voltage across the resistor increases, and the voltage across the LED drops back to a safe level.

Let's look at your red LEDs:

V[battery] = 3.0 V
V[LED] = 1.9V
I = 36mA

Therefore:

V[resistor] = 3.0 - 1.9 = 1.1V

And solving for the resistance:

V[resistor] = I * R
1.1V = (36mA) * R
R = 1.1V / 36mA
R = 30.6 ohms

You may end up having to buy 33 ohm resistors, since they come in certain standard sizes.

Your white LEDs have a higher voltage (3.6V) than the pair of 2 AAA batteries (3.0V, assuming alkaline batteries). So you can connect them directly. They won't be as bright as they would be with full voltage, but they'll probably light up with fresh batteries. If not, then you'll need a higher voltage, perhaps 3 AAA batteries (4.5V) or a 9V battery. But the higher the battery voltage, the more power you're wasting across the resistor.

[Derby Dad]

Thanks terryep,

Just to make sure I understand, are you indicating that a single 9V battery is sufficient to supply power for the 2 circuits? Or is it a 9V battery for each circuit?

Also, for my own edification, how did you arrive at the rating for the resistor (180 ohms)?

Again, thanks very much.

[Derby Dad]

Thanks Darin,

I will definitely ensure there is no rule against this. I do have a couple of questions regarding your suggestions:

As you indicated, your V=IR example is only for a single LED (the red one). So, if I understand correctly, I'd need 4 separate circuits, one for each LED?

circuit 1:
red LED, 33 ohm resistor, 2 AAA alkaline batteries

circuit 2:
red LED, 33 ohm resistor, 2 AAA alkaline batteries

circuit 3:
white LED, 2 or 3 AAA alkaline batteries or a 9V battery

circuit 4:
white LED, 2 or 3 AAA alkaline batteries or a 9V battery


Thanks very very much.

[Derby Dad]

thanks guys,

Just trying to fully understand before I go buying more components (and frying them) from Radio Shack.

[gpraceman]

Just to make sure I understand, are you indicating that a single 9V battery is sufficient to supply power for the 2 circuits? Or is it a 9V battery for each circuit?

A single 9V or a couple of AA or AAA batteries would work, you just need to make sure that you are using the correct resistance depending on the battery voltage you go with.

For my police car, that I did last year, I used 2 AA batteries to power the headlights, siren and flashing lights.

I would suggest adding in a simple on/off switch going to one of the battery terminals so you can easily turn things off and save battery power between heats.

[Darin McGrew]

As you indicated, your V=IR example is only for a single LED (the red one). So, if I understand correctly, I'd need 4 separate circuits, one for each LED?

You need 4 separate resistor-LED circuits, but they can all share the same battery. And taking Randy's very good advice about adding a switch, you end up with:

battery -> switch -> 4 resistor-LED circuits -> battery

So you connect the battery to the switch, and then you connect each resistor-LED circuit to the switch's other contact and the battery's other contact.

Assuming you're using 2 AAA alkaline batteries (3.0V), you have:

circuit 1: red LED & 33 ohm resistor
circuit 2: red LED & 33 ohm resistor
circuit 3: white LED (no resistor)
circuit 4: white LED (no resistor)

Assuming you're using 3 AAA alkaline batteries (4.5V), you have:

circuit 1: red LED & 72 ohm resistor*
circuit 2: red LED & 72 ohm resistor*
circuit 3: white LED & 45 ohm resistor**
circuit 4: white LED & 45 ohm resistor**

* (4.5 - 1.9) / .036 = 72
** (4.5 - 3.6) / .020 = 45

Personally, I'd go with 3 AAA batteries. I don't like connecting LEDs directly to a power supply, but I don't like dropping more voltage than necessary across the resistor.

If you use 3 AAA batteries, then you could also combine the red LEDs into a single circuit:

circuit A: red LED & red LED & 19 ohm resistor***
circuit B: white LED & 45 ohm resistor
circuit C: white LED & 45 ohm resistor

*** (4.5 - 1.9 - 1.9) / .036 = 19

[Colibri]

Here is a couple of sites to shopw you whats happening in slot cars with lights.

http://www.slotservice.com./Home/Pictur ... llery.html

http://members.optusnet.com.au/bpatto/lights.html

Granted these are outside the country. You can see what can be done with lights

[terryep]

Hi Derby dad
Here is a schematic for you. I put in the common 5% resistor values. Daren gave you the theory nicely. I too would not recommend direct connection of an LED to a battery.. you allready know the result.



Terry

[terryep]

Derby Dad,

Your choice of battery will hinge on mechanical considerations since:

One 9V alkaline battery has a capacity of 595mAhr so it should last 15 hours at a draw of 40mA (2 x 20mA). Weight is 45.6g plus connector.

One (same as 3 series connected) AAA alkaline battery has a capacity of 1150mAhr so should last 14 hours at a draw of 80mA (4 x 20mA). Weight is 3 X 11.5g plus battery holder

Terry

[SPEEDBUGGY]

WHAT IF YOU USED 1.5VOLT LIGHTS LIKE IN A HESS TRUCK.PAINT TWO CLEAR RED AND LEAVE THE OTHER TWO CLEAR. YOU WOULD USE LESS BATTERIES TO POWER THEM.DON'T FORGET TO GROUND OUT OFF THE BATTERY SO IT DOSE NOT OVER HEAT. I DID THIS ONCE AND IT GOT TOO HOT TO TOUCH.

[Da Graphite Kid]

Derby Dad,
While the information many have given you has been correct, you have never mentioned one thing: How bright you want these LEDs to shine. You did not mention the "mcd" rating for the LEDs you bought (this is a measure of the amount of light the LED outputs - just can't remember what it stands for right now!). I've bought LEDs from Radio Shack and have found that there is sometimes a difference in the specifications of one LED to another although they share the same part number. Could just be a mixture of old and new stock but it can happen. I would hate for you to go through all of this and end up with the wrong resistor values for the new LEDs you bought.

You are going to have to go with either the 9-volt battery of the 3 AAA sized battery configuration as the white LEDs you spec'd will not shine well at the lower 3-volt voltage (2 AAA batteries). I'm thinking that the amount of room there is in your son's pwd car will dictate which battery configuration you go with. Me, I'd go with the 9-volt battery and here's why: I would want my LEDs to be very visible. I would over-drive the LEDs I used to make them brighter. Now you over-drove the LED that you fried but I'm wouldn't go as far as you did. Depending on the mcd rating of the LEDs you use and what affect you want from them, you can run them at 1.5 or 2 times the current rating. Sure this will cause them to burn out quicker but with life ratings of over 100,000 (and most over 250,000) hours, who cares if they only last 50,000 hours? That would be like running them continuously for over 2,000 days! We have done this quite successfully in building film canister 9-volt flashlights with the Cub Scouts before so I know that it can be done.

Just wanted you to know that there were more LED options out there. Contact me at: gwalters@techsonic.com if you have any particular questions about this. I think I even have a spreadsheet that will calculate the resistor values for you if you want it.

Da Graphite Kid

[terryep]

Before overdriving an LED you should check the specs.... if you exceed the max die temperature you risk failure in a short time. Since Radio Shack parts are not spec'd properly or completely I'd stick with safe values.

Also, if you choose ballast resistors that use a small portion of the available voltage your load regulation will be poor. One LED may get much brighter than the other since the forward voltage varies between individual LEDs and you want the LEDs to work well at the battery's end voltage. After all, battery life is important too.
Terry

[Derby Dad]

Thanks to everyone. I'm off to Radio Shack to buy some more LEDs and a breadboard so we can experiment a little.

Graphite Kid -- I think my son just wants the lights to look "cool". I'll inquire about this MCD rating and try to ensure that all the LEDs of a certain color are the same in that regard.

Thanks again everyone. I'll let you know how it goes.