Wiring multiple LED's

[RC_Racer]

Hi all, I am a relative newbie to the electronics world and I have a project I want to do but need some direction. I have an 1/8th scale remote controlled nitro buggy. I want to add some lights to my car but I have a few factors that need to be addressed. I have already put 2- 3.6v led's in the front for headlights. No problems there until I added a few lower voltage bulbs as tail lights. Contrary to what the "professional" at Radio Shack said, I can't run all the lights from 2 "AA" batteries.
What I would really like to do is have the headlights & tail lights on one switch and a light bar on top of the car with probably 4 more lights on a separate switch. My biggest problem is that I am very limited for space to mount multiple power supplies. I was wondering if it was possible to use a 9 volt battery to power everything? I'm sure I need resistors or something but I don't know how to calculate what size I need. I really don't even know for sure if what I want to do is possible. Will a 9v battery be enough for 6-3.6v led's and 4-others that are a lower voltage bulb? How long would it burn the led's before the battery runs out?
Any help you can give me will be greatly appreciated. I have done a lot of electrical work on residential homes so the switches and everything aren't a problem.

 

[andynerd]

i used a 9v battery on a single 1.5v led with a 380hm: resistor so really you should have no problem just use a low voltage led hook em up in parallel and give each light it's own resistor.

in a resent project i had resistor issues and i was told this:

Well no you can leave it in...you just have to take into account that a resistor needs to be put in front or behind of it to reduce the current going through it. Otherwise you'll burn it out.

Like lets say you have 9V going through the led but you drop 2 on the buzzer (I have no idea what you drop). Well assuming that the led is 1.2V (also no idea) and you want 25mA (a guess)...then the resistor value would be: (9 - 2 - 1.2)/(.025) = 232 ohm so 240 ohm resistor needed for that situation.

 

[iONic]

If I get this straight you want:
2 White LED's for the fromt headlights
2 Red LED's for the tail lights
4 Red? for an LED bar on top of the car.

Do you know the voltage rating and current rating for all these LEDs?

Here's a simple circuit that might get the job done.
The values are gueses.

Here's a resistor calculator link **broken link removed**

 

[shaneshane1]

Hi all, I am a relative newbie to the electronics world and I have a project I want to do but need some direction. I have an 1/8th scale remote controlled nitro buggy. I want to add some lights to my car but I have a few factors that need to be addressed. I have already put 2- 3.6v led's in the front for headlights. No problems there until I added a few lower voltage bulbs as tail lights. Contrary to what the "professional" at Radio Shack said, I can't run all the lights from 2 "AA" batteries.
What I would really like to do is have the headlights & tail lights on one switch and a light bar on top of the car with probably 4 more lights on a separate switch. My biggest problem is that I am very limited for space to mount multiple power supplies. I was wondering if it was possible to use a 9 volt battery to power everything? I'm sure I need resistors or something but I don't know how to calculate what size I need. I really don't even know for sure if what I want to do is possible. Will a 9v battery be enough for 6-3.6v led's and 4-others that are a lower voltage bulb? How long would it burn the led's before the battery runs out?
Any help you can give me will be greatly appreciated. I have done a lot of electrical work on residential homes so the switches and everything aren't a problem.

Using the 9V battery should be fine, the less mA you run through your LED'S the longer the battery will last, lets say you want to run a 2.1V 15mA LED, that means that the LED draws 15mA an hour, so if you times that by all the LED'S you will be using, lets say 8 LED'S, so that will be 120mA an hour if they are all running at the same time, so now it depends on how much mA the battery contains if it conains 1000mA (1Amp) then running your LED'S non stop should last around 7-8 hours

9V - 2.1V = 6.9V

6.9V / 0.015mA = 460ohms resistor

so you could use one 460ohms resistor for each LED

Hope this helps you.

 

[audioguru]

A 9V alkaline battery has 6 tiny skinny AAAA cells inside so it doesn't have much capacity.
8 LEDs connected with 2 in series and in series with a current-limiting resistor as shown with 18mA per string has a total current of 72mA.

Go to a battery manufacturer's website to see the datasheet for a 9V alkaline battery. Energizer shows that with 75mA, a 9V alkaline battery voltage drops to 7V in about 1 hour.
The white LEDs will dim during the hour then not light anymore. The red LEDs will dim slower and not light after about 8 hours.

 

[RC_Racer]

Thanks for the calculators they help alot. What can I do for a power supply that will last? I am really limited on space for a battery pack. Right now I have a 2-AA side by side holder and it just barely fits without interfering with any of the moving parts. Also, is there any chance that any of this could interfere with the radio controls?

 

[shaneshane1]

Thanks for the calculators they help alot. What can I do for a power supply that will last? I am really limited on space for a battery pack. Right now I have a 2-AA side by side holder and it just barely fits without interfering with any of the moving parts. Also, is there any chance that any of this could interfere with the radio controls?

you can get rechargable batteries AA that are 1.7V and 2700mA if you get 2 of them and connect them in parallel(not series) then i would assume that you would have 1.7V and 5400mA (5.4Amps)

and that would save on space and last alot longer

 

[jmoorsie]

I agree with the rechargeable AAs, but have you ever thrown one between the leads of a multi-meter? There's NO WAY you're gonna pull that kind of current, but thats not to say that it won't be enough... I think they are your best bet though, considering they are pretty cheap.

 

[Torben]

Using the 9V battery should be fine, the less mA you run through your LED'S the longer the battery will last, lets say you want to run a 2.1V 15mA LED, that means that the LED draws 15mA an hour, so if you times that by all the LED'S you will be using, lets say 8 LED'S, so that will be 120mA an hour if they are all running at the same time, so now it depends on how much mA the battery contains if it conains 1000mA (1Amp) then running your LED'S non stop should last around 7-8 hours

Hi Shane,

I think you mean that the battery contains 1000mAH, not 1000mA. Amperes are a measure of current flow (coulombs per second) and you can't store flow--but you can store whatever it is that is flowing, which is charge in this case.

9V batteries can store a surprisingly small amount of charge. AAs are better for this.

9V - 2.1V = 6.9V

6.9V / 0.015mA = 460ohms resistor

That should be 6.9V/15mA, or 6.9V/0.015A. Gotta watch those units. (Yeah, I know that was probably just a typo).

so you could use one 460ohms resistor for each LED

Hope this helps you.

That's it for my session of the Picking of Nits. Back to your regularly scheduled thread.


Torben

 

[audioguru]

My RC electric motor airplane uses AAA Ni-MH cells. At full power the current is about 6A. AA cells can supply 10A easily.

 

[shaneshane1]

Torban

thanks for pointing those things out,

but if you dont get technical about it, doesnt it still make sense

eg: if i have a battery (1000mAH) and i run something at 100mA for one hour, then arnt i left with only 900mAH left to use?

and as for the 0.015mA, yes i should have said 0.015A

 

[Torben]

Torban

thanks for pointing those things out,

but if you dont get technical about it, doesnt it still make sense

eg: if i have a battery (1000mAH) and i run something at 100mA for one hour, then arnt i left with only 900mAH left to use?

More or less, yes. But it's also more complicated than that, as the capacity rating is related to the rate of discharge--so the faster the discharge rate, the lower the capacity will appear to be.

The problem with technical subjects is that if you don't get technical about them then you end up with guesses.


Torben

 

[shaneshane1]

More or less, yes. But it's also more complicated than that, as the capacity rating is related to the rate of discharge--so the faster the discharge rate, the lower the capacity will appear to be.

The problem with technical subjects is that if you don't get technical about them then you end up with guesses.


Torben

really?

if i have a 9V battery (1000mAH) then when it gets down to 4.5 (half voltage) then wouldn't i still have around 500mAH at 4.5V?, i thought this is how it works, i never really thought of it any other way

 

[Torben]

really?

if i have a 9V battery (1000mAH) then when it gets down to 4.5 (half voltage) then wouldn't i still have around 500mAH at 4.5V?, i thought this is how it works, i never really thought of it any other way

Not the way I understand it. One thing to clear up: it's not like we're draining the voltage, which is kind of what it sounds like you're saying with the "gets down to 4.5 (half voltage)" thing. We're draining the charge at a certain rate, which creates a current of a certain amperage. The closer to full capacity the battery is, the better able it is to generate the expected voltage across its terminals.

For instance, check out this datasheet for an Energizer 9V NiMH (really 7.2V but it's called a 9V) rechargeable: https://www.electro-tech-online.com/custompdfs/2007/12/nh22-175.pdf

Look at the discharge graphs to the right of the datasheet page and you'll see what kind of voltage it can produce across its terminals for certain amounts of remaining charges and different discharge rates. At a 175mA rate of discharge you get about 30 mins out of the thing before it drops below 5.5V; at 17.5mA you get just about 10 hours. And the voltage plots aren't straight lines, as you might expect.

The rated capacity is 175mAH. So at a 17.5mA discharge rate, the math would say you get 10 hours out of it. So far so good; that's what the graph shows. But at ten times that discharge rate, the math would say you get 1 hour out of the battery--as the '175mAH' would suggest. But look at the lower graph: you get 30 minutes, and the whole time, the voltage is dropping like a stone.

A battery which comes closer to approaching the ideal (at least, for lowish currents) is the LR55 Lithium 9V: https://www.electro-tech-online.com/custompdfs/2007/12/l522.pdf : With this, the load can draw up to 120mA and the voltage will stay above 5.5V for over seven hours. The same load on the other 9V would drop it to 5.5V in less than an hour. According to the datasheet, trusting the math and figuring that one could pull, say, 1.2A for 42 minutes will result in the perpetrator needing a new battery.

These numbers of course only deal with a drop to 5.5V across the terminals, but that's because we don't speak in terms of absolutes like the battery being "empty" or "full"--you determine what good values for "full" and "empty" are and use those. Sure, at 175mA you might still have a voltage at one hour, but it would be so low as to be useless to a circuit expecting a 9V source, even if that circuit were designed to work down to, say, 5.5V. We have to decide where we draw the line when designing a circuit: i.e. "this should operate OK with a supply between X and Y volts".


Torben

 

[shaneshane1]

Torben

thanks for that very informed infomation, i have learn't alot just now, and have a better understanding of batteries and there rated mA, i used to assum, up until now that if a battery had 1000mAH, then i could run something for 1hour at 1000mA and then the battery would need replacing, i had no idea that a batteries performace could be permanently affected by exeeding mA ratings (infact i didn't know there was a rated mA)

So once again, I Thank you for making that easy to understand.

 

[Torben]

Torben

thanks for that very informed infomation, i have learn't alot just now, and have a better understanding of batteries and there rated mA, i used to assum, up until now that if a battery had 1000mAH, then i could run something for 1hour at 1000mA and then the battery would need replacing, i had no idea that a batteries performace could be permanently affected by exeeding mA ratings (infact i didn't know there was a rated mA)

So once again, I Thank you for making that easy to understand.

Glad I could help, and what you assume is true for the most part--as long as you ensure that you're pulling no more than the current that the battery maker was pulling when they calculated their mAH rating. There are other factors such as temperature, but for general room temperature the rules should apply according to the battery datasheets. That said, your 1000mA example would indeed be way too high for a 9V battery.

Pulling too much can either cause the battery to burst or even explode, or (in the case of the Lithium 9V) can permanently trip a safety circuit.


Torben

 

[audioguru]

A woman was giving a speech to an audience, then began to dance and shout.
The spare 9V alkaline battery in her pocket was shorted by her keys and was getting very hot!
The spare battery was for her wireless microphone.

 

[Andy1845c]

The spare 9V alkaline battery in her pocket was shorted by her keys and was getting very hot!

This happened to me once. Up till then I would have never thought a 9v could get that hot.

 

[OutToLunch]

is there video of this dance on youtube?