Hi guys,

I have a very beginner question.
For my RC car, I want to add 2 red LEDs and 2 white LEDs in to the car. I want to use 9V battery to avoid using 1.5V battery pack. Is there any way to have the circuit avoiding using resistor or do I have to use a resistor no matter what? I'd appreciate your guidance.

 

You can have a compromise and put all four of the LEDs in series and use just one resistor to limit the current to the string. That will provide better efficiency because there will be less power wasted in the current limiting resistor.

If I select the current as 20 milliamps, which has been a "standard" for many years, and then guess the forward voltage of the red LEDs to be 1.8 volts and the forward voltage of the white LEDs to be 2.7 volts, the total voltage drop will be 1.8+1.8+2.7+2.7=9.0, so if those numbers match your LEDs you will not need a series resistor. And the actual current will be a bit less because the battery voltage will drop a small amount in a fairly short time.

 

Those voltages are well below what you normally get for LEDs, with red and green being closer to 2.0 and white being over 3V. I am not even sure it would light with all 4 and a 9V. And it would certainly dim quickly as the battery was depleted.

Bob

 

So, what do you recommend to do?
Group two red and white LEDs in series and add a resistor?

 

No mention of GREEN was ever made. And many red LEDs are less than 2 volts forward drop.
So I suggest that the TS do an experiment before installing them in the vehicle. Connect them all in series and see what you get!
CERTAINLY they will light at some brightness.
If it is not bright enough, then two strings, one red and one white, in parallel with one resistor to limit the current, which would be 40 milliamps in this arrangement. The resistor would need to drop about 4 volts at 40mA., so try 100 ohms.

 

red and green

No mention of GREEN was ever made.

Probably just mis-spoke.

would be 40 milliamps in this arrangement.

Isn't 40mA a bit high for standard 5mm LED's? I'd opt for never greater than 30mA and would likely prefer 20mA. At the most, 25mA. But I'd stay away from 40mA. Even if the battery is going to drop voltage quickly, a fresh battery will put the hurts to the LED's and shorten lifespan. I would also suspect the TS merely wants the appearance of working headlights and tail lights. If so, 15, or even 10mA would probably suit the bill.

 

As for the proper way to calculate the correct resistance for an LED circuit; you start with known factors such as the actual Vf (forward voltage) of the LED's in question. Red will certainly have a lower Vf than white. For sake of education lets assume you have a single white LED with a Vf of 3 volts and a starting voltage of 9V.

(9V - 3Vf)÷15mA (or whatever current you choose)
6V÷0.015 (15mA) = 400Ω.

So if you want to light a single white LED with 15mA you'd need a 400Ω resistor. I know you want to use more than one LED and more than one color. As @MisterBill2 suggests, you can put some of them in series. In the case of using two white LED's in series, the total Vf would be 6V. Vf represents the amount of voltage the LED will drop. Since you're starting with a 9V battery your net voltage you need to drop would be the remaining 3V (9V-6V=3V). So 3V÷0.015A=200Ω For the white LED's in series you'd use a 200Ω resistor. The battery will last longer running the LED's at 15mA than at 30 or 40mA. A LOT longer.

As for the red LED's the same approach would be needed. But I would assume you don't want the red LED's as bright as the white LED's as tail lights are significantly dimmer than headlights. Run them at a much lower current, maybe 5mA. So let's assume the red LED has a Vf of 2 volts each. That's 4Vf.
So (9V-4Vf)÷0.005A=1000Ω.
That would give you a well balanced look for lighting AND a MUCH MUCH longer battery life.

One last thing that MUST be taken into consideration, and that's the wattage the resistors need to handle. In this case 1/4W resistors will be well more than enough. But let's make sure.

9V (times) 0.015A = 0.135W (or 135mW). A 1/8 watt resistor will be too small but a 1/4 watt resistor will do nicely on the white LED's. (for the headlights)

9V (times 0.005A = 0.045W (or 45mW). In this case a 1/8 watt resistor will do just fine. (for the tail lights)

In conclusion, my approach would be two white LED's in series with a 9V battery and 200Ω resistor for 15mA, which would be plenty bright. For the red LED's in series with a 9V battery and a 1KΩ (1000Ω) resistor for 5mA, which is still plenty bright but nowhere as bright as the headlights in your model. That would be how I would approach this.

Your approach may vary.

 

Hi guys,

I have a very beginner question.
For my RC car, I want to add 2 red LEDs and 2 white LEDs in to the car. I want to use 9V battery to avoid using 1.5V battery pack. Is there any way to have the circuit avoiding using resistor or do I have to use a resistor no matter what? I'd appreciate your guidance.

Resistors are needed. 2 ways to do it, in this case right side is more simple. Current is set to around 20mA but brightness depends what sort of LEDS you're using. Tune resistors according to your brightness needs but don't exceed max current.

 

Probably just mis-spoke.

Isn't 40mA a bit high for standard 5mm LED's? I'd opt for never greater than 30mA and would likely prefer 20mA. At the most, 25mA. But I'd stay away from 40mA. Even if the battery is going to drop voltage quickly, a fresh battery will put the hurts to the LED's and shorten lifespan. I would also suspect the TS merely wants the appearance of working headlights and tail lights. If so, 15, or even 10mA would probably suit the bill.

If you read it and understood, that 40 mA was two 20mA strings in parallel. read post#5 again.

Probably if it is just for show, a series string with all four in one string will work, no resistor needed.

 

two strings, one red and one white, in parallel with one resistor to limit the current, which would be 40 milliamps

Unless I still misunderstand you - the Red and White LED's won't share current equally. One set of series LED's will hog all the current, thus making 40mA too high. Each set (left series Red LED's on their own resistor) and (right series White LED's on THEIR own resistor). That way each leg can be tuned for the desired brightness. otherwise the reds will hog all the current and burn out. Then the whites will start to glow brightly and possibly burn out.

Hey! I may be wrong, I may still misunderstand what you're saying, but here's how I see it:

To borrow Juhahoo's diagram: where each "White" (not blue) LED's in series with a current limiting resistor and the Red LED's in series with its own current limiting resistor. The Red and the White strings are parallel to each other. As Juhahoo has drawn, each has a different value resistor.

By the way - thanks Juhahoo for allowing me to steal your drawing. It's faster than me drawing my own schematic.

 

Unless I still misunderstand you - the Red and White LED's won't share current equally. One set of series LED's will hog all the current, thus making 40mA too high. Each set (left series Red LED's on their own resistor) and (right series White LED's on THEIR own resistor). That way each leg can be tuned for the desired brightness. otherwise the reds will hog all the current and burn out. Then the whites will start to glow brightly and possibly burn out.

Hey! I may be wrong, I may still misunderstand what you're saying, but here's how I see it:

To borrow Juhahoo's diagram: where each "White" (not blue) LED's in series with a current limiting resistor and the Red LED's in series with its own current limiting resistor. The Red and the White strings are parallel to each other. As Juhahoo has drawn, each has a different value resistor.
View attachment 253010
By the way - thanks Juhahoo for allowing me to steal your drawing. It's faster than me drawing my own schematic.

My thinking would be one red and one white in each series string. Then the currents would be very close to equal while the voltage drops across the two colors could be different. Just a bit more wire. And now I see that I was not adequately clear. I should have stated Two strings, EACH WITH one red and one white LED, but I accidentally implied a red string and a white string, not my intention at all.

 

The only problem with one red and one white in each string would be relative brightness.

 

The only problem with one red and one white in each string would be relative brightness.

Possibly, but given the application, headlights should be brighter than tail lights anyway.

 

True

 

headlights should be brighter than tail lights anyway.

In conclusion, my approach would be two white LED's in series with a 9V battery and 200Ω resistor for 15mA, which would be plenty bright. For the red LED's in series with a 9V battery and a 1KΩ (1000Ω) resistor for 5mA, which is still plenty bright but nowhere as bright as the headlights in your model. That would be how I would approach this.

I knew I read that somewhere before. 15mA for the headlights, 5mA for the tail lights.

 

@ThePanMan yeah, that's exactly what I was saying; headlights should be brighter than tail lights.

I should have stated Two strings, EACH WITH one red and one white LED, but I accidentally implied a red string and a white string, not my intention at all.

You can have two different color LED's in series - that's not a problem. However, I suspect the red LED in series with the white would make the red much brighter than the white. I could be wrong about that though.

Two series white LED's with 200Ω for a current of 15mA and a parallel circuit with two series red LED's on 1KΩ for 5mA should look fairly close to authentic. And perhaps using a regular red LED versus a super-bright red LED would further enhance the appearance. But at 5mA it might not stand out.

 

You can have two different color LED's in series - that's not a problem. However, I suspect the red LED in series with the white would make the red much brighter than the white. I could be wrong about that though.

That would depend on the LEDs used, for the same family of LEDs the mcd of a white is much higher than red. (usually)

Another issue I see with the white and red in series is not really a problem, but it would involve 3 wires going to the back of the car and 3 wires going the the front instead of 2 each. (not really a problem, just a nit)