Making LED light UP????

[gpw]

Bought some LEDS from RS, and guess what ,they're not like a light bulb!!!I guess I need a special circuit to make them light....the goal was to have some ultra bright "landing lights"( like those new LED flashlights)......can anybody help with a simple circuit??????

[steve lewin]

What voltage are you running them from and exactly what type of LED are they ? Usually they just need a resistor in series to drop the excess voltage that's all. OTOH if you've been trying them on more than about 1.5V you may by now have some ex-LEDs :(.

Steve

[rcav8r2]

Typical voltage drop across a LED is 1.5-1.7 volts.... You then need the resister to limit the current. (I forget the exact current,and it varies for different LEDs, but that should be on the LED package.)
Take source voltage - voltage drop across LED. Remaining voltage will then be dropped across the resister. Use this voltage and the current you need to calculate the value of the resister.

[Andy W]

We'll assume 2V drop across each LED. A little high but it works fine. Each diode has a recommended current (I) - 20mA is typical - that's 0.02A.

Vin is the source voltage.

(a) R = ( Vin - 2V ) / I

eg. 5V in, 20mA suggested LED current:

R = (5 - 2) / 0.02
R = 150 Ohms

(b) R = ( Vin - 4V ) / I

(You add the voltage drop of each diode because they're in series.)

eg. 5V in, 20mA suggested LED current:

R = (5 - 4) / 0.02
R = 50 Ohms

(c) R = ( Vin - 2V ) / ( I * 2 )

(You double the current draw as the diodes are now in parallel - does this remind you of anything?)

eg. 5V in, 20mA suggested LED current:

R = (5 - 2) / (0.02 * 2)
R = 75 Ohms

Resistors come in various values but often one 10-15% away from the calculated value will work just fine. Use 1/4W or better resistors (heat dissipation which would just confuse the issue if I explained that here)..
..a

[Capt. Electron]

Wow, good job Andy, nice examples and image.

For those that may still be a little confused, 'A', 'B' and 'C' text examples correspond to the three circuits in the image so you can see and pick your situation. Single, Series or Parallel.

Jimmy

[escapee]

GPW
Did you get "hi intensity" (white/blue?) LEDs?
If so, they generally are ~3.5 volts Vf (forward voltage).
It's probably easiest to rough the resister #'s, given battery/LED voltages vary a bit, then measure & tweak with the quantities of LEDs, etc. to get what you want.
Also it's better to connect LEDs in series due to mismatching.
The new flashlights have pulsing drivers, like ESCs.
How do you plan to activate them - servo output / remote switch?

[gpw]

I'm afraid I'm still in the dark!!! The one I bought on sale was the Cat. no. 276-206...Orange Jumbo ,high brightness , LED....10 mm round ,12000mcd brightness.........forward (supply ) voltage 2.6V .....reverse voltage 4.1V .....forward current 50 ma ....peak 100 ma ......power dissipation 130 mw
opto -electrical characteristics.......forward V (typ) 1.9V ....forward current 20 ma ....luminous intensity(typ) 12000mcd
peak emission wavelength 620 nm (did I buy the wrong thing???)
Allthough having a basic electronics background in the service (35 years ago), I'm somewhat foxed by the equations presented ......I very much appreciate your responses, but I'll need to go slowly...Pulsing Drivers ??? flashlights??? would it be easier just to take some flashlights apart and servo actuate the switch manually.....IN THE DARK!!!!!

[rcav8r2]

Sure pulsing would be more efficient, and I'm sure if you search google you could find a circuit for one or two, BUT for simplicity sake you can power it with just a constant source...say a 9volt battery.

So using Andy's explanation ( Very nice BTW), take source voltage (9v)- the voltage drop across the LED ( 2.6) and you get 6.4volts. This is the voltage drop across the resister. Now let's use a 75ma (.075amps) current as it is in the middle of the range shown for this LED. so 6.4/.075 and you get 85.333 which is the value of the resister you need. I've been out of the loop on this sort of thing for years, but I know that this value is not a standard value. Just look for a value that is close. A lower value will give you a brighter LED., a higher value a dimmer LED. YOu want to stay in the range of 128 ohms ( 50ma), and 64 ohms (100ma)

[gpw]

Starting to make sense now...will try digging around for parts...and put something together ...will post results here asaf...thanks again!!!!!

[escapee]

Yes, lots of good info by Andy & others.
BTW, I did not mean to suggest using a pulsing circuit, those circuits make some assumptions about the battery & are not really trivial but that is why they are efficient & bright.
I would suggest using at least 2 LEDs in series as the resistor is wasting power which can be used for lighting.

Here is a (more/less) one piece equation, if you want to plug in different numbers:
(max battery volts - (# of LEDs * 2.6 volts)) / .1 amps = minimum resistor ohms

The resistor has to dissipate:
.1 amps * .1 amps * resistor ohms = minimum resistor power rating watts

The max & mins are just to meet specs which are usually a bit conservative.

For 12 volts max (8 cell pack just off the charger) & 1 LED, it works out (round #s) to 94 ohms @ 1 watt; 2 LEDs, 68 ohms @ 1 watt; 3 LEDs, 42 ohms @ 1/2 watt. Things get sloppy for more than 3 LEDs.

For 5 volts max (BEC output) & 1 LED: 24 ohms @ 1/4 watt.

BTW, a resistor dissipating 1 watt could use a little free air around it though not absolutely necessary considering what happens to the rest of the stuff in the plane.

[Dsegal]

For a complete introduction to the subject of lighting with LEDs see Bob Kopski's column in the February, 2001 issue of Model Aviation.

Dave Segal

[DanSavage]

Originally posted by gpw
Starting to make sense now...will try digging around for parts...and put something together ...will post results here asaf...thanks again!!!!!

Something that wasn't mentioned by anyone is that in addition to the correct voltage, the positive and negative need to be connected correctly to the LED in order to make them light. It is, after all, a (L)ight (E)mitting (D)iode. Accent on the diode part.

Dan

[rcav8r2]

Originally posted by DanSavage


Something that wasn't mentioned by anyone is that in addition to the correct voltage, the positive and negative need to be connected correctly to the LED in order to make them light. It is, after all, a (L)ight (E)mitting (D)iode. Accent on the diode part.

Dan


That was covered in the drawing provided earlier. Maybe not emphasized, but was there :)

[DanSavage]

Originally posted by rcav8r2
That was covered in the drawing provided earlier. Maybe not emphasized, but was there :)

I just looked again and you're right. Thanks for pointing it out for me.

Dan

[Pierre Audette]

An easy and 'crude' solution is to simply straddle the LED on a 1.5V coin battery. It's usually enough to get the flashing LED's going. You then just tape them to wing tips or tail feathers as needed.

I also used a small filament bulb connected and hanging below the motor. Of course the brightness varies with the speed control, but that's just an added feature! ;)