Afternoon all,

I am currently working on a project where I need to control various Leds and I could do with a bit of technical assistance.

The outline is as follow.
I will have 22 objects, that each has been embedded with two strands of fine 0.2mm coated coper wire. Onto this I have soldered 7x 0.25mm White Leadframe-Based Surface Mount Chip LED's in parallel. (This took some time, he he)

http://www.farnell.com/datasheets/80903.pdf

Due to the size and nature of this project there was no space for resistors before each led, so I hope that I can get away with it. Each unit or object embedded with 7 Led's works fine when connected to a 3v (2x1.5v) power source.

My main aim is to control or trigger each object so that all 7 leds should receive power at various intervals. Ie when all are placed together to form a random pattern consisting of 7 leds each.

To achieve this I am proposing to connect a switching transistor to the ground lead of each object, where each trigger pin on the transistor is connector to a output of a Arduino micro controller.

Thus in theory the Arduino can complete the switch for each object to close the power circuit, thus triggering the on/off of the leds. The leds will mainly be blinking at various intervals and won't stay permanently on for too long.

This is where I need some help to determine what transistor I should use and some advice on the general setup. Any assistance would be much appreciated!
I would like to use a transformer to power the setup as batteries might drain fairly soon under this constraint. I am fairly new to working with the Arduino, but I am sure that this basic stuff.

Thank you in advance!

 

You need the resistors friend.

You will soon experience thermal runaway. The resistance of an LED changes as it gets warmer, causing it to draw more current and get warmer still. This is an loop that ends with you buying new LEDs AND resistors.

Have you looked into SMD resistors? The 0603 are about the size of a hyphen ' - ' If you cant find room for that, you will need to make it. Route the trace to an area where you have room, or simply make the resistor part of the LED. solder it directly to the anode. (or cathode)

If you are using PWM to light the LEDs, you may get away without the resistors for longer, but you will be killing them much faster than by not using resistors.

LEDs are CURRENT controlled devices. Resistors are 'current LIMITING' devices.

I dont consider it an option.

 

Retched is right on. You might be able to share a resistor between several LEDs, but the results won't be satisfactory. There has to be current limiting in there somewhere.

A CMOS 555 operating off 3V limits current, but the moment you increase power supply voltage pop goes the LED.

LEDs, 555s, Flashers, and Light Chasers

 

Thank you for the advice! I know that i should use the resistors although I am a bit stuck now.

My challenge is that the object is accentually knitted with 2 strands of wire, thus each stitch is about the size of a 0603 led, this makes it tricky to add a resistor. Except if I solder the 0603 led to the 0603 resistor and then to the stitch. The whole piece is then casted into a resin block. Not sure what this would do to the temperature of the leds.

My problem is that I have already made 9 objects, ie soldered 63 0603 leds and casted the object into resin. I take there would be no advantage that I only want the leds to flash and not stay on permanently. I could maby add a longer delay in the "flashing state" between these that I have made without the resistors, at they might cool down in between "flashes"

I might connect one to a 3v batt and see how long the leds will last, as I cant change any of the circuits on the 9 that has been casted, do you think that because they are casted into resin, would it help with the temp.

Ok, so I will have to do the rest with resistors. If I can salvage the 9 and use them maby with a 10 ohm resistor in Series with the the power source it might help? then what size 0603 resistors should I then use if I add them directly to the 0603 leds. Would a 15ohm do behind each led.
Then on adding a 2N222 switching transistor, what size resistor should I add infront of it? 3K3 resistor.

Thank you for all your help, it really helps me!

 

You have a 3v Vcc, and the Vf of the leds is 2.85 typical for 5mA current. That means you need a (3v - 2.85v)/5mA = 30 ohm resistor per led.

It says 3.15v max Vf, however, so that leds won't turn on at all on a 3v supply.

Did you consider buying a prebuilt led bar designed for some particular voltage?

 

You take your voltage(V) and subtract the LEDs forward voltage(Vf)

9v - 2.2 = 7.8

Then you multiply that number by the current you want to use. The answer will be in OHMS. If the number is not a standard resistor value, use the next one UP.

So if you want 15mA: (15mA = .015)

7.8 x .015 = 117ohms

117 isnt a 5% value so use 120ohm (close enough)

 

Gents,

I have taken all aboard and I think that I will have to bite the bullet, do it right and start from scratch.
Seeing that I expect this to be working for a week on display.

First a big thank you for all your comments and Please review my proposed solution and help me before I end up messing it up again.
Diagram

SMD LED 0603 Details
http://www.farnell.com/datasheets/80903.pdf
2.85 Vf
5mA

SMD Resistors 0603 470R
http://www.farnell.com/datasheets/328504.pdf

Then the 2N222A. Would the following Do?
http://www.farnell.com/datasheets/328504.pdf

All that is then left is the resistor before the 2N222A.
Any advice on that size should be appreciated.

Each array should draws current of 35 mA from the source, there will be 22 so I would have to make sure that my 5 Vcc supply is capable of 800mA or 1A to be safe. I have changed to 5v as then I can power my Arduino and the leds off the same power source. Ie a phone charger or adapter with 5v and 1A

Thus I will end up with 22 objects, with the capability of enabling them for brief moments via the Arduino.

Thank you again.

PS: I ended up calculating the resistor size via the online tool at http://led.linear1.org/led.wiz hope this is fine, else I might look at retched's calculations.

 

That tool uses the same calculations (its OHMS law).

That will work. It is good to heat that you decided to do it the right way.

 

Thank you to all for your input.

Loads have changed from the start of the project, and its been a good learning experience for me, even if most of it is improving my soldering skills.

In the end I did solder the 0603 Leds to 1K2 0603 resistors, connected these all in parallel tunning of a 12v supply. Switching is done with a logic level mosfet.

Now I just need to work on the controlling of this. Started playing with PWM, but will start a new thread, as thats a beast of a different kind, as I dont think the Arduino can Control multiple pins (22), at different duty cycles.