# LED Name Display Project Box

Discussion in 'The Projects Forum' started by sloetyme, Dec 12, 2010.

1. ### sloetyme Thread Starter Member

Jan 13, 2009
11
0
Glad to be back! Sorry for my long absence. Hope everyone’s Holidays are going well!

I tried to reply to my post that was already started, but it had been 91 days old and not allowed to post there. So, I'll try this again, and the ideas were great there.

So, I want to build a name sign display to wrap up into a project box. Meanwhile, looking at the complexities with this project, I had found myself with very little knowledge to tackle this. Although, I have forced myself to learn more (because I love electronics) about components needed to conclude this project.

To make this short, I’m gonna use a safe (from what I have researched, correct me at will) amount of specs to complete this project. I have written them below to see if it even makes since (LOL) along with pics I have made with tracks. All help is and will be considered.

What I have come up with:

1. L.E.D. Blue Vf = 3.4
mA = 10

Now for the math:

Originally I needed 47 leds to flash, now only 45. So, below is what I know now. Source will be 12V (wall-wart or x--xx--xxx Cells). 555 timer to flash on/off for one second each still working on this. I chose the IRF510 as the driver portion still working on this too. Still under construction and pardon the mess.

I will need 22 chains of 2 leds with a resistor.

LED + LED = 6.8v (Source – 6.8 = 5.2v)

5.2v / .010mA = 520ohms 560ohms

5.2v X .010 = .052watts ¼ watt tolerance

And 1 LED with 1 resistor.

LED = 3.4v (Source - 3.4 = 8.6v)

8.6v / .010 = 860ohms ≠ 1K ohms (E12 values)

860 = .086watts ¼ watt tolerance

Two Main Questions:
Am I on the right track? Don’t be afraid to let me know where I’m at.

Where should a resistor go in a series? Cathode or Anode and whether it’s with a mosfet or not.

Thank you.

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Last edited: Dec 12, 2010
2. ### bertus Administrator

Apr 5, 2008
15,798
2,384
Hello,

Your calculations seem to be OK.
You could also use 15 strings of 3 leds.
Then you would have a resistor of (12 - (3 * 3.4)) / .01 =
(12 - 10.2) / 0.01 = 1.2 / 0.01 = 180 Ohms.
This would work fine if the leds are really 3.4 Volts.

You can also take a look at Bill_Marsdens thread on 555 and leds:
LEDs, 555s, Flashers, and Light Chasers

Bertus

3. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
LEDs, 555s, Flashers, and Light Chasers (blog, current edition)

Chapter 10 - Transistor Drivers

What is your power supply voltage again? As long as it is 12V I would recommend the MOSFET circuit, but less than that you should go with a conventional (BJT) transistor. Radio Shack sells both, though their MOSFET (IR510) is not among the best.

4. ### sloetyme Thread Starter Member

Jan 13, 2009
11
0
Here the manufactureers specs for these leds:

5MM waterclear, blue, 465-470nm, 280-420mcd, 2. 8~3. 4v, 20mA.

This all I have and I was going with 10mA just for safety. I will try to set it up with 3 leds and a 100ohm resistor in each string @ 20mA, and see what happens.

((12v-(3.4*3) (1.8v/.020mA) = 90ohms ≠ 100ohms )

Thank you for the input.

Last edited: Dec 12, 2010
5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,900
874
The OP has stated two different current requirements for the LEDs.
and
Double check your calculations using a current draw of 20mA... (It's not 20mV)

6. ### sloetyme Thread Starter Member

Jan 13, 2009
11
0
Thank you Bill for your response and the links, I am referring to your knowledge here...

Yes I will be using 12v for the source.

I am trying to figure out now having a 300mA draw just for the leds, and what ever I'll need for the 555 pulse circuit, whether I can just use a 12v 500mA wall-wart for this. Do I want to connect 555 and leds with 12v or keep them separate?

What I know:

Leds will be powered from the 12v via resistors for each string, then all the grounds connected to the drain on a irf510. But figuring out the gate resistance is a tough one for me.

Last edited: Dec 12, 2010
7. ### sloetyme Thread Starter Member

Jan 13, 2009
11
0
Sorry....Typo's on my part...I know what it is and typed it wrong. The 10 mA was just to be safe, but I have found that it can be 20mA. Corrections were made.

Thanks

Last edited: Dec 12, 2010
8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
There is no reason to separate voltages, the 555 can control the MOSFET directly (don't forget a small gate resistor). It is actually a simple circuit. Anything unusual about the flash pattern?

Wall warts can be unregulated. Be sure to check the voltage across the circuit with it operating.