a project...

[jrz126]

I'm currently going to Penn State for EE (I'm going into my 3rd year, so I have a pretty good concept of stuff). I'm working on a project (not school related) involving 62 led's and my car. https://www.cardomain.com/memberpage/435572/2 here's some pics of my old version. I had some problems with my circuit and the LED's just arent bright enough, so I'm gonna re-do it.

This time I'm going with some 74hct299 shift registers to drive the uln2803's. I chose the shift registers because I'm hoping to get a microcontroller hooked up to it some day. (and I got 360 of them off ebay for $4.95). I'm also strongly concidering learning how to make my own printed circuit board. I'm still going to use the lm3915 to drive the clock for the S.R.'s and then I'll go with a 555 to drive the serial data in. It should give me the 'random' lighting I'm looking for.

Now being a college student, I'm working with limited funds here. I need some 2 pin female connectors to connect to each led. anyone know of a good source for these? I found some that will work from digikey for about $20.00 (it's for 100 connector housings and 200 crimp pins.

Also, does every LED need it's own current limiting resistor? I think they do, because I didnt have one for the old project and I had a prett high falure rate.

Let me know what you think.

EDIT: sorry, this is in the wrong section, I just found this board and I had to get my post up

 

[k7elp60]

I have found rather that use connectors for lots of LED's it is easier to solder wires to the leads and cover the joint with heatshrink tubing.
As far as current limiting resistors for each LED, it depends upon the circuit.
I assume you are using the 2803's to drive the LED's, and thus you are using a common + buss for the LED's.
How are the LED's connected to the 2803's and how many 2803's are on at the same time? If all the LED circuits for one 2803 are the same and only one driver is on at a time, you could get by with only 1 current limiting resistor for that driver. Or if more that one driver ckt is the same and only 1 driver is on at a time you could get by with only 1 resistor.
You can also parallel LED's if they have the same Vf and have similar IF,
and get by with only one resistor per configuration.

 

[jrz126]

for the leds i have in there now, I soldered them and I used hot glue on the leads. I'll probably just go the heatshrink route. it'll be much cheaper. I dont have the specific datasheet for these led's, whats the typical soldering time before I have to worry about damaging the LED's (I'm using a cheap Home Depot iron)

Also, for the led's in there now, I have 20 of them in parallel with one +5 reg. (3 vregs total). the Vf of those leds is pretty high, I think it was around 4.75V so I just have a .1 ohm 5W resistor in series with each v-reg (there really wasnt much calculation involved with this, so this probably contributes a lot to my dead led problem).

I'm going to have the Shift registers connected in series, the Dsr of the second SR is connected to the q7 of the first, then i'll shift in a random stream of data, so pretty much any led could be lit at any time.



Would it be safe to assume that each LED has the same Vf and similar If? They are from the same manufacturer.

 

[k7elp60]

These are typical forward voltages for LED's.
These were determined by experiment. I put a resistor approximately 1K ohm in series with the LED, and adjusted the power supply until the specified current was flowing, then I measured the voltage across the LED.
I have sucessfully paralled red,yellow and green in strings of as many as
50 LED's. I would determine the forward voltage for sure before you calculate the value of the resistor. The actuall current for each LED may vary a little with the specified forward voltage, but that shouldn't matter.

The more the current the brighter the LED. But there is a limit to the maximum current before the LED will fail. I have some superbright LED's that are extremely bright with only 10 to 15 Ma of current.
Hope this helps.