Modifying LED Schematic to Use 20mA LEDs Instead of 10mA for Star Trek Panel

Greetings!
I grabbed a schematic online that should make a little PCB that will randomly flash LED lights. Basically, I am building some Star Trek: TOS style panels for on and about my home bar. I have ordered some epoxy repo 'lights' and can do the panels up, but I need a simple driver to blink the LEDs. The schematic was fairly simple, and I grabbed all of the components 'EXCEPT' 10 mA LEDs. All I can find is 20 mA ones (and they are dirt cheap!). Can someone take a look at this schematic and see if there is an easy way to convert this to run 20 mA LEDs? Or is there an easier way to do it? I am going to order an Arduino board for messing around with this, but I would like something that would either run on a wall adapter, or battery (either is fine). Any help would be greatly appreciated. I'm a definite newbie at electronics, but a fair bit of it is easy to follow. Some is harder than others. Parts are certainly bountiful. Thanks in advance

Hi Zack, schematic is fairly straightforward enough.
The voltage output on the CMOS binary counter/divider is roughly the same as the voltage input. In this case you've got 5V. Using Ohm's law V=IR (Voltage = Current x Resistance) you can calculate the value of the current limiting resistor (the 1K resistor attached to the LEDs). In the schematics you have Vin=5V, Rlimit = 1K, this gives a current of 5mA. To get a limit of 20mA, you'd want a resistance of 250ohms. However I'd recommend trying 500ohms instead, giving half the light and half the power. Sure test both and see what kind of brightness you prefer. Just as long as you don't go any higher tan 20mA (the limit of the LED itself).

All you have to change is the resistor value.

I've no experience with the Arduino myself, but I'm hoping to get into it sometime. Maybe someone else can recommend something on this.

Ah, okay, thank you. So...at 5V what would my limit be on # of 20mA LED's ? The OP of that little diagram had said that it was set up for 1K resistor per 2 LEDs, which completely lost me (sorry). Here's the LED's I was hoping to use (nice and small, they'll fit behind the lenses well)
Color:White
Size: 3 mm
Lens Color:Water Clear
Forward Voltage: 3.2-3.4 V
Forward Current: 20 mA

So you're saying with the schematic there, I should be able to run these using 500 - 250 ohm resistors? Let's say I wanted 5 or 6 LED's on each leg of the counter? (which I assume run on separate clock times). Sorry if these are stupid questions, I -kind- of have a grasp on this, well at least I think I have. I have 95% of it assembled on a breadboard, sort of stuck here before I wire it all to 5V

When you say "10ma LED" or "20ma LED" you're probably talking about the maximum continuous current of the LED. In actual practice, the current in an LED is the current it is driven at. That is determined by the series resistor, as Mike pointed out, or it can be determined by current regulation (a more "exotic" way of driving an LED).

Thus, to "convert a 20ma LED" to a "10ma LED" its merely a matter of driving the LED at 10ma instead of 20ma (using the technique described by Mike).

Mike also suggested the cautious practice of driving the LED at "half power". This has merit in a circuit like this where there is no current regulation. But, I doubt there is an need to be so conservative. 75% of 20ma is probably sufficient.

But, there is one thing that Mike left out of the equation: the forward voltage of the LED. So, if the overall voltage difference between the Q outputs is 5 volts, then the formula for determining the current limit resistor is thus:

R = (V-Vf)/I where R is the limit resistor, V is the voltage across the R:LED circuit (in this case 5V) and Vf is the forward voltage of the LED and I is the current.

Now, the forward voltage of the LED depends on several things:

1. The color of the LED (also the brightness, to some degree -- such as a regular brightness LED vs a super-bright LED)
2. The current level that the LED is being driven at (small variance in terms of current)
3. Age (the forward voltage can drift with age, tending to increase)
4. Lot (i.e. the forward voltage will vary slightly between batches of LEDs)
5. Temperature (not a significant factor for you application because indoor temps have a minuscule span)

It's because of all of these factors that Mike was suggesting halving the current (to make sure the max current isn't exceeded by one of these variances)

If you look at the datasheet for the LED(s) you plan to use, it will tell you the range of forward voltages to design for -- always use the highest voltage listed for Vf and then set I to the max continuous current (in this case 20ma) (note: often the datasheet will quote 30ma as the maximum continuous current, but I usually limit the current to 20ma, regardless).

If there is no datasheet, for the LEDs you acquire, then create a test circuit with an ammeter and a variable resistor and socket where you can plug in test LEDs and use the variable resistor to vary the current to the level you want to run the LED at, and then measure it's forward voltage. Then use this formula:

R = (V-Vf)/(I * 0.75)

BTW: sounds like a fun project! And I think you would get the best result using a microcontroller (i.e. the Arduino). That way you can use a pseudo random number generator [RNG] function to produce a more "realistic" or at least interesting result. Patterns are easily recognized and if the panels "blinky light" patterns repeat it will make your panels monotonous. Also, you can program the microcontroller to cause the lights to suddenly "go wild" for short periods of time, or suddenly change their flash pattern as if something activated, or something was detected. And, if you add sensors, such as a PIR or microphone, or other such way of making the panels interactive, then it gets even more fun.

One thing I like to do is use the signal from a proximity sensor (such as a PIR) to reseed the RNG so it executes a completely different pattern (because it's using a different set of numbers). Then the RNG becomes more like truly random!

I ordered up a solderless breadboard some time ago, just waiting for it to show up, I guess when I start getting capacitors involved I sometimes worry I'm building a tiny M80 on the board. That's why I'm asking questions I did this first board mainly to test my soldering skills on such a small scale (I replaced all the capacitors on an arcade monitor recently so I am all proud of myself).

I knew this would invovle math though. :)

If I can get a working model, I can mess around with the resistors and LED I suppose, so as it stands, this little board should work, it's more or less down to using the right resistor on each set of LED's, am I reading that right?

Thanks! The only benefit to this setup over an arduino... cost! This thing is ~$4 in parts, and the arduino board I'm looking at is ~$50. However, it reminds me of the old radio shack toys that let you put wires on terminals and make a radio (etc) so I'm stoked about trying one. My friend made R2-D2 lights with one, but he's way beyond my comprehension until I get one in my hands to play around with. If I can get these little board running I can get more creative (I ordered a voice recorder board off ebay, I hope to get TOS sounds working on it...a motion sensor might be cool too.... the possibilities are endless really...

BTW the panels are going into my home bar, which already features a few lights:
www.somersetbar.ca

Blinky lights are just too cool to pass up.

Hi Zac you dont have to spend 50.00 $ on an ardiuono bourd you know you could opt for a 16F877 which give you more than enough ports available from ebay and then put pcb together your self fraction of the cost In fact check ebay first you might even get a full development pic bourd from there Or there is the amicus18 bourd now plus free ide available from www.crownhill.co.uk. Or if you fancy a bit of challenge then use 16F88 and two 74LS373 this will give you 16 bit output

Mark Harrington

Heh, I don't mind the $$$ spent, but to sort of waste it on a blinky-light bit of artwork seems a bit wasteful. The thing about the arduino is there is so much support for guys (like me) that didn't take electronics (dammit). 'a challenge' to you might be a 'lets just set it on fire and have a beer' to me :)

Besides, it remains to be seen if I can even get this little board together and working!

But, even if he goes with the 16F877, he still needs to get a programmer, such as the Pickit 2 or 3 which will set him back $40 to $50 bucks -- but, once bought, it can be used for PIC projects in the future.

Then there's the development environment. Check out MikroElectronica -- they have free software that is fully functional for small projects. They have Basic, Pascal and C. They also support various other microcontrollers such as Arm, Avr and 8051. And the Pickit 2 or 3 will work with the Mikroe IDEs

http://www.mikroe.com/

You can also go with the Microchip MPLAB IDE [free] and one of their C compilers [the "lite" versions are free] But, that system is harder to learn because it's a lot more complex. The Mikroe system is far more "entry level".

Yup, it's all in the resistor.

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Hi Steve there are sites on the net which show you how to build a programmer The 16F677 also has isp "in circuit programming facilty ". There are IDE,s available as well now also free for download which include bootloader code used to enable isp programming No one minds supplying a chip with ISP code its no big deal to put this into the chip these days simply build you board with a low cost level convertor circuit Not too difficult Theres Stack of support for microchip as well

Microchip themselves are extremely helpfull Theyve even made long distance phonecalls to myself to provide assistance in the past

Ive got no complaints about microchip at all

Atmel Not so helpfull at times

MD Harrington

Yeah, I know, but I figured that would all be too complicated for Zack since he's confessed being challenged by the CMOS circuit. Also, even with ISP you need a programmer (such as the PICkit).

But, you have a point with all the support that exists for the PIC! So, maybe it _is_ a go ;)

And, a PIC with bootloader already installed sounds like a way to go... which brings to mind the PICAxe:

* http://www.picaxe.com/
* https://www.sparkfun.com/categories/125
* http://www.solarbotics.com/catalog/microcontrollers/picaxe/
* http://elecurls.tripod.com/picaxe.htm

or the BasicStamp:

* http://www.parallax.com/tabid/295/Default.aspx
* http://www.digikey.com/product-search/en/inte...ler-or-microprocessor-modules/2556371?k=basic stamp

So how would these compare to an arduino? They definitely look similar, however it's been 25+ years since I tried to learn any pascal, and probably 30 since I wrote any basic lol. Just how 'good for beginners' is this?

I was definitely disappointed by my last visit to 'the source'. My how radio shack has died. The electronics section was one tiny wall divider, everything else was cell phones! Thank god for the internet or these sort of projects would be basement and back alley at best!

I haven't played with the PICaxe, but I've read about it. Looks VERY simple. There's a "Starter Kit" but bare minimum you need the following:

* A "Download Cable"
* A stereo mini-jack (also called "Download Socket" by the PICaxe people)
* A PICaxe (there are several varieties).

The manual [sort of] shows you how to connect the Download Cable to the PICaxe -- just match the cables Tx, Rx & Gnd [or "0"] leads to the pins on the PICaxe with the same names.

The basic language used by the PICaxe is _very_ simple. Here's an example of a simple Basic language program for flashing an LED on and off at 1 Hz (from the manual):

bq). The following program switches output 4 on and off every second. When you
download this program the LED should flash on and off every second.
main:
high B.4
pause 500
low B.4
+
pause 500
goto main
This program uses the high and low commands to control output pin 4, and uses
the pause command to make a delay (1000 ms = 1 second).
The last goto main command makes the program ‘jump’ back to the label main:
at the start of the program. This means the program loops forever. Note that the
first time the label is used it must be followed by the colon ( symbol. This tells
the computer the word is a new label.

Well after a fair bit of trial and error, I gave up on this board, I couldn't get the little board to work, and after talking to my buddy that makes up the R2-D2 light packs talked me into grabbing an arduino kit, his reasoning was good...I can take the main unit out of the panel and plug it into something else, or run other things in the bar. I'll still post some updates as I get the parts in place and the panel built. Who knows, I might not have any more luck with the new board

The PICaxe chips are cheaper, but the Arduino might be easier to use, and since your friend is familiar with them, and thus can likely help you, that's probably the way to go

I have started playing with Picaxes in the last year and I would recommend them as by far the easiest microcontroller to start on.

The PICAXE microcontroller is a PIC microcontroller that can be programmed using the BASIC (Beginner's All-purpose Symbolic Instruction Code) language (as used in schools), thereby making it by far the easiest chip to program and use.
It is available in versions from 8-pin to 40-pin, just choose how many inputs and outputs you need.

The only necessary hardware is the USB programming cable (about £12) and a breadboard with a couple of sockets and resistors. All the necessary programming software is available as free downloads.

In the UK:
Information: rev-ed.co.uk
Supplier: techsupplies.co.uk (= shop for rev-ed)
Technical Support Forum: www.picaxeforum.co.uk

In the US:
http://www.phanderson.com/picaxe/ Supplies both the chips and helpful information.

Frank is correct about this picaxe is by far the easiest to start with since it utilses basic and unless you have done any c then id stick to pic and pic basic C can be difficult although a very powerfull langauge mistakes are easily made

You would also have to understand various other parts of c such as pointers, pointer maths, pointers to pointers, unions, structures, templates, pass by reference, pass by value and other difficulties with C not highly recommended if youve havent programmed for a while or are new to the subject


Plus of course there is the assembler side of the story as well which is a good starting point simply to get to grips with hardware control and debugging your code not to much of a challenge with Pic micro but arduino difficult but can be done

However not easy when writing lengthly code nevermind integration with MP lab or other Ides such as eclipse and which eclipse version to use, what you have to download, how you set eclipse up and any other langauges you might also need to install just to get the ide to run etc

Thats excluding updates etc

I remember it took me some 2 years and more and i still find this difficult now

Im now into year 15 with this and many other topics encompassing IT

Although I am self taught though due to exorbitant costs of training never the less It can be done despite all manor of difficulties

Example of minor easy to understand differences

In Basic testing equality is just simply "if x= SOMEVALUE" where as in C you can write the above example or the correct example which is

"if x == somevalue"

Both will compile in C.
The difference being of course let x = SOMVAL one equals sign and boolean equality == double equals sign

The latter method will assign a value to variable whist boolean tests equality of a variable

Also whilst on the subject of variables or containers you have to test in c for unsigned and signed You dont have to in basic i.e you cannot assign a result of an expression or function to unsigned container in c if the result is negative You can ignore this in basic Pic basic takes care of this for you Can be an advantage could be a disadvantage but at your entry level into this subjuct given your overall experiance not recommended

Hence Picaxe easy choice Pic basic easy choice

C for experianced programmers once you have got the basics right

Mark Harrington

No question there seems to be a ton of stuff for PICaxe, it just seems like there's a lot more support for the arduino out there once I started looking, and yeah, it looks easier for dumb newbs like me.

HI ZAC
Guess what ive just found for you after lots of searching bingo This I think will solve your problems

http://profmason.com/?p=218 go check this out It get you started at least

Mark Harrington

Truly, the C language is more "esoteric", but you can "dumb it down". It's not necessary to use the more advanced features, such as pointers. You can get away with a more "Basic like" syntax. For instance use _arrays_ instead of _pointers_.

C has all of the branching structures, such as _if_ and _if-else_ and _switch_ -- looping structures, such as _for_, _while_, _do-while_ and even _goto_. The names may not be the same, but the concepts are similar enough to be accessible. One merely needs to learn to use brackets rather than words to contain a statement.

For instance:

[code lang="c" line="none"]
// Basic:
IF b1< 3 THEN
c1 = b1 + c1
ENDIF

// C:
if (b1 < 3) {
c1 = b1 + c1;
}
[/code]

To get around the type checking issue, just always use signed integers. Yes, it's inefficient, but for this level of usage, who cares.

They would also have to learn to put semicolons after every statement. But, the bottom line is, just because C has things like pointers and structures and such, they don't have to be used in order to get things done. I mean, all the guy wants to do is blink some LEDs.

BUT, don't get me wrong, I still think something like the PICaxe is the best way to go for a beginner, but if other factors, such as having a friend who is versed in a different technology, I'm just saying that C can be used at a beginner level -- not as easily as BASIC, but it can be done.

I forgot about the complication of case sensitivity, so that would add yet another layer of difficulty for the beginner, but just do everything in lower case.

Also, “if x == somevalue” will not compile in C. This is the correct syntax in C:


[code lang="c" line="none"]
if (x == somevalue)
[/code]

In other words, you need the parenthesis, whereas in BASIC you don't.

So, I've been doing a lot of "ragging" on you, Mark, so permit me to take this opportunity to say.

Excellent find!!!

No worries Steve lol
Im not always quite presise sometimes or dont write subjects as I should do sometimes I think thats what the problem is

If I were speaking to you youd know exactly what I was talking about “live in other word” but sometimes I dont quite express on paper what i actually mean I should have put this into brackets your right

Example using boost c compiler in MPLAB

[code]
if(x== 2)
{
// equality test will compile is object a equal to object b

}

if(x=2) // one equals sign not equality testing
// but always returns true since x=2 equivalent to
// if(true)
{
// THIS CODE ALWAYS EXECUTES ABOVE CODE EQUATES TO
// IF AND ONLY IF
// will also compile but assigns value 2 to x
// as in basic let x =2

}// both compile both entirely different meaning
// so can be highly confusing

// basic code

if x=2 Then

' do events ; will compile equality testing

end if

[/code]


Not to worrry I shall get it right

Hi Zack!
If you have a Arduinoboard try it out it is really easy starting point. Think you will find it fun and there are help and LOTS of programming examples. It is easier than programming that old BASIC used 25 yrs ago. It use its own language which is a simplified C type of programming. Think that you will be blinking LEDs within half an hour.
Good Luck

Okay, that is something I can follow, it's not much different from the board I put together (from my original post). I actually made a second copy of it today on a solderless breadboard. I have everything lighting up, if not blinking randomly :)

I do have an arduino board coming, but I do like the simplicity of this (except the programming part). I think I need to take baby steps here, but all of this is pretty intriguing. This year I made a DIY CREE LED lighting system for my home reef tank, and since that has had success I have been amazed at the possibilities of the future of LED lighting.

Cool!

And I'm the other way around -- I talk like an idiot but sound intelligent when I write