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Audioguru

Joined Dec 20, 2007
11,251
With the red LED's you must use 5 LED's in series and with the extra 2V and you require 20mA then use a (R=V/I R=2/0.02=100Ohms).
No.
On another website the spec's for the LEDs were shown. The "2V" red and yellow LEDs have a maximum forward voltage of 2.4V (not 2.0V) so five in series will cause NO CURRENT in the current-limiting resistor and in them.

On the other website I made many calculations with 2.0V LEDs and 2.4V LEDs and with 3 and 4 in series.

The spec's for the blue and green LEDs were also shown on the other website. Their maximum forward voltage is 3.8V (not 3.3V) so I also showed many calculations if the LEDs were both voltages.
 

elec_mech

Joined Nov 12, 2008
1,501
Try adding the Vf voltages, 5 is an illegal number. If the red Vf is 2.5 it can't go to 4 either. Those little changes in voltage matter a lot.

As for blue, if it is 3.3Vf 3 in a chain is it. If it is 3.6Vf 3 is pushing it with a 12V power supply.
Ah, I assumed the typical forward voltage was gospel and did not consider it could change. Thank you for the lesson.

12.0V - 8.8Vf = 3.2V (Red)
3.2V / 0.02A = 160Ω ≈ 150Ω

12.0V - 9.9V = 2.1V (Blue)
2.1 / 0.02A = 105Ω ≈ 100Ω
For this red LED, 2.0V is given as typical, 2.4V is given as max, so it appears you selected 2.2V, a value halfway between the two. However, 3.3V is given as standard for the blue LED which you appear to have used, but the max is 3.8V. How do you select the forward voltage to use in calculations to be safe?

I'm going slow to try to teach.
My apologizes, I thought you were working toward a design (I know you've been busy in your new role, congrats again), not teaching a lesson, otherwise I wouldn't have jumped in with a circuit. This just looked like something I could offer some help with.

Love the design. I would never have considered using the switches the way you did. Will the LEDs be affected in the least if the switches are closed and the 555 is still sending pulses? Not from an operational standpoint, but rather I'd be worried about seeing a just-perceptible dimming/brightening of the LEDs with the switches closed.

I added a 1uF and 0.1uF capacitors as SgtWookie told me they help with noise/filtering of the 555. May I presume you did not include them because there are no digital logic ICs in the circuit?

For the OP, you are using a computer power supply if I looked up the part number correctly. I would assume your 12VDC supply is going to be close to 12.0V, not 12.6V. If you were using an unregulated wall wart, that would be a different subject altogether.

I have no preferred parts source here in greater Denver, but a recommendation is very welcome. We have Radio Shacks but the too closest ones are staffed with newbies (like me).
Ah, Denver used to have a Tech America, but RadioShack bought it and in two years closed it down, alas. A quick search turns up a couple of places:

http://www.fistells.com/ - looks promising, but it appears you need to call ahead to make sure they're open when you head over.

http://www.merchantcircle.com/business/Waco.Electronics.Inc.303-289-4743 - not sure what they sell, but they may well carry LEDs. Suggest calling first.

If you're ever out in Boulder, check out Sparkfun. Look at their website first to make sure they carry what you're looking for: http://www.sparkfun.com/categories/172

Superbrightleds has some cool LEDs and I've done business with them a few times. I'd expect to check your order a few times once you place it, because they haven't been great with taking and filling an order in my limited dealings with them (server went down and lost an order, didn't get confirmation when I ordered online and when I called the person who handles credit cards was gone for the day, etc.). I'm not saying don't use them, just don't expect great service if you need parts in a hurry.

If you just want to tinker, I've used www.ledshoppe.com for really cheap bright LEDs. Note that you should probably expect 5% to be bad when you get them (these are cheap for a reason), but overall these are great to play with on a tight budget. They come from SE Asia, so it will take a week or so to get.
 

Wendy

Joined Mar 24, 2008
21,840
LEDs in a family tend to be pretty close, enough to design with at least. But until you have them in hand you really don't know what you're going to get.

If you read the tutorial I mention there is the imperative to get as many LEDs on a chain as you can, but this can have very negative consequences if you get it wrong.

Thing is, I have every resistor you can get, so test selecting is easy for me. Not so for many people though.

Resistor Parts Storage

Since this was a beginners project (but expensive in parts count) I was conservative. It is easy to be adventurous with other people's money.

I generally teach while I'm designing, in an open ended sort of way. The idea is for people to design something for themselves next time, and get in on the real fun. Generally there is no one right way to do something.

The simpleness of this oscillator design is why caps are not needed. It is one chip alone, there is nothing to interfere with it. A lot of the protoboard designs I've published are just like it, super simple but capable of interesting things.

A large part of design is looking for where things will go wrong. Do that and the things that will go right take care of themselves. People tend to oversimplify Murphy and LEDs, because LEDs are so simple. They do have hidden traps.
 

Audioguru

Joined Dec 20, 2007
11,251
On the other website the power supply voltage was measured and reported to be 12.6V, not 12V.

It should be obvious that when one current-limiting resistor value is used, LEDs with a low forward voltage will produce a high current and LEDs with a high forward voltage will produce a low current.

It should also be obvious that the amount of voltage across the current-limiting resistor determines how much or how little the current changes when the supply voltage or the actual LED forward voltages are different.
On the other website I showed that too many LEDs in series use a current-limiting resistor with a small voltage across it. When the LEDs have a low forward voltage then the current is extremely high and the LEDs will soon burn out. When the LEDs have a high forward voltage then they have very low current and are dim or no current and do not light.
I showed the same thing when the supply voltage and current-limiting resistor value were changed 5% due to tolerance.
 

Audioguru

Joined Dec 20, 2007
11,251

Thread Starter

Frank88

Joined Feb 6, 2012
12
Thanks Elec Mech. I stopped at Fistells today, they have what I need. Also I've bought some low voltage items, LED's, switches etc. from All Electronics out of CA. Everything went/worked fine (they are surplus resellers). For this project I was looking for as much simularity as possible between the LED's and wound up with Superbrights website. They've shipped my small sample order already so I should get that in a few days.

Bill, the design work is extraordinary and highly appreciated. I should pay you for it.

And AudioGuru was very instrumental in helping push this project forward. A tough instructor, I wound up understanding not to trust the wizards (ie, 1 ohm?) and with his help I was able to create my own excel wks that computes resistors for multiple LEDs (as long as I keep them even, none hanging on their own parallel line).

I'm a 1greenbar newbie over there but feel like I got special help for this hobby project I'm developing and I'm very grateful for the attention from each of you.

I have 3 more objectives, two I think will be simplier than this one and one where I would like to develop a countdown timer (from a few seconds to under 5 minutes) where a fixed number of seconds is added to the accumulation of the countdown when a different single-pole switch is thrown. And then a switch starts the countdown. More to come.

Thank you all,
Frank
 

Wendy

Joined Mar 24, 2008
21,840
I've been thinking of designing a martial arts timer myself.

At some point you may want to get into µC. The circuits are simpler, though the programming is not. Depends on how you want to do it.
 

Thread Starter

Frank88

Joined Feb 6, 2012
12
Quick follow-up: When I drop the LED quantity frpm 36 down to 30, I come up with:

Reds & Yellows: 12V power / 2.2Vf / 20mA use a 270 ohm resistor in the legs of a 10 x 3 array?

For Blues & Greens: 12V power / 3.3Vf / 20mA use a 100 ohm resister in the legs of a 10 x 3 array?

Will the circuit controlling the blinking (page 2) remain same for the reduced quantity of LEDs?

Thank you.
Frank

Heck with it, no sense in pulling teeth.

 
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Wendy

Joined Mar 24, 2008
21,840
One nice thing, the larger the resistor the more stable and predictable everything becomes, and it will require less tweaking.
 
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