LED flasher circuit resistor question

SgtWookie

Joined Jul 17, 2007
22,230
Stick this in your favorites/bookmarks:
http://www.qsl.net/in3otd/parallr.html
Very handy for figuring out which resistors you can use in series/parallel to get a certain value of resistance.

This page is handy, too:
http://www.logwell.com/tech/components/resistor_values.html
Standard decade tables of resistance values. E12 and E24 are the most common values. You can get the E48 and higher values, but you will usually pay a good bit more for them, and have to order them.

Capacitors are typically available in just the E6 values.
 

Thread Starter

danleigh57

Joined Apr 21, 2010
31
A question then off to wish mom a happy day.

Working on the back half of this circuit, before trying Bill's idea for flushing the delay out of the system.


I initially hooked up 8 LED-resistor pairs without the transistors, and while they would count up and latch, the only method I found for unlatching them was to turn off the switch and wait for them to count back down. I surmise this is the 'store' part of shift and store. I didn't see an obvious reset function (acknowledging that I would know a reset if it bit me...)

While looking for a method to reset the registers I came across the 4015B chip. Since the function I am looking for is 4 stage - 1, 12, 123, off, four registers is plenty and dual 4 registers maybe better. Since this chip has a reset function, and assuming I can initiate the LED's on high, this seems like a better option.

Feel free to tell me where I am wrong, and I will back up and take another run at the wall.
 

SgtWookie

Joined Jul 17, 2007
22,230
For the 4015;
Connect the memory reset pin to the Q3 output using a 47k resistor.

Connect a 10nF to 0.1uF resistor from the memory reset pin to Vdd (+V).

Connect the data in pin to Vdd (V+).

Connect the CK (clock in) pin to your 555 out.

Q0, Q1, Q2 will turn on in sequence. When Q3 goes high, the IC is reset (all low).

The resistor and cap ensure that when power is turned on, all outputs go low.

Wire any unused inputs to Vdd or ground.

See the attached.
 

Attachments

SgtWookie

Joined Jul 17, 2007
22,230
You won't be able to use that array design. You need separate current control for each string of LEDs. Otherwise, if one string fails, you'll wind up with the Chernobyl effect.
 

Thread Starter

danleigh57

Joined Apr 21, 2010
31
Thanks for the diagram, that will be a big help.

I got mixed replies on the resistor thing. I assume you mean I need a resistor for each leg of each array. That's the way i was leaning.

How about the transistor pair (I'm sure it has a name, but it escapes me now.) Is there a better/simpler way to accomplish the same task? I think it was the transistors being backwards from each other that confused me. I'm pretty sure I wired them correctly, but was worried about trying to do it correctly a half dozen times.
 

SgtWookie

Joined Jul 17, 2007
22,230
If you have a regulated voltage source, you can use a number of transistors as current mirrors - providing that you use PWM so that the transistors have a cool-down period. Thermal bonding of the transistors is also a good idea.
 

SgtWookie

Joined Jul 17, 2007
22,230
Here's what I'm talking about. See the attached.

V1 is a regulated voltage supply.
VR1 adjusts the current for ALL of the LEDs.

Note that I have a different mix of LED's in each string; they have a different Vf, yet the current differs by only a small percentage.
 

Attachments

Ron H

Joined Apr 14, 2005
7,063
Here's what I'm talking about. See the attached.

V1 is a regulated voltage supply.
VR1 adjusts the current for ALL of the LEDs.

Note that I have a different mix of LED's in each string; they have a different Vf, yet the current differs by only a small percentage.
Even small Vbe mismatches will result in relatively large current differences. A delta Vbe of only 5mV will result in in a current mismatch of about 20%, IIRC.
Some low-value emitter ballast resistors will help equalize the currents.
 
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Thread Starter

danleigh57

Joined Apr 21, 2010
31
Lets see if I follow.

I'll need a 78xx regulator The transistors unconfuse things a bit.

Guess I'm not getting it all. What are Q(x)c, how do you thermally bond the transistors, how am I, or will I, use pwm?

And I don't follow this part of your diagram:


I apologize for being dense. :rolleyes:

Oh, and edit: is this circuit for each of the three outputs? Once I get my head wrapped around it and parts ordered I'll try to draw something up.
 

SgtWookie

Joined Jul 17, 2007
22,230
I'll need a 78xx regulator.
VR1/Q1/R1 must have a voltage regulated supply; the voltage between the upper side of VR1 and the lower side of R1 must remain constant.

The transistors unconfuse things a bit.
That's good. :)

Guess I'm not getting it all.
What are Q(x)c,
Those are "node labels". It is a way to give meaningful names to the lines on plots when a simulation is run. A "node" is a signal name in a Spice network list. By default, node names in LTSpice are "Nnnn", where "nnn" is some number from 001 to 999 - or higher, if you have that many nodes.
how do you thermally bond the transistors,
You can epoxy their flat faces to a strip of copper or aluminum. Big Orange and Big Blue hardware stores carry aluminum angle that would work well.
how am I, or will I, use pwm?
PWM is Pulse Width Modulation. Such a circuit can be made from a 555 timer and a few commonly available parts.

And I don't follow this part of your diagram:

VR1 is a 500 Ohm potentiometer, aka "pot", wired as a rheostat.
A rheostat is a variable resistor with just two terminals.
A potentiometer normally has three terminals; two end terminals and a "wiper". The ends of the pot are connected to two different voltage levels, and the wiper, which is controlled by a rotary shaft or screw, can select any voltage potential between the two end terminals.

Oh, and edit: is this circuit for each of the three outputs?
I've simply drawn it as one would control multiple LED strings. I have no idea how many LEDs you plan on driving, or what the specifications of your LEDs are.
Once I get my head wrapped around it and parts ordered I'll try to draw something up.
It would be a good idea to decide on the LEDs that you are going to use first. That will drive the rest of the design.

You should get LEDs that are a red-orange color, 620-626nm wavelength.
They should have a wide focus angle, at least 65°. 90° or more would be preferable.

You will have to figure out a way to dissipate the heat that the LEDs will generate. However, you have to figure out which LEDs to get first.
 

Thread Starter

danleigh57

Joined Apr 21, 2010
31
Trying to take in all the information/advice I've gotten and come up with a plan with the least moving parts - especially in the elements.
Don't know if this makes sense or not, and I suppose I still need a regulator here somewhere.

I welcome criticism as a learning tool. Of the design anyway. I am a sensitive soul. Ask any of my ex-wives.
 

Wendy

Joined Mar 24, 2008
23,415
That would work, but it is not a good design for cars, due to the variability of auto voltage. Figure around 12.0V to 13.7 V, which is why the original current regulators were a good idea. You can still use PWM on them for what it's worth.

The articles I recommended in post #25 cover a lot of that ground.

Another way is to reduce the number of LEDs from 4 to 3 per chain, and add another chain. That way variances in the voltage will affect it less, since the resistors are proportionally larger.

Pins 2/6 need connected to C2, it is a dead circuit the way you drew it.
 

Thread Starter

danleigh57

Joined Apr 21, 2010
31
That would work, but it is not a good design for cars, due to the variability of auto voltage. Figure around 12.0V to 13.7 V, which is why the original current regulators were a good idea. You can still use PWM on them for what it's worth.
By current regulators are you referring to SgtWookie's transistors, or something on the original design?

I went and did some math instead of using calculators, and I think I get a glimmering now of the effect of the voltage changes on LED brightness, if nothing else. I have a better feel for regulating voltage of the LED supply. I haven't had luck finding a low drop out regulator for my wish list yet. Then again, if I source different LEDs I can get by with 10V.

I am also wondering about the brightness of the LEDs. The mcd ratings are all over the place, and I don't really know what they mean anyway. I have no reference. I am sure these lights will not be too dim. I don't want them to get too bright.

While I understand the function of PWM in controlling the brightness of the LED's, I am unsure of what you guys mean in this context. Does it call for an additonal 555 chip? And if so, why? I always feel thick when I have to ask a question more than one way. Means I can't grasp what I'm being told.


Oops. Got lost in the cut and paste. Fixed now. Thanks for the catch.
 

Thread Starter

danleigh57

Joined Apr 21, 2010
31
C4 needs to connect +12V (pin 8 on the 4015), not pin 7.

Shouldn't it connect to 16? Vdd? The Vdd - Vss thing still confuses me. And of course Vcc.
Google is my friend.

Unless 4015 and 4015B are different. Or I've read something incorrectly. More things to watch.
 
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