LEDs, 555s, Flashers, and Light Chasers

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
Mr. Marsden

Could you modify your Fig. 8 circuit with the Two LEDs ? I need one that will work on a battery voltage of 6v to 12v and I know the 555 I.C will handle that range. Also I need a variable resistor of 50k, I have lots of them in stock, so that I can have Either One LED or use Two but can adjust the flashing rate to simulate a flame in a wood stove. Can you please include all the parts in a circuit that I can put together ?

P.S. I am the same person who sent you a P.M about this.

Thanks For Time and Help.

Jure George
Do you want to vary the pulse rate (frequency), or the duty cycle? If you just want to adjust the frequency check this thread out, I think it will do what you want...

http://forum.allaboutcircuits.com/showthread.php?t=15342

Use the formula in Figure 8 along with this chapter two adjust it to whatever you want. :)

If you want to adjust frequency and duty cycle both it gets a little harder, but not too bad. Think two 555 timers, a oscillator followed by a monostable.
 
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R!f@@

Joined Apr 2, 2009
9,918
Very nice circuits for a beginner these are.
referring to the fig 22 in post 11, is it a single led chaser cascaded, I mean one LED run's from top to bottom,if so, I was trying to find something like that, and I ended up designing one using NAND gates, your's is much simpler, never thought of using diodes.
One eye opener after another.
Thanks

Rifaa
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
This going to be quite a read and educational too.


I was at an electronics store these caught my eye.

Not being advanced enough to make up my own circuit maybe you can direct me to something simple and basic to incorporate these components and learn:
"hands on"
This site (AAC) is full of good ideas. One I've been working with another gentleman on concerns using 555's (or 556's, or CD40106's) to make 3 different LED throbbers. The idea is if done correctly it can make millions of colors.

Check this thread out...

The Fading RGB LED

I've reserved a bunch more slots to keep my article, which looks like it will evolve in a long term plan, up front. This is my working area, my blog is where the contiguous copy will be.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
Yep, you got it right. This is where I will work on the articles, and take comments on them. You want to see the pretty copy you read the blog. :)
 

SgtWookie

Joined Jul 17, 2007
22,230
Chapter 3: The LED / Resistor Only Bargraph

LEDs tend to drop a constant voltage when they are conducting. It’s not perfect, but it can be used. Take the following schematic in figure 3.1. I’ve included a schematic for a simple variable power supply using transistors and two 9V batteries if you want to experiment with them.


..........................Figure 3.1
Bill,
Did you build this circuit?
It does not work well in simulation, as the LEDs take the brunt of the current.
When the TIP120's emitter follower output is at the peak (around 17v), current through all of the LEDs is about 60mA.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
The problem is probably the assumed Vf. I've built plenty of these units, but generally I measure the LEDs individually. What is you're Vf in this circuit?

***************************

OK, I've added a blub to reflect how critical using the correct Vf is. Thanks.
 
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bakayara

Joined Dec 9, 2009
3
I've been Google-ing for what I'm trying to do and this appears really close so I'll try asking here... :D

I'm capable enough with building circuits but not really with the design... What I'm trying to do for my son's motorcycle is build a circuit that will fade-on/fade-off the LED accent lighting he already has on his bike when it's parked... The LEDs are in strips of 6 SMDs in a silicon tube with appropriate resisters to allow them to be run off a 12v circuit... there are total of 6 strips that I need to control... It looks like a timer circuit based off a 555 chip is my best bet but I'm unsure of the math to adapt the various circuits presented here to 12v operation and how to calculate values for the delays... Ideally I'd like cycle of -> 2-3 seconds of ramp up to full brightness, 2-3 seconds at full, 5-6 seconds dimming to off, 2-3 seconds off & repeat...

Can someone take a bit of pity on a non-mathematical sort and help me out with the values that I'd need? The kiddo always has a bit too much faith in the old man's abilities, but I think that I can fake it if I can get a little direction.. ;)

Thanx!
 
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Thread Starter

Wendy

Joined Mar 24, 2008
23,408
Sure, but start a new thread, and show us what you already have. You'll be surprised at the the numbers of replies you get.
 

talking

Joined Dec 10, 2009
12
Chapter 11: Making Patterns

Light chasers are cool, but sometimes you want to do more. By steering the current from the 4017 IC sequence other special effects can be created, such as this one.


...................................................................Figure 11.1

This circuit will make the LED light sweep back and forth, a popular Hollywood effect. We have also added transistor drivers that will give the LEDs 20ma without significantly loading U2, which means this particular circuit should last. You may need to add some power supply capacitors, but in general battery circuits are pretty stable without them, as the batteries share some of the same characteristics as capacitors. The voltages from 9V batteries tend to drop fast, down to 7.5 volts, and then stabilize, so be aware. The 555 oscillator will go as low as one cycle every 3 seconds, with the other end being faster than the eye can follow, so it is very open ended for the user.

Another popular design shown in Figure 11.2 is the flasher used in emergency vehicles. This can get you a ticket if you try to use it on a street vehicle, but the basic design is pretty simple.

..

.................................................Figure 11.2.....................................................................................................Figure 11.3

Of course, a design like this practically screams bright lights, so I've shown several options in Figure 11.3. Toys usually use 9V batteries, which can drop as low as 7.5, so this limits what can be done. Some blue LEDs can have a Vf of 3.8V (and 3.8V X 2 = 7.6V). I'd use single transistor in Common Collector configuration shown in Figure 11.3 (also shown in Figure 9.1) to drive individual chains. If the circuit is drawing 100ma for the LEDs, and the transistor has a gain of 50, the current pulled from the CD40XX chips is around 2ma. At 9V and a Vf of 3.8V the LED current is are getting 21ma, if the Vf is 3.5V the LED current is 22ma. At 7.5 and Vf of 3.8 the LED current is 14ma. These calculations show this circuit tries to minimize the current variance for the LEDs. This was covered in the Current Limiting chapter.

If you have a stable 12V then the options are more open. Since you can put more LEDs per chain the total current per LED is reduced a bit. The calculated current for this layout is 21ma. If the Vf is 3.5 then the current would be 24ma. Again, the variation is minimized.

But what if you want a lot of LEDs, say 100 of them (50 chains)? This would be a current of 1 amp. A transistor with a gain of 50 would use 20ma through the base, more than the CMOS IC could provide. This would be a good time to use a Sziklai pair as shown. Q2 would definitely have to be a power transistor, but other than that it is pretty straight forward. This would bring the CMOS requirement to 0.4ma, which solves the problem.

I mentioned earlier that the CD40XX ICs could go above their individual counts. The datasheet shows how to do this, as well as Bill Bowden's Website. Figure 11.4 shows how this is done.


.................................................................Figure 11.4

The number of transistors and resistors used makes the method shown in Figure 9.1 to drive LEDs more appealing, doesn't it? U3 can be repeated for even more counts, if need be.

You do not need any gating diodes when two lines from a 4017 are taken to a common point on the base of a transistor. Just use a 3k3 from each output to the base, and the transistor will turn on when either line is active "HIGH." The mid-point of the two 3k3 resistors will be "half-rail voltage" but the base will drop this to 0.6v and the base will see sufficient current to turn on.
If you wan a higher current, just use 2k2 resistors.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
I'll stand by the circuit as is. There are very good reasons for it being drawn that way, least of which is a CMOS chip doesn't have very good drive. Plus, what if you want to have 5 or 6 going to a transistor? Diodes in this application are just plain better. This is a form of DTL logic.
 

talking

Joined Dec 10, 2009
12
This circuit Chapter 3: The LED / Resistor Only Bargraph has two faults. Firstly the darlington transistor is not needed and it doesn't have to be a "power" transistor.
Secondly, the 75R current-limiting resistor does not give you very much tolerance for changes in supply voltage.
A change of 1v will alter the current through the LEDs by 13mA. Most LEDs run on 15-25mA MAX and if the voltage drops by 1v, they will dim considerably.
In addition, the 470R resistors do not serve any purpose AT ALL.
 
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talking

Joined Dec 10, 2009
12
The 4017 will deliver 10mA per output and they can drive LEDs directly, so I do not see why you have said there is any reason why gating diodes are needed.
You have also produced a circuit Chapter 3: The LED / Resistor Only Bargraph to which I have taken you to task as you have obvioulsy not tested the circuit at all.
In both cases you have not tested or tried the circuits and I have put them on my website under: "Spot the Mistake" to show others how faulty circuits are appearing on the web.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
Actually, I have, many times. In trying to explain how to design it I showed the assumptions, and how to work around them. If you don't want to read what is written as it is written, this I have no control over. It is OK to disagree how you would design something, there is usually no one correct way. You are trying to make this personal, but it won't fly.

BTW, the other circuit was built with good results by several other people on this site for their own use. I also used the same scheme that Bill Bowden used, so you aren't just argueing with me. The multichip 4017 sequencer is a tested and proven design, diodes and all.

Betcha you haven't tried building the LED bargraph. Like I said, I've build quite a few. The weakness in predicting values revolves around individual Vf and their variations.
 

Thread Starter

Wendy

Joined Mar 24, 2008
23,408
I just reread your posts, you really don't read do you?

This circuit Chapter 3: The LED / Resistor Only Bargraph has two faults. Firstly the darlington transistor is not needed and it doesn't have to be a "power" transistor.
This was meant to be a quick and dirty power supply thrown together using common parts. This was explained in the first paragraph.

Secondly, the 75R current-limiting resistor does not give you very much tolerance for changes in supply voltage.
Wasn't meant to, it is designed for the voltage the power was supplied for. The math explained this, if you had read the article.

A change of 1v will alter the current through the LEDs by 13mA. Most LEDs run on 15-25mA MAX and if the voltage drops by 1v, they will dim considerably.
Actually, this is incorrect. Many LEDs appear as bright at 10ma as at 20ma. They are very nonlinear devices where brightness is concerned. These are indicators, not illuminators. The whole point of being a bargraph is some will be off while other will be on. Voltage, once Vf is achieved, in not important. They are current devices.

In addition, the 470R resistors do not serve any purpose AT ALL.
Again, you prove you didn't read what was written, it was the setup for the following circuit, which was explained in paragraph 3.

The 4017 will deliver 10mA per output and they can drive LEDs directly, so I do not see why you have said there is any reason why gating diodes are needed.
Actually, a 4017 is rated at 6.8ma, which was explained in the transistor driver section. Reading datasheets isn't your thing either.

You have also produced a circuit Chapter 3: The LED / Resistor Only Bargraph to which I have taken you to task as you have obvioulsy not tested the circuit at all.
Depends on your definition of tested, I've built well over 20 LED bargraphs in my career, and I stated such in my later posts. Again, that reading issue. I did not build the power supply, nor did I feel the need to test it and see if it worked.

In both cases you have not tested or tried the circuits and I have put them on my website under: "Spot the Mistake" to show others how faulty circuits are appearing on the web.
A threat! Of course, you neglected to mention your website, which is OK with me. My reputation is intact, with or without your misinformation. Wonder how much of what you say will be out of context?
 
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