LEDs, 555s, PWM, Flashers, and Light Chasers Index

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Posted 09-20-2009 at 04:39 AM by Bill_Marsden
Updated 10-10-2009 at 12:04 PM by Bill_Marsden

LEDs, 555s, PWM, Flashers, and Light Chasers

I'm moving this article to blog format to allow me greater flexability in editing. Please, do not comment in the blog, if you do I will delete it. If you want to leave a comment do it here.

One of the most common requests at All About Circuits is various methods of flashing LEDs. I'll try to show most of the techniques used for this purpose that have been covered on this site, explaining how and why along the way.

Index

1....LEDs
2....Current Limiting
3....The LED / Resistor Only Bargraph
4....The 555 Integrated Circuit
5....The 555 and PWM
6....Low Power Applications
7....The Joule Thief
8....From Four, Twenty
9....Light Chasers
10.l.Transistor Drivers
11.l.Making Patterns
l.....Conclusion

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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.

Posted 09-29-2009 at 01:39 PM by Bill_Marsden Bill_Marsden is offline

Updated 10-10-2009 at 11:48 AM by Bill_Marsden

Conclusion

LEDs are among the more fun circuits to build. They are easy to construct, give instant feedback when they work, and can be tweaked in many different ways. Your imagination can take these basic ideas even further. The field is still advancing very quickly, future models of LEDs will probably replace light bulbs, and we'll be building circuits to make them do wild and crazy things right along side. If you are interested in the history of these LEDs, I would recommend the online LED Museum.

Posted 10-10-2009 at 11:47 AM by Bill_Marsden Bill_Marsden is offline

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