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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 6: Low Power Applications

While the 555 isn't a power hog, it is a product of the 70's. It has 15KΩ resistance, not counting the rest of the circuitry. It will drain a battery very quickly, in days if not hours. Several manufacturers have come out with low power CMOS versions, such as the TLC555 and the 7555. These parts are pretty similar to each other, though not exact. They can both drive an LED going to ground (low), but have about 10% the current capability going to Vcc (high). As the power supply voltage drops the current they can provide radically reduces, so with really low voltages you will have to use a transistor to light an LED to full brightness. On the other hand the CMOS versions draw about one hundredth the current for its internal circuitry, so they definitely have their uses.

Figure 6.1 shows some low power long duration flashers.

.................................................. ..Figure 6.1

Oscillator #4 uses a capacitor voltage multiplication to boost the 3V from the battery to almost double that, enough to drive the 3.5V Vf of the blue LED. The Schottky diode drops a fraction of what a conventional diode does, or a Germanium diode could be used for much the same reason.

Capacitor C2 was added after experimentation showed that it was necessary for maximum life. Without it the circuit basically dimmed and died after two weeks, using AAA alkaline batteries. Adding the capacitor extends the flash life, my test circuit has worked more than 3 months using AAA batteries. This is because the circuit is only on 3% of the time, the remaining 97% the capacitor takes on a charge. I suspect this is a unique case, but it is interesting.

Posted 09-29-2009 at 01:31 PM by Bill_Marsden Bill_Marsden is offline

Updated 10-10-2009 at 11:52 AM by Bill_Marsden

Chapter 7: The Joule Thief

The classic Joule Thief uses transistors. The basic principle, using an inductor to kick the voltage from the battery up until it will power an LED has also been applied to the 555 also. Figure 7.1 is a redrawn schematic, the original source was uploaded on another thread.

.............................................Figure 7.1

The 555 has been so useful over time that a dual version, two complete 555s, have come out. They also have their CMOS versions. I applied this to the following schematic.

.................................................. ...........................Figure 7.2

These schematics use a feature that hasn't been shown to date. Pin 4 is an Enable pin for the 555, it is possible to use a 555 oscillator to control the second one, the voltage booster. This design works, and should make a battery or two last a very long time, but it could be improved quite a bit. Using two batteries to make 3V improves the brightness of the LEDs substantially. You may notice there is no current limiting resistor. This is because at 3V there simply isn't enough voltage to turn the LEDs on, all the current driving these LEDs is coming from the inductive kick of the coil.

Posted 09-29-2009 at 01:32 PM by Bill_Marsden Bill_Marsden is offline

Updated 10-10-2009 at 11:52 AM by Bill_Marsden

Chapter 8: From Four, Twenty

There is a way to flash 20 different LEDs from 4 555 ICs. Each LED would have it's own flash pattern, no two alike (though some are inverted from others), half of the LEDs will be on at any time for a total of 100ma. Basically we're merging Circuit #1 and Circuit #2 together, and using the way the 555s switch on the outputs for this effect. This could be used in a Christmas Tree, or just a light panel for a kinetic sculpture, or some other special effect. The base idea could be expanded even further for more LEDs, however the current draw on the 555s quickly approaches their limit. For 10ma per LED, 5 would be the max (150ma, 30 LEDs). At 6 would be 42 LEDs (210ma). The colors shown in Figure 8.1 were selected at random, and are by way of example.

.................................................. ...........................................Figure 8.1

Posted 09-29-2009 at 01:34 PM by Bill_Marsden Bill_Marsden is offline

Updated 10-10-2009 at 11:51 AM by Bill_Marsden

Chapter 9: Light Chasers

Light Chasers take a flasher to the next step. Many cases they are done with microcontrollers, small computers, but that isn't really necessary unless some kind of computation for the display is really needed. Two nifty ICs, the CD4017 and CD4022, are perfect for this kind of application. They will sequence almost any number of outputs. The data sheet shows how to cascade even more 4017s for more than 10 outputs, and one 4017 can do 2-10 outputs. For CMOS this chip has incredible drive, rated up to 6.8ma best case! I have designed it using 10ma for direct drive of LEDs, though this is definitely not recommended by the manufacturer, and may not work in everyones build.

Figure 9.1 is an old design of mine. This circuit has worked for over 25 years, though not continuously (figure several months on that level). Again, the CD4022 is very stressed, so this isn't a recommended design (but I would use it again in non critical uses).

.................................................. ........................................Figure 9.1

The thing to note about this design is it makes absolutely no difference how many LEDs are in each chain, as long as you are under the Vcc/Vf limit (and don't forget the LM317 3V drop). Why is this important? Take the following circuit in Figure 9.2 as an example.

.................................................. ............................Figure 9.2

With this circuit there are 3 lights apparently chasing around the square. We have all seen variations of this effect on signs and in supermarkets. The thing to remember is this was done by how the LEDs were arranged and wired. It could have as easily been runway lights. I have done this in friends cigarette ashtray with good effect. The arrangement of the lights is more important that the circuit driving them in many cases.

Note how the CD4022 was limited to 4 counts. This is a common theme in using these chips. The 4017 is probably more popular, but it can be limited in a similar way. This is important when you want to generate patterns, which will be discussed later.

Posted 09-29-2009 at 01:35 PM by Bill_Marsden Bill_Marsden is offline

Updated 10-10-2009 at 11:50 AM by Bill_Marsden

Chapter 10: Transistor Drivers

The CD4017/4022 low current output means we have to have some means of increasing this drive. It is easy to become spoiled by the 555, with its relatively huge output currents. It can be fun to cheat a little with something like the 4017, forcing it to go beyond it's ratings, but at some point everything will go permanently dark. These chips can work for decades if kept within their ratings. Fortunately it is easy to use transistors as simple switches, to fully drive modern LEDs. A lot of the schematics have already shown this to one degree or another. Most moderate LEDs seem to focus around 20ma. In some cases much more current is needed, either because the LED requires it or there is a large quantity of LEDs.

The humble 2N2222A NPN transistor has been around for many decades. It performs admirably as a switching transistor, with a rated max of 0.6A. If we derate it to 0.3A this will still drive a lot of LEDs. If a job comes up that is too big for this part there are many other much higher rated transistors to choose from.

There are two ways of using a transistor. The common collector mode shown previously and in Figure 10.1 is a variation of the voltage regulator. It works because CMOS tends to get quite close to the power supplies rails (the plus or minus voltages). The loading on the CMOS chip is the LED current divided by the gain of the transistor. So if a LED array is pulling 100ma, and the gain of the transistor is 50 (which is pretty low, a minimum spec) the current from the CMOS device is 2ma. This design will generate some heat, since the emitter is 0.6V below Vcc (at a minimum). 0.6V X 100ma is 0.06 watts. In extreme cases the transistor can get a lot hotter.

.................................................. ...................Figure 10.1

The common emitter mode has a different bag of advantages and disadvantages. The transistor acts like a switch because the collector is very close to the emitter voltage, so it generates very little heat. The two most efficient states for any transistor in terms of wattage is when they are fully on (dropping almost no voltage) or fully off (drawing almost no current). Since wattage is voltage times current (V X I), and you have moved one of the variables close to zero the wattage is a very low number. The disadvantage of this configuration is input current, which has to be controlled by R6. A general rule of thumb is the base current should 1/10 the collector current. This isn't always practical, and the collector current should be the base current times the gain of the transistor (Ic = ßIb), but since gain is such a wildly variable number even within a family, the rule of thumb exists.

The way around this is to increase the gain of the transistors. Fortunately this is pretty easy to do with only minor drawbacks. Darlington transistors (aka Darlington pair) and a Sziklai pair. The gain is the two transistors gains times each other, and the only major drawback is the collector emitter will have a minimum of 0.6 volts (as opposed to less than 0.1V for a single transistor in common emitter mode). Shown in Figure 10.2 are examples of the two types in use. In both cases the value of R6 can be increased dramatically.