LED Strobe

[Mike6158]

I need to fire a sequence of LED's. I'm guess 6 - 10 of them at this point. I've got the sequencer figured out. I can use a 555 to clock an up/down counter and that can drive a BCD decoder driver. Anyway... enough of that... Each LED is rated at (typ) 3.6v and If= 20mA (3000 MCD. They have T1-3/4's that go all the way to 10,000 MCD) Since I plant to pulse the output from the 74LS42 do you think that I could get away with driving the LED's directly (no transistor)? I guess that what I'm saying is that I'm not sure that I know how to make sure that my IC's can handle the load that I might put on them. The I0L output current of the BCD driver is 16mA. Some of the current will be "eaten up" by the dropping resistor... I'm rambling... I need a little guidance please...<p>I can hang with the boolean part of this... making it work without letting the magic smoke out is another story...<p>You can find the LED's that I am thinking of using here: Superbright LED

[gerty]

If you calculate your dropping resistor for about 16ma you should be ok. The dropping resistor will not "eat up" any current since the current remains the same across the components in a series circuit..You might go a little higher on the current if it's pulsed fast enough and the on time is short..

[Enzo]

If the chip can max out at 16ma, do you really want to run it at 16ma? Would you drive your car all over town revved up right to the red line? If you had a 50 pound rated wall bracket, would you hang a 50 pound plant container from it right over your head? Give the poor thing a break and give it some breathing room. Getting away with something doesn't make it the right way.<p>A healthy general purpose transistor like the MPSA06 costs maybe 6 cents, and a nice darlington like the MPSA13 certainly costs under a dime. Why not use a transistor and have a reliable circuit? I just checked Jameco, and those types are so cheap they don't even sell them one at a time, the MPSA06 (One of my all time favorites for projects)sells for 46 cents for ten, and the MPSA13 sells for 75 cents for ten. CHEAP.<p>A transistor will invert your logic, but even then, a 29 cent inverter chip or two will fix that.<p>The LED wants no more than 20 ma. The resistor is there to limit the current to that or less. The current capability of the chip doesn't limit what the LED will draw. The transistors in the output of the chip are just switches. If you connect the LED to +power and to the chip, the chip transistor will turn on. If the LED tries to draw more than 16ma, that transistor inside will burn up.<p>Even a transistor outside needs a limiting resistor. If you connect the LED to a 9v battery and have a transistor in series, what is there to limit current? That LED might have a 3v drop across it, but beyond that it is just a diode, and you would not want to put a diode across a battery. The transistor is just there acting as a switch. The <MPSA06 can handle 500ma, but if the supply can provide that much, the LED will burn even that out without a resistor.<p>For each LED, you would need a resistor to limit its current, a transistor to act as a switch, and a base resistor to limit base current in the transistor. The darlington has so much gain, you can use almost no current and the base resistor can be thousands of ohms. The LED resistor is calculated from the supply voltage.<p>There are chips intended to drive LEDs directly, but they have current limiting circuitry included.

[rshayes]

If you are using CMOS, a simpler solution might be an emitter follower driver. A single transistor will have a current gain of well over 20, probably at least 40. The base current will probably be under 1/2 milliamp. A CMOS gate can supply this current with only a slight voltage drop from the supply rail. <p>Basically, the base connects to the CMOS output with no limiting resistor. The collector connects to the positive rail. The emitter is connected through a resistor to the LED anode. The LED cathode is connected to ground. The resistor provides the current limiting. There is a voltage drop across the transistor of about .7 volts in addition to the LED voltage.<p>The resistor is calculated by taking the supply voltage, subtracting the base-emitter voltage of the transistor (about .7 volt), subtracting the LED voltage, and dividing the result by the desired LED current.<p>The emitter doesn't invert and it saves a resistor. If you have a lot of LED's that may be significant.

[rshayes]

Sorry, didn't notice that you mentioned LS TTL circuits. These pull down much harder than they pull up, but even so the output for the 74LS42 is specified at .4 volts when sinking 4 milliamp and .5 volts when sinking 8 milliamps. The output on a TTL gate is pulled down by the collector of a transistor and the current is limited by the base drive to the transistor. This isn't very predictable. The pull up current is much less and is limited by a resistor inside the IC whose value may not be tightly controlled.<p>Consider adding a TTL buffer such as the 74LS244. This will sink 24 milliamps at .5 volt. There are 8 per package. The LED anodes would be connected to 5 volts. The LED cathodes would be at about 1.4 volts. The resistor would be .9 volts divided by 20 milliamps, or about 47 ohms.<p>With the voltage drop across the limiting resistor at only .9 volts, small changes in the TTL gate voltage or the LED voltage will have a large effect. A higher logic voltage swing would help this problem. One possibility would be 4000 series CMOS operated at 6 or 7 volts. This would definitely need buffer stages of some kind, but would allow better current control.

[Engineer1138]

There is one other thing you could try, but it's risky. Use no transistor or no resistor!
If you are pulsing the LEDs fast enough and at a low duty cycle, then the TTL chip won't heat up enough to be damaged by the overcurrent, and you'll end up with a short, intensely bright flash from the LED (also watch out for its max current rating).<p>I doubt the datasheets will give you the information you need to calculate just how short the flash would have to be, so it's something you'd have to determine by trial and error, hence the risk!<p>Wonder if putting a large capacitor in series would help? I should try that sometime.

[Mike6158]

Wow- what a great place to come for good advice... I put all of your responses in a txt file and saved it as a tech note (ie my version of brain cell replacement technology). I should know this stuff... make that remember... but, in my defense, it's been a decade or two and things have changed just a bit since I last worked with electronics.<p>Well, my project, has taken a turn for the "now what?" The original idea was to build a replica of a rotating beacon of sorts. I went with strobing led's as opposed to gradually increasing and then decreasing light intensity because I thought that a circular train of led's would look more realistic. However, I now have dimensional constraints. 3/8" to less than 1/2" in diameter for the light emmitter. I've been "arranging" T1's in a circle all morning and the absolute best that I can get is 5/8" diameter and even that is questionable for actual usage.<p>Thanks for your reply's<p>BTW- I wonder if there is any way to rotate a single LED in the space of 1/2 an inch diameter and 5/8" tall. And power the circuit. And vary the rotation speed Magic... That's what I need...

[Chris Smith]

Darlingtons are too slow to use with strobing. Saturation times delay the flow of current.
Fets are better.
Also store your energy in a cap, and dump it through a switch. <p>If you manage real fast times, both on and off, you can dump an amp easily through a led with out burning it out as long as the Duty cycle is around 5%.<p>[ December 08, 2004: Message edited by: Chris Smith ]</p>

[Engineer1138]

Instead of rotating the LED, can you just rotate a reflector around a stationary LED?<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by NE5U:

BTW- I wonder if there is any way to rotate a single LED in the space of 1/2 an inch diameter and 5/8" tall. And power the circuit. And vary the rotation speed Magic... That's what I need...<hr></blockquote>

[Mike6158]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Engineer1138:
Instead of rotating the LED, can you just rotate a reflector around a stationary LED?<p> <hr></blockquote><p>If I could find something tiny enough. Think "Attention KmArt Shoppers there is a blue light special on aisle...." only much much smaller. Lens diameter of around 1/2" or less. Scale model type stuff. Needs to be low weight, low power.<p>The propeller clock that I posted about in another thread did something interesting with supplying power to the rotating board from the motor. That might me something doable if I could find a small enough DC motor and modify it...

[Chris Smith]

Vibrator motors out of pagers are that small. A ½ inch rotating tube and mirror set at 45 degrees will give you the flashing light effect.

[jollyrgr]

I started a thread similar to this almost a year ago. I wanted to make an LED strobe similar to something I saw at a ham fest. The strobe uses several white LEDs fired together. They are SUPER bright and are as bright or brighter than a xenon strobe light. I've been able to flash LEDs before; this is real easy. But I wanted a true strobe effect like the thing I saw at the ham fest. The thing I saw was a home made device and the guy that had it did not make it (so he claimed) so did not know how it worked. <p>I've tried to duplicate the same type of thing using a 555 timer and power transistor. I drove the LEDs using a 10% on square wave @15Hz. While it flashed, it did not have the "flashtube strobe" appearance. I drove the LEDs directly off of a 9 volt battery. The transistor was driven by the 555 timer with no bias or current limiting resistors. The LEDs did not seem to suffer any ill effects but I did not the circuit long as it did not provide the effect I was looking for.<p>Ramsey sells a kit for $45 for a white LED stobe. Maybe it would be of help to you but it is way more than I'm willing to pay to flash a group of LEDs! Another thing I've seen is the "UFO" hover cars (the Hondas, Taurus', etc. that have the body glow lights and strobes). They make some flashing LED circuits for highlighting cars but I have not played with any of them yet.

[jollyrgr]

Chris, <p>I just re-read your post. I think I've found part of my error. I may have to retry the circuit using an FET instead of a bipolar. Also, for such a simple circuit, I only used a small cap for the 555 and didn't go with a bigger cap for the LEDs. And I think I'd better lower my duty cycle.

[Mike6158]

I put this schematic together last night. After I finished it I realized the the high intensity LED's that I wanted to use might be too much for the driver. The idea was to form the LED's, looking outward, into a circle and fire them sequentially. It would give the illusion of rotation. At least I think that it would. I think I'll build it even though it won't work for what I wanted it for.<p>

[Chris Smith]

Jolly Roger
The secrete of sucess is low on resistance, high voltage, and short switching times. <p>Bi polar transistors dont offer either, unless reversed and in the break down mode, where they are pushed, until failure. <p>You can break down the diode junction of a Bipolar with out breaking the transistor, and it will do so in the nano seconds, instead of the milli or micro seconds. <p>BUT, High voltage is needed. <p>Fets on the other hand can operate quite fast, and handle large loads.<p>[ December 08, 2004: Message edited by: Chris Smith ]</p>