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runklem

LED pulser help

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Hi, just built the LED Pulser project from your site, but am having problems.  I added a 5 VDC regulator to the V+ as my intention is for this to drive some LEDs on my motorcycle to produce what can best be described as a "breathing" effect for red LEDs around the engine.  Anyway, it works fine when hooked up to a 9V battery and one of my LED modules (2 red leds in parallel), however when I hook it up to the motorcycle and 10 modules (in parallel with each other), I get a constant on.  No dimming or pulsing effect at all.  Not being nearly as smart as I think I am, I am at a loss of what to do to fix this.  Obviously changing the amount of LEDs has caused something to go awry but I don't know what to change/modify to overcome this.  Please help.

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You should post a link to the project. There are a few different blinking LED projects in the projects area. Also, provide a sketch of the changes you have made. I am sure you will get better help if you provide these items. Otherwise, it is hard for everyone to visualize the problem.

MP

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Doh! just lost the whole email 'cause I couldn't attach a .bmp..... ok, take two:

Here is the hyperlink to the original project:
http://www.electronics-lab.com/projects/motor_light/020/index.html

Attached, hopefully, is a schematic of what I did..or my best attempt at a correct schematic.  In case its unclear here is what is different from my project and the original.  I replaced the LM358 op-amp with a 272 dual op amp (because its all I could find).  I also added a 7805 voltage regulator on the incoming voltage, since I've got 12v in.  This steps it down to +5v.  Finally, the load is 10 LED "modules" wired in parallel.  Each module is actually two LEDs w/ resistor wired in parallel.  Disregard the R6 in my schematic, that is meant to represent the built in resistor on each LED, not an additional resistor before each module.

My voltage source is the 12v battery on my motorcycle.  I have the leads screwed directly on the battery with inline 15 amp fuse for safety.

Again, the thing works fine on my workbench when hooked to just one LED module.  Its stays constant on when hooked to the 10 modules.

Any help is appreciated.
Thanks

post-31548-14279143543697_thumb.jpg

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I and most datasheet sites don't know what is an LM272. They have an L272 which is very different from an LM358 dual opamp ued in the project and won't work in this circuit.

Maybe the very heavy load on the transistor melted it so that now it is shorted?

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Your LEDs are pulling too much curent for this little transistor. Since we do not know the built in resistor value and the specs on these LEDs it will require a little research and calculation of your own. If you do not have this information, you can remove the LEDs one at a time on your bench to see how many you can actually fuel from this circuit. As mentioned above, do not be surprised if you have  burned out the transistor and need to replace it. I think your L272 op-amp combination is ok since you have tested it to work with only one LED. Basically, you are drawing too much current with the added load of more LEDs. Remember that even though they will all take the same voltage, you need to add up the current used in all the branches of your circuit. In this case, you have several branches where the original circuit had only one. You can either re-design this circuit or you can eliminate some LEDs and build more than one of these circuits to sit side by side when you determine how many of your LEDs it will actually power.

MP

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Thanks for the replies.  As for the amp, I don't know anything beyond it being called a 272.  It came with my sons electronics kit, which I have ransacked for this project (don't tell him).
As for the transistor.  It can't be completely fried as the circuit still works now, with a single LED module.  But, that fact that it is failing (non-permanently) with the larger load is certainly possible.  Assuming its the transistor, does anyone have a recommendation on what to replace it with?  I am not an electrical engineer, but merely someone who can follow directions fairly well and have enought electronics knowledge to just get myself into trouble (case in point, now).  I'm not much good beyond V=IR.... So if my current requirement has gone up, then I need more voltage(?), which means taking out the voltage regulator and letting the 12v flow in?  Would this not kill the transitor and/or amp?.....Thinking as I type now...what if I let in the 12v, but put a resistor of some magnitude in front of the transitor?......
Thanks

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If you take out the regulator there will be other problems since the voltage over the transistor will be much higher (more heat)! Can you measure the voltage over the LED/resistor while fully lit? It would be very helpful to know some data for your LED

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The overloaded transistor needs to have a high base current to turn on. The TLC272 is a Cmos opamp that has a low amount of output high current. The opamp's output is trying to go high enough to switch the comparator (first opamp) but the high base current holds down its voltage so that it continues to go high.

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The overloaded transistor needs to have a high base current to turn on. The TLC272 is a Cmos opamp that has a low amount of output high current. The opamp's output is trying to go high enough to switch the comparator (first opamp) but the high base current holds down its voltage so that it continues to go high.


Wow, talk about overloading a transistor! (my brain transistor) :o  I haven't any idea what you just said...but do you have a solution?

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