Reverse Polarity on 555 timer???

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Joseph
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Re: Reverse Polarity on 555 timer???

Post by Joseph » Tue Aug 26, 2003 6:40 pm

Well, you can modulate the signal of a transistor by applying another signal to its emitter, probably through a resistor to add or remove current from its existing emitter resistor. You could also do it by adding both signals at the base through two seperate resistors, I suppose.<p>Do you mean you want the flashes to start slowly and grdually speed up? Or do you want to do the fading in and out with the brightness?

N9AOK
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Re: Reverse Polarity on 555 timer???

Post by N9AOK » Wed Aug 27, 2003 8:12 am

I'm wanting to keep the strobing action (same rate) but slowly fade it in and out.<p>In the interest of saving battery I would be more inclined to drive the base to a "more off" state for the fade<p>OK, This is what I'm thinking...Does this have merit?
My problem is to first replace the base resistor (R5)in there (the one used for the strobe action) so that it will just bring the transistor into saturation when the strobe pulse is HIGH. (rather than the way I have it now where I didn't do any calculations to determine how hard I'm driving the transistor).<p>Then for the FADE action tap another signal into the base just beyond R5 (between R5 and the transistor).
http://www.homefires.us/bluemarble/PAGE ... STROBE.jpg<p>I suppose the output of my OP-AMP generated triangle wave will have to be negative ??? and should be able to sink the base current at this tap to linearly fade out the LED current path. I'll have to determine a resistor that will pull current away from the base correctly.<p>Do most people determine this stuff mathimatically first or do they they "try" values until the things does what they want???<p>Thanks<p>Todd Snyder<p> <blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Joseph Meisenhelder:
Well, you can modulate the signal of a transistor by applying another signal to its emitter, probably through a resistor to add or remove current from its existing emitter resistor. You could also do it by adding both signals at the base through two seperate resistors, I suppose.<p>Do you mean you want the flashes to start slowly and grdually speed up? Or do you want to do the fading in and out with the brightness?<hr></blockquote>

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