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Waveform Generator Answered

Hi guys!

So actually i wanted to make a circuit that can turn on and off an LED array at a particular rate (say 10 seconds time period). I want the LEDs to gradually brighten and dim. At first i thought i can implement it using a multi vibrator, but it it turning off and on too fast, since the output of the MV is a square wave. I want to design a simple circuit that can generate more slowly rising waveform (like sine or triangular) and can also drive a load of at least 100 mA and have a time period of 10 seconds or so. Any suggestions? 

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user
Orngrimm (author)2012-12-26

How long should it phase in and out?
If only a second or so, you can add a resistor and Capacitor to dampen the transients.

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charmquark (author)Orngrimm2012-12-28

well i can do it, but the resistor will limit the current drawn too much. say, i want a time of 10 seconds, that makes 1Mega ohm * 10 micro farad. The resistor will drastically reduce the current that reaches the load. This very limitation makes me look for alternatives

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Orngrimm (author)charmquark2012-12-28

10sec of fading with 10sec period? So always-fade... OK... In this case the RC-configuration wont work.
Would a simple sawtooth / triangular-waveform (with offset) fit your needs or does it have to be a sine with offset?

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charmquark (author)Orngrimm2012-12-29

I am making certain assumptions here. If the waveform is slowly rising, with a time period of say 10 seconds, then i can make my LED array such that when it reaches the peak of its waveform, it glows with maximum intensity and when it falls below, the LEDs will dim out. So any slowly rising waveform like sine or saw-tooth/triangular will do. The square waveform reaches its peak too fast, so i want to avoid it.

Also, can i use a BJT instead of the FET? The FET may not be available where i live, so do suggest me some alternatives

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Orngrimm (author)charmquark2012-12-30

Also the quite common 2N7002 works...
http://www.bilderload.com/bild/276753/10secFA2T1.png

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Orngrimm (author)charmquark2012-12-30

What exactly is a BJT?
You can use whatever FET you have around... Simply check for the specs if it is capable of supplying the current, being able to be switched by the voltages you have here and if it is the correct type (N-channel enrichment-type).
I simply took the FDS6614A in the simulation because i just had no other type at hand...

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Orngrimm (author)charmquark2012-12-28

So why not simply use a FET to change the impedance and enable the drive of huge loads?

See the quickly simulated circuit @ http://www.bilderload.com/bild/276215/2ampsawtoothV272R.png i made for you.
See the 2 different voltages and the resulting current in the top-part of the image.

It is split into 3 parts:
Frequenzy-generation
Waveform-changing
Impedance-changing

You can drive many amps with that if you want...

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Orngrimm (author)2012-12-26

Oh: The formula to charge a capacitor to its voltage is
t=5*R*C

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