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Author Topic: Trying to understand difference between two circuits supposedly doing same thing  (Read 2790 times)

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bcsteeve

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I have a board I sell that does something (not important what) and also provides an LED out so the end user can attach a 3rd party LED backlight.

I wanted to introduce dimming.  For various reasons, it can't be done by a micro, so I needed an analog method.

Attached is the circuit I have been using to drive the LED.  It provides a constant current output to the connector in the top left (that the user attaches an LED backlight directly to).  Where it is labeled "PWM"... that currently gets a high signal (+5v) and the circuit is either on or off.  However, as implied, a PWM signal could be applied allowing for dimming.

So I search for a means of generating that PWM without a micro.  I found this instructable and it seemed reasonable enough.  But then I found this circuit on pcbheaven.  Similar... but different.  I have never used a 555 before, but I immediately notice that one has PWM output from pin 3 while the other's output is on pin 7.  Same chip, different output.  OK, sure... but which is better for my purposes?

I think I still want the constant current driver circuit.  The instructables circuit is more straight forward in that it has a PWM out, so I just connect that to my PWM in and Bob, as they say, is my uncle.

I imagine if I wanted to use the PCBheaven ciruit, I would cut off T1 from the diagram and insert my circuit (@ PWM) in its place.

But then I started to question if my constant current circuit adds anything at this point?

Thoughts?


notes re: constant current circuit attached, in case it matters.

Q1: DTC114TKAT146 NPN Transistor
Q2: MMBT3904 NPN Transistor
FET1: 2N7002 N-ch MOSFET
« Last Edit: May 24, 2015, 18:58:46 PM by bcsteeve »

cheerio

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can you link in the other circuits you were talking about?

bcsteeve

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Did this forum not allow the links (anti-spam??)?  I'm sure where I typed "this Instructables..." and "this circuit" I created a link to the pages in question.  I can see making a mistake and maybe I left one out... but two?  I'm not exactly new to bbcode. [edit: now that I've gone back and added them, I distinctly remember not only adding the links, but previewing them to make sure it worked]

Annoying because now I have to dig them up again... and that's after it took 4 days to get any response.

Now that I have dug it up, I see also that the comment I placed on the pcbheaven blog post regarding the circuit was never "approved" - which explains why I never got an answer there.

Immediately after posting this, I will attempt to edit the original post to include the links.  If it doesn't let me, then never mind.

[edit:  ok, it seems to have let me add the links after-the-fact.  note to mod:  if you have some anti-spam measure not allowing new members to post links, then let them know that up front!  Better yet, update your Simple Machines beyond 2013 for the myriad of fixes they've put out relating to spam problems. ]
« Last Edit: May 24, 2015, 19:02:06 PM by bcsteeve »

cheerio

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i am currently very busy. i will have a look at this weekend

cheerio

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The difference between the 2 circuits is that the PWM polarity is changed.
http://www.electronics.dit.ie/staff/mtully/555%20folder/555%20timer.htm
As you can see on this page the output and discharge pins are just inverted.

So basically both circuits can be used for your needs.
You are right, you can use them as the pwm input in your circuit.
If you use the PCB-Heaven circuit you do not need the T1. The current load is handled by your circuit.


and by the way:
welcome to the pcbheaven :)