[sdiy] Op Amp LED driver / Current Source problem/solution

[Justin Owen]

Yeah - that was one of the first things I tried. Fine if you're feeding it a bipolar +/-V signal (you only get one half) but a signal that only swings between 0 and +5V - you get the same problem where the LED doesn't fully dim.

To clarify - that 1M to the rail definitely sorts it properly - just not sure where on the 'ugly hack <> elegant solution' scale it is!

Ta,

- Justin

-----Original Message-----
From: Tim Ressel [timr at circuitabbey.com]
Received: 26.11.2014 16:55:47
To: Justin Owen; SDIY List
Subject: Re: [sdiy] Op Amp LED driver / Current Source problem/solution

Have you tried replacing the LED with a diode?

Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




________________________________
From: Justin Owen <juzowen at gmail.com>
To: SDIY List <synth-diy at dropmix.xs4all.nl> 
Sent: Wednesday, November 26, 2014 8:06 AM
Subject: [sdiy] Op Amp LED driver / Current Source problem/solution


Hello,

I've previously used this: http://www.sdiy.org/juz/bipolar_LED.png as a bipolar LED driver where each LED shows either the positive or negative-going portion of a signal.

Have a need for a nice linear driver for a varying signal e.g. sine wave LFO going between 0 and +5V and assumed I could use the same circuit and just remove the bottom, negative LED.

That version doesn't turn the LED fully off when the input signal gets to 0V...

...but adding a big old 1M R to the supply rail fixes that:

http://www.sdiy.org/juz/unipolar_LED.png

Is that a valid fix or is this potentially going to cause problems elsewhere in the curcuit/make bad things happen?

Thanks.

- Justin

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