[sdiy] LED's on negative rail??

[Grant Richter]

This is the technique I used on all the Wiard modules. All the voltmeters,
VU meters and the like are LM339s quad comparators powered from +/- 15
volts. The outputs are open collector and you can run the LEDs between noisy
(digital) ground and -15 volts (use two ground systems in any analog design,
one for a quiet voltage reference, another for noisy current demands).

Then you run a trace from the -15 volt pin on the LM339 to the incoming -15
volt terminal BEFORE your power supply input filters or regulators. The
noise generated on the -15 volt line is now filtered out with the rest of
the rail noise, and doesn't get inside your circuit. Relatively little
current noise appears on the LM339 positive supply pin.

The LM339s are very cheap and reasonably precision quad comparators. Connect
all 4 positive inputs to the 1K source signal. You can use simple resistor
dividers on the negative inputs to set thresholds. A linear string such as
GND>2.2K,2.2K,2.2K,2.2K,8.2K>+15 makes a linear voltmeter with thresholds at
around 2,4,6 and 8 volts. Diodes can be used to create a "dot" mode display
by pulling down the previous LED anode when the next higher stage turns on.
A log weighted string such as GND>330,680,2.2K,6.8K,5.1K>15V makes a VU
meter with thresholds at 0.33, 1 volt, 3.3 volts and 10 volts (very roughly
-10dB, 0db, +10dB, +20dB).

To sum up, an LM339 quad comparator can be used to make inexpensive bargraph
displays like a 4 segment LM3914 or LM3916. This creates an independent
subcircuit to allow control of switching noise on the supply rails.  The
comparators also form a high impedance buffer between the audio output and
LEDs which further reduces noise and distortion.

> From: media at mail1.nai.net
> Reply-To: owner-synth-diy at dropmix.xs4all.nl
> Date: Fri, 25 Jan 2002 14:27:46 -0500 (EST)
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] LED's on negative rail??
> 
> 
> I'm working on a few things that run almost entirely off the postive rail.
> Since LED's draw a significant amount of current (the light has to come
> from somewhere), I'm thinking that I should run the LED's off the negative
> rail to balance things out.  However, I've never seen that done.  Is there
> any reason why??  Is it a noise issue??  As far as I can tell, the LED's
> would "see" the same difference.
> 
> Any suggestions on what is the best way to drive an LED??  I've seen
> circuits that use op-amps or FET, but wouldn't regular transistors work
> just as well for less money??