Bi color Led VU meter

[FireAce]

Greetings gentlemens

Would love some help in designing a simple circuit that I've been seeing more and more in most audio equipment with any form of audio presence/ clip capability. I'm more of a builder then a designer, so any input is appreciated.

The circuit does as follows. A bi color LED (Green/ Red with shared ground), is used to allow user to monitor line level audio levels on audio device input. Green LED turns on at -30db audio presence. Red clip LED comes on around or just over 0 db.

A simple transistor circuit would benefit. At the moment audio comes through 10k pot and into 5532 opamp for gain and buffering. I have dual color LEDS installed on unit, LEDS share the ground pin.

Thoughts?

 

[ronsimpson]

Help me understand.
There is a single green LED that comes on when audio is above -30db.
There is a single red LED when audio gets above 0db.

If you are using a two color LED then when green and red are on together it turns yellow. So you might want to turn green off when red turns on.

 

[FireAce]

That is correct. The red is somewhat powerful enough to almost override the green. Currently doing some testing on my bench and actually seem to be getting somewhere. Will post once I have something. Gonna take some brain digging.

Currently I have 2 PNP transistors driving each of the LEDS. A single NPN transistor uses the audio level at gate, to pull the PNP's to ground. The PNP's are connected to the NPN, each with diff resistor values to cause them to come on at diff volumes. Once I'm happy with performance I'll upload a diagram, but the problem will be to turn off the green LED when red comes on.

 

[fernando_g]

I've seen the circuits you are talking about. They are very useful to monitor many audio channels without the undue complexity and clutter of full-blown bargraphs.

What you need to do is add circuitry that will deprive the green-diving PNP from base current when the red-driving PNP turns on.

 

[MikeMl]

Or carefully blend the Red/Green current(s) such that the amber is distinctive. This makes a Green=OK, Amber=a bit high; Red=too high indicator possible. I have build a battery discharge monitor like that.

 

[FireAce]

I've seen the circuits you are talking about. They are very useful to monitor many audio channels without the undue complexity and clutter of full-blown bargraphs.

What you need to do is add circuitry that will deprive the green-diving PNP from base current when the red-driving PNP turns on.

I managed to do that, but it effected what level my green would come on. So it backfired on me. This will take just the right transistors, and just the right resistor values. At the moment it may be out of my hands unless I open my Fast Track Ultra 8R, and I'm not about to do that.

 

[fernando_g]

Well; if you want ot do it with an all-transistor circuit, one must find the boolean expressions that dictate when each LED should light up.

If we call C1 the high level threshold and C2 the low level threshold, then the boolean expressions for the green and red LEDs are respectively:

G = C2 * C1
R = C2 * C1

The AND function can be realized by two transistors in totem pole.
And of course, the NOT function is a single transistor inverter.

 

[FireAce]

Great Fernando the great, I will be awaiting your completed diagram.

 

[FireAce]

Ok, I think I found something buried deep inside of the google search engine.

https://www.electroniq.net/led-drivers/bicolor-led-driver-circuit.html

Appears to use TLC272 IC which I'll have to order up to try. I use mostly 5532 and TL082 Opamps. May or may not need to be modified for audio use. Not exactly fond of the Single Voltage Supply either.