DISCLAIMER: I am new to building electronic circuits. Any and all criticism is accepted and appreciated.

Hello Everyone,

I have been working on making an Audio VU meter with the LM3916 chip, and I have been able to get everything working in Dot Mode according to the Datasheet.

What I want to do is add an additional LED that will act as a "resting" LED. So when there is no audio signal present I want the LED to be on and then I want it to turn off when any of the other LEDs are on due to an audio signal being present.

I feel this should be a simple task but I've been working on it for months and I've come up with 1 or 2 ideas but when I test them, they don't work. One of my ideas was to add an op-amp or transistor to switch the LED on & off when the voltage changed at pin 1 but it didn't seem to work.

I think my mind is focusing on the wrong aspect of it and I was hoping to get someone else's opinion.

Attached is my working schematic with and without the "resting" LED.

I thank you guys in advance for any suggestions, and if any of this is unclear or more information is needed, please let me know

Thanks

 

If you want this to work in dot mode and you want a normally on LED to go off when ANY signal LED goes on, you might build a circuit such that when any output pin goes a volt below the supply voltage the "resting" LED is off. This is going to require 10 inputs to something that is normally on. You might do this several ways, like a summing junction, a NOR gate with 10 inputs, or a diode matrix.

Another approach might be to monitor the current draw of the 3916 chip so that when it draws enough current to light an LED, an amplifier turns off the resting LED.

Have I just gone completely over your head?

 

The first part was a little over my head and seems like a little to much work for turning an LED off when another one turns on.

The second approach with the amp makes sense to me however I'm not sure what pin(s) (+, -, Sig, Ref High, Ref Low, Ref Out, Adjust, or any of the led pins) to monitor and compare the current that will accurately switch the normally on LED off when a strong enough signal is detected or when the first led on pin 1 is on. I maybe overlooking how that is done but can you clarify that for me please?

 

It's not a pin on the chip. You add a resistor in series with the power and use the voltage across that to trigger a comparator or op-amp that runs the "resting" LED. Somewhere in the range of 10 ohms to 100 ohms should work.

Make sense now?

Make a voltage divider to set one pin of the amplifier to 8.5 volts. When the voltage to the display LEDs drops to 8 volts, the op-amp switches the resting LED off.

This is the concept:

 

One option would be not to use an additional LED but instead to simply apply a slight positive bias to the SIG input so that LED1 is on with zero additional signal input. The other 9 LEDs would work as normal.