# Switching 3 different voltages with one input

I have a VU meter project I am working on using an LM3916 chip and a series of 4 RGB LED's. Each set of colors is varied in intensity by varying the voltage from ~0-15V. Each set of colors draws about 30mA so when all are fully illuminated it exceeds the maximum rating for the LM3916.

So I need to come up with a "relay" of some sort that can be triggered by 12 or 15 volts (preferably) and light up all three colors of LED's each with their independent voltages. For instance R=3.5V G=12V B=15v - but they should all come on at the same time when the "relay" is triggered.

The easy solution is 3 solid state relays - one for each color - but as there are 10 segments of 3 LED's I would need 30 SSRs which is just too expensive (not to mention needing a second 30 for stereo).

My second thought was transistors but I'm not sure how to get them to work correctly - especially with varying voltages.

What I'd like is a SSR with one input but three isolated outputs but such a thing seems not to exist. Ideally I could find a single chip with 10 inputs and 30 outputs but again I can't find anything.

Thoughts?

Thanks!

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I'm confused how many RGB LEDs you are trying to drive independently and how many are ganged together somehow. 0-15V makes no sense for driving LEDs, so I have no idea what to believe in the rest of your question. Show a diagram. –  Olin Lathrop Jan 17 '12 at 20:38
A rough diagram would be good. Also, why do you need isolation? –  Oli Glaser Jan 17 '12 at 21:08

The way to increase the LM3916's output current is to drive a PNP transistor with it:

That will work as a switch which switches the LEDs on and off. R1 prevents the transistor from conducting through the LM3916's leakage current. You can use a 10k$\Omega$ here.

If you want to control the brightness with your 0-15V you'll need to convert that voltage to a current; LEDs are current controlled devices. Place the following circuit between the LEDs and ground:

The LED's current will be $\dfrac{V_{IN}}{R_{SENSE}}$

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The description of your problem is not consistent with your diagram, and that's why nobody is answering you I believe. Each output of the LM3916 should go to exactly one led (the other end of which goes to power or gnd depending on how that particular chip works) in a typical scenario. Your diagram has all of the led's in series. I believe you mean for each output of the LM3916 to go to either one of 3 led's, in which case you will need a type of analog multiplexor...or a system of switches.

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Yes, that is typically how the LM3916 works. But I want to drive 12 LEDs instead of one. The LM3916 can handle about 15mA of current but each set of 4 LEDs draws about 25mA when fully illuminated. I also want each series of 4 to be supplied different amounts of voltage in order to vary their intensity. –  Chris Jan 18 '12 at 13:16