# 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!

• 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. Commented Jan 17, 2012 at 20:38
• A rough diagram would be good. Also, why do you need isolation? Commented Jan 17, 2012 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 $\dfrac{V_{IN}}{R_{SENSE}}$

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.

• 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. Commented Jan 18, 2012 at 13:16

According to your diagram it seems you want to drive 3 rows of 4 LEDs in series from each output of the LM3916.

One way to do this might be to use a current mirror like this:

I1 represents the open drain output of your LM3916 set to 30mA. Q2 forms the reference for Q1, Q3 and Q4 (all bases tied together) so the same current that flows through Q2 will also flow through Q1, Q3, Q4. So you can say Q2s collector-emitter current "sets" the current for the rest of the transistors.
30mA will flow through each series chain of LEDs, as long as the supply voltage is high enough for the diode drops + a bit for transistor (say at least 0.5V)
Note that the transistor shown is not a "recommendation", rather the first I clicked on in LTSPICE - any typical small PNP should be suitable though. Same with the LEDs.
Also note that you cannot vary the current linearly by altering supply voltage as it's a constant current setup. To do this you would need to vary the LM3916 current setting.

• That gets me close but each set of 4 LEDs needs their own supply voltage or another way to vary their intensity. Commented Jan 18, 2012 at 13:18