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!
Best Answer
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}}\$