# What transistor do I need to drive many LEDs and how should I hook it up?

I know that a transistor amplifies a signal, but I've never used one. I have this design here and I want to add 15 LEDs connected to each pin, instead of 1.

How I could incorporate a transistor to help me achieve a strong enough current to power the LEDs ?

• This question is kind of a duplicate of this one: electronics.stackexchange.com/q/6093/3803 – Tall Jeff Aug 8 '11 at 0:36
• This question adds the unique constraint of the vu-meter ic's active-low open collector outputs, vs. the PIC's output and active hi/low flexibility. That means directly driving NPN transistors is out. – Chris Stratton Aug 8 '11 at 0:45
• Would a PNP transistor work instead of a NPN? – jsolarski Aug 8 '11 at 1:09
• ...PNP: Yes. The original LEDs are Vdd (V+ , Vhigh, Vsupply ...) referenced. If you don't mind them being ground referenced instead then a single PNP per channel would work.LM3915 drive PNP base via a resistor. Each PNP emitter to Vdd. Each PNP collector via a resistor to LEDs. LED cathodes to ground. Resistor per LED is required as, while LM3915 current limits the LED current, the transistors do not. While it is notionally possible to common transistor emitters and use a single resistor for 2 or more transistors, this is a bad idea for several reasons. – Russell McMahon Aug 8 '11 at 5:30
• Don't think of a transistor as "amplifying a signal". Think of it as a valve that can be opened or closed. It's used as a component of amplifiers, but it can be used in switches or other things, too. – endolith Sep 7 '11 at 21:22

The components' values will depend on V+ and the current required for the LEDs. For V+ = 12V you could make R1 = 1k$\Omega$ and R2 = 4.7k$\Omega$. This will give a base current of 2mA, which is enough to saturate most transistors, so T1 can be a PNP type like a BC556.