NPN transistors, LED strip and Arduino

Question

I'm trying to control an LED-strip with an Arduino like an 8x8 matrix with a common anode (pic 1).

I put 2 NPN-transistors in the circuit and a LED-strip between them. The first transistors in pic 2 are 2N3904s, each one connected to an Arduino digital pin; the second one is a TIP120. The maximum current in the circuit is 0.4 A; 0.08 A for each strip.

I use simple Arduino code for it, like digitalWrite(4, HIGH) (for any of the 2N3904s) and digitalWrite(5, HIGH) (for the TIP120) to turn it ON, but after digitalWrite(4, LOW) and digitalWrite(5, LOW) the LEDs are still on. So I need to fix my circuit, but I don't know how.

Answer 1

Rows are connected to anodes, and columns are connected to cathodes. Since you are using only NPN transistors, any transistor on the anode side of a diode is "high-side" and will have to be an emitter follower. Emitter followers should not have a base resistor.

On the low side, at diode cathodes, all transistors are operating in common emitter configuration, and will require base resistors to limit base current.

The diodes all sit between high-side emitters and low-side collectors. For 3 columns and rows, the circuit will resemble this:

^{simulate this circuit – Schematic created using CircuitLab}

Extending this for all 8 rows and 8 columns will require 16 transistors.

Don't forget the LED current limiting resistors Ra1, Rb1 etc.