RGB led strip is too dim

Question

I'm following this tutorial to build an Arduino powered RGB led strip controller but I'm facing two problems:

1. When I use digitalOutput the lights are too dim.
2. When I use PWM the leds change so slightly that I can barely detect a change.

These are the parts I'm using:

• Wemos D1 mini
• a USB cable to power the Wemos
• 1k ohm resistors
• TIP31c transistors
• A power outlet 12V 1A to power the strip (which works fine with the original controller unit)

The circuit connected as presented in the tutorial (like the circuit below) doesn't work, the board does not start.

If I disconnect the board GND from the outlet GND then the board starts but the leds are too dim to be visible

What am I doing wrong?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

The Wemos D1 Mini is a 3.3V device. With a 1k base resistor, the most base current you're going to get is (3.3V / 1k ohm) = 3.3mA.

Looking at the datasheet, the current gain (h_FE) for the TIP31 transistor is no more than 50. So the maximum current you will pass from collector to emitter is 50*3.3mA = 165mA.

Your base resistor is too large and is limiting the current to your LED strip to no more than 165mA per channel. Try reducing the base resistor to 150 ohm. The TIP31 can handle continuous collector current of 3A, so you ought to be safe with a 1A power supply, and 150 ohm resistor.

UPDATE:

Per @Gerben's comment, the ESP8266 can source a maximum of 12mA from a GPIO pin, so you should not use lower than a 225 ohm base resistor on the initial TIP31. A Darlington pair would look something like:

^{simulate this circuit – Schematic created using CircuitLab}