Unipolar stepper motor and a MCU
Did you read the transistor rating of BC546 in the datasheet, Collector Current − Continuous IC 100 mA dc. 

Your mcu specs mention an absolute max or 4mA as output current (you have already exceeded that with the base resistor you have used).

You need to select a more powerful transistor and a high gain Darlington would suit your needs, a device like TIP120

Your mcu is a 3.3v device and a Darlington transistor needs about 1.5v between base-emitter so a resistor of about (3.3v - 1.5v)/0.003A = 600 ohm should do or slightly higher to 620 Ohm



If you don't have a Darlington available you can use two discrete transistors in a Darlindton configuration to increase the current gain.







simulate this circuit &ndash; Schematic created using CircuitLab

Note that I've uses a 1A transistor for Q1.
You'll need four of these, one for each coil.