driving a medium power darlington transistor with the output of a 555 timer
You've got a circuit like this







simulate this circuit &ndash; Schematic created using CircuitLab

Ratings of components do not tell us how to design the circuit.

Here, if the 555 can provide a HIGH output into 100 ohms in series with 1.5 or 2.0 volts of the Darlington two-diode base input, then this circuit will work.

However, the TI 555 datasheet, (if powered with +5v) promises only 2.75 volts output
with NO current promised. At +12 volts, you can pull 0.2 amps.