The Current is pretty stable but:
1A in the TIP 41, the more voltage you have to loose in the transistor, the more it will heat, so your better to keep the supply voltage as low as possible.
BTW i did not draw the control ON/OFF transistor, it is another 2n3904 actually in parallel of Q2, but the base is control by a PWM output (with a 10K resistor) of the arduino.