username - you don't need BJT for triggering at all. Input is 3.3V compatible, so you can trigger it directly from MCU. Actually the sensor is 3.3V Vcc compatible, but max distance is dramatically reduced (to about 0.5m).
AFAIK this sensor has minimum distance of about 20mm. OTOH I tried multiple sensors and all of them were performing with about 3mm accuracy up to 4 meters (I was using the Energia library for measurements). What I had to add was temperature coefficient (I'm measuring liquid levels outdoors).