Photo by Anesh Prasai
|A phototransistor is a special
transistor that is sensitive to light. We can see the actual transistor
semiconductor by looking through the window that admits the
light. The square gray block in the center of the device is the
actual active part of the transistor - less than 1 mm across. The
picture below (also taken by Anesh Prasai) shows the total phototransistor beside a pencil to indicate
te size. As can be seen the active part of the transistor is the
size of the dull pencil point. To make the close-up photo at left Anesh
supported a small hi power magnifier in front of the camera lens in
order to obtailn a magnified image of the transistor.
picture at right
shows a photogate in the Physics Lab. An infrared-emitting LED is
placed behind the sector-wheel. The receiving phototransistor is
located on the near side of the sector wheel facing the LED. A
attached to moving masses and passes over the wheel. The
beam is alternatively blocked and un-blocked by the spokes of the
sector wheel as the wheel turns.
Each time the beam is blocked the photogate sends a signal to a
computer to record the time to the nearest milli-second, thus
permitting very accurate motion-time measurements. The photogate
was assembled by physics students at Warren Wilson College. The
students assembled another circuit using an ordinary transistor in
order to amplify the photogate signal to a level compatible to the
computer for data acquisition. A transistor is featured in PPOW
over a year ago on Nov. 18, 2005.
The photogate/transistor assembly was used by students to test the conservation of energy principle - specifically the conversion of gravitational potential energy of a hanging weight into kinetic energy of moving masses.