Physics Photo of the Week

December 8, 2006

Photo Transistor

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.

A transistor consists of three parts: the emitter, base, and collector.  In the photo transistor the emitter is the small circular part near the center of the square with a gold wire leading to it.  The base consists of most of the square with the wire leading to it from the bottom of the device.  The collector surrounds the other two parts of the transistor (underneath the visible square).  The case of the package is connected to the collector.  Light of the proper wavelength striking the base of the transistor "creates" electrons and holes in the base which in turn makes the transistor act as a switch.  The light striking the base "turns on" the switch, thus creating an electrical connection between the other two terminals.  The utility of a phototransistor is to make a switch with no mechanical moving parts.  If the light beam is blocked the circuitry connected to the transistor can turn on-or off a light, open an automatic door, work a TV remote, sense a computer mouse position, or can  be used for timing physics experiments.  This phototransistor is sensitive to infrared light instead of visible light.  This allows the transistor to be used in the presence of ambient light.  When a remote control is used to control a home TV, the remote unit contains an infrared emitting light emitting diode (LED) which flashes a code sequence so that the matching transistor in the TV receiver can receive the code and change the channel for the channel-surfing viewer from the comfort of the couch.

The 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 string is attached to moving masses and passes over the wheel.   The infrared 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.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

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