_______________________________________________________________________ Special Devices
3-59. Figure 3-33 shows the schematic symbols for the different types of
phototransistors. Phototransistors may be of the two-terminal type, in which the light
intensity on the photodiode alone determines the amount of conduction. They may also be
of the three-terminal type, which have an added base lead that allows an electrical bias to
be applied to the base. The bias allows an optimum transistor conduction level, and
therefore compensates for ambient (normal room) light intensity.
3-60. An older device that uses light similar to the photodiode is the photoconductive
cell, or PHOTOCELL. Figure 3-34 shows the cell along with its schematic symbol. Like
the photodiode, the photocell is a light-controlled variable resistor. However, a typical light
to dark resistance ratio for a photocell is 1:1,000. This means that its resistance could range
from 1,000 ohms in the light to 1,000 kilohms in the dark, or from 2,000 ohms in the light
to 2,000 kilohms in the dark, and so forth. Of course, other ratios are also available.
Photocells are used in many types of control and timing circuits (for example, the
automatic street light controllers in most cities).
Figure 3-33. Two-terminal and three-terminal Phototransistors
Figure 3-34. Photocell
23 June 2005