Suppose there is another 20-ms light pulse that may strike the cell in given Problem, but it is of lower intensity, so that the final resistance would be . Call this one and that of Problem P2. Devise a system that will latch on a red LED if strikes the cell, or a green LED if strikes the cell. Each time a pulse strikes the cell, the latches are cleared and the appropriate LED turned on.
Problem
A CdS cell has a time constant of 73 ms and a dark resistance of 150 kΩ. A light pulse that is only 20 ms in length strikes the cell. If the intensity of the light pulse is such that the final resistance would be , plot the resistance versus time for 100 ms. What is the resistance at 20 ms?