An automatic security camera must operate as follows when an intrusion occurs: 1. If the ambient light level is greater than 20mW/cm2, the shutter is activated. 2. If the ambient light is less than 20mW/cm2 a light source is triggered. 3. When the source lighting reaches 20mW/cm2, the shutter is triggered. Design the system and signal conditioning using the following assumptions:
a. The shutter and light source are triggered by TTL logic high signals.
b. A CdS cell with characteristics given in Figure 6.9 is used to monitor ambient light level.
c. A photodiode with the characteristics given in Figure 6.16b will be used to measure light intensity after the light source is triggered. Explain why the CdS cell could not be used here also.