Several different flame-detection technologies exist: optical (detection of light), thermal (detection of high temperature), and electrical conduction (detection of ionized particles in the flame path), each one with its unique advantages and disadvantages. What we need in this system is a sure way of detecting the presence of a flame, and permitting waste to be injected only if a flame is “proven” by the flame detection system. If the flame were to be extinguished, however, it would be unsafe to continue to inject waste into the combustion chamber, as it would exit the exhaust un-neutralized, and pose a health threat to anyone in close proximity to the exhaust. So long as a flame is maintained in the incinerator, it is safe to inject waste into it to be neutralized. Such combustion-based techniques are commonly used to neutralize medical waste, which may be infected with deadly viruses or bacteria: The intense heat of the fire is intended to neutralize the toxicity of the waste introduced into the incinerator. Suppose we were given the task of designing a flame detection circuit for a toxic waste incinerator. To illustrate this procedural method, we should begin with a realistic design problem. Here, Boolean algebra proves its utility in a most dramatic way. While some people seem to have a natural ability to look at a truth table and immediately envision the necessary logic gate or relay logic circuitry for the task, there are procedural techniques available for the rest of us. The design task is largely to determine what type of circuit will perform the function described in the truth table. In designing digital circuits, the designer often begins with a truth table describing what the circuit should do.
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