With the flooding of New Orleans in the wake of Hurricane Katrina, people around the world became all too aware of human dependency on technology to manage the forces of nature and maintain a high quality of life. New Orleans' sewage and water control systems depend on water pumps to move sewage, storm-water runoff, and fresh water around the city. Under normal conditions, when it rains in New Orleans, a city that sits below sea level, massive water pumps are automatically engaged, and rain water is pumped into Lake Pontchartrain, a nearby reservoir. However, when Katrina hit and electrical power to the city was cut, the system failed.
The transport of drinking water is one of the technological marvels that people do not think about until they have to go without. Water systems need several important components in order to operate efficiently, such as pumps and pump motors and reliable automation.
At the heart of the system that automates water pumping is an electronic control circuit. Electronic control circuits are used in a wide variety of applications, from controlling pump motors to making automobiles more safe and efficient. The same technology that forms the essence of this control and automation is also the basic building block of central processing units in computers. This technology is so pervasive in everyday life that it is truly a fundamental of technology that middle school and high school students need to understand.
This technology is known as the binary logic gate.
Logic Control Circuits
Often, logic circuits are interfaced with other devices, such that the electronic circuits control the behavior of the other devices. For example, if there is water present in a storm drain, a water pump should automatically turn on. In order to do this, the logic circuit must be able to tell if water is present. This is accomplished by using sensors. There is a variety of ways to sense the presence of moisture. On some sensors, if water is present, the water changes resistance to electrical current flow. In turn, the sensor device will output a specific level of electrical voltage. It is the sensor's output voltage that becomes the input to the binary logic gate. In this case, if the sensor's output voltage is low (say 0 to 1VDC), then the output is low or zero. If the output voltage is high (say 4.7 to 5 VDC), then the output is high or 1.
Binary logic gates may be thought of as configurations of simple switches (see Tokheim, 2003). Suppose an electrical switch is being used to control a light. When a switch is closed, the light is on. When the switch is open, the light is off. In the world of logic, "on" is represented by 1, and "off" is represented by 0. On/off, 1/0, this is why the gates are referred to as "binary." They work on a base 2 number system like the central processing unit in a computer.
Logic gates are systems within themselves. They receive input. They process that input and change it into an output. When the sensor sends a 1 (or high voltage) to the input of a logic gate, the logic gate processes that input signal into an output. Whether or not the output of the logic gate is a 1 or 0 (high or low) depends on how the logic gate is designed.
In Figure 2, an AND gate has been constructed from simple switches. Only when switch 1 ($1) AND switch 2 ($2) are both closed will the light-emitting diode (LED) illuminate.
[FIGURE 2 OMITTED]
Two conditions must be met before the device being controlled can be turned on. There must be moisture present AND the water valve leading to the reservoir must be open before the pump will turn on. In the truth table for the AND gate, notice the inputs (A and B). If any input is 0 (low), then the output will remain 0 (low). Only when both inputs are 1 (high) will the output be 1 (high). That is the condition under which the logic control circuit will cause the device (such as a water pump) to be turned on. …