MCS-170 The Nature of Computer Science

Fall 2006

Lecture 16

1. Chapter 16 - Logic Circuits and Switches

2. Switches

  • In the above relay circuit, the switch is open by default. We could reverse this and have a relay that is by default closed. If the switch on the relay is labeled "P" then when P is TRUE the bulb is extinguished; we could say that the bulb is FALSE, or NOT P.
not relay
  • Generation 0 Computers (1940's) - Vacuum Tubes as switches. Tubes could control current, just like a relay, but could switch on and off much faster. However, they generated lots of heat and burned out frequently.

3. Transistors (Generation 1 Computers)

  • In the n-type transistor, both the source and the drain are negatively charged and sit on a positively charged well of p-silicon.  When positive voltage is applied to the gate, electrons in the p-silicon are attracted to the area under the gate, forming an electron channel between the source and the drain.
  • This opens the flow from the drain to the source, so that when a positive voltage is applied to the drain, the electrons are pulled from the source to the drain. In this state the transistor is on.

4. Transistors as Building Blocks of Logic Gates