We investigate the basic chemical processes involved in building the basic switching unit of computers, the transistor. These devices may be composed to build complex computers that operate digitally. We investigate, for example, how to build a simple counter.

1. The transistor.
   1. The transistor is a physical device that is the basis for the first logical layer in building complex decision-making machines.
   2. The potential at a gate of a transistor controls the flow of current across the transistor's other two leads. A large potential difference will cause a bridge to develop under the gate; the bridge facilitates the flow of electrons.
   3. Transistors make use of the semi-conducting property of doped silicon. Other atoms in the same column as silicon are semi-conducting as well.
   4. Computer scientists make use of the transistor in its extreme or saturated modes. Physicists use the non-saturated region of the transistor to develop simple amplifiers of the signal presented at the gate. The transistor radio made use of these inexpensive amplifiers to improve portability and to reduce power consumption.
2. The gates (review).
   1. Logic levels. We think of 0 volts as false, all other values as true.
   2. The inverter makes use of a single transistor.
   3. The NAND gate makes use of two. It generates a true output when at least one of the inputs is false.
   4. The AND is its inverse (and can be made of three transistors). It generates a true output exactly when both inputs are true.
   5. The NOR is true exactly when both inputs are false.
   6. The OR is true whenever at least one of the inputs is true.
   7. The XOR (exclusive or) is true exactly when one input is true.
3. Making use of the delay in gates.
   1. A leading edge detector---generates a short spike when the input moves from low to high. (Thought: A sort of one-sided "contrast indicator".)
   2. A trailing edge detector---generates a short spike when the input moves from high to low.
4. Feedback: adds complexity to otherwise simple systems.
   1. A debouncing device. Damps uncertainty in signals.
   2. (review) A bistable device. Happy when it's in one of two different states.
   3. A clock. Generates heart-beat pulses for driving computers.
   4. A Set-Reset Latch.
   5. A clocked SR-latch. Two make a "flip-flop".
   6. A T-type latch. Always remembers something different...or, counts from 0 to 1, based on leading edge of the clock.

Resources needed for this week's classes (please read!):

• von Neumann's The Computer and the Brain, 1958. His final lecture. Please read at least pages 39-60.
• An article on The Neuron from Scientific American's The Brain, 1979.

These items require Acrobat Reader.