A Full Adder - Using Analog Components for Digital Logic

Abstract

This paper details the process of building a full adder out of 2N3904 NPN transistors and resistors. Sure, there are ASICs (Application Specific Integrated Circuit) such as the 16 pin DIP 4-Bit Full Adder that performs the job; however, a group of students asked themselves what truly makes up a full adder? and came up with this wonderful design. Their answer is to use Analog components to build digital logic. Aside from the NPN transistors, resistors are used for limiting the base current to the transistors and for pulling up the outputs to the source voltage. Toggle switches work as bit inputs to the adder, while green LEDs display the output. Logisim is used to simulate different combinations of gates to build the full adder circuits and to show some additional challenges. Based on the various design outcomes, the final full adder design was chosen to use combinations of the universal gate NOR which needs fewer transistors to produce the same result as other gate combinations would. An oscilloscope is used to measure the delay between the input and output of the full adder. The project allowed Engineering Technology students at Sam Houston State University to engage in a challenging hand-on project to brainstorm, think creatively, make simulations to back up the theory, assemble the components on a circuit level schematic that included soldering, do measurement results, and finally build a PCB prototype. The focus of the Electronics and Computer Engineering Technology (ECET) curriculum is to have our students acquire hands-on skills, be able to understand and grasp multidisciplinary knowledge and be industry ready upon graduation. The students did an oral presentation and prototype demonstration at the end of semester and submitted a technical paper in IEEE format. This built prototype has earned strong attraction from attendees in various recruiting events regardless of age, sector, or orientation. The success of the project and real time presentation has encouraged students to participate in more active learning and innovative challenging projects integrated in many other courses in Engineering Technology department at SHSU by offering new and challenging concepts to the curriculum.