Tulley Family Tinkering Lab Website

DIY logic gates – Science Fair Project progress

by on Apr.04, 2010, under science fair

One huge problem we have with Robbie’s science / invention fair project is that we are relying on RadioShack for parts. Yeah. That one store that USED to be a good supplier of electronics. Now they have a small part of one row, and a few drawers with SOME components.

Unfortunately they lack the following:

  • Any logic ICs
  • A useful set of assorted capacitors (the one bunch we bought was almost useless!)
  • Small potentiometers
  • Employees who have any knowledge of what these components are.

We can deal with large pots, and change some circuit values to work around their meager capacitor supply. But, the first one was a real problem. We needed some AND, NOT, and OR chips 74LS chips for our circuit to be successful.

Fortunately it hit me at some point that these circuits should be easy to build with simple transistors. So, I looked them up on the Internet. Oh duh — dead simple. In fact, I had accidentally built a working NOT gate using a transistor while trying some stuff out.

There is a good article on Making Logic Gates with Transistors on Squidoo, so I won’t repeat it in depth here.

In summary:
For an OR gate — two transistors in parallel (collectors are tied to each other and emitters are tied to each other). If either one is “on”, voltage follows through the transistor and to the output (taken at the emitter). Makes sense.

AND gate — Two transistors in series — the emitter of one is hooked to the collector of the next one. So, BOTH have to be “ON” (with a voltage on the base), for the voltage to flow through the circuit. Output is taken from the second transistor’s emitter.

NOT gate — one transistor, and a resistor between the emitter and ground. The “output” of the circuit is taken right from the collector, which is tied to VCC. So, basically that point drops to zero when the transistor is on, since all the voltage drop is across the resistor at that point.

It was kind of fun doing this ourselves, and used less room on the breadboard too. We WERE contemplating rigging our NAND 74LS20 chip to do the NOT function, but that would’ve been a huge waste of space. And, when we found we needed OR gates instead of AND gates (we had 74LS08 AND chips), we realized that transistor-based logic was inevitable.

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