# How to intuitively explain the role of transistors in boolean logic and switching?

Boolean logic is easily explained using truth tabels, but the underlying hardware is rarely mentioned in the same class. I would like to explain boolean logic from the point of view of actual transistor. My target audience is educated adults with no background in engineering, for a one-time public lecture. The goal is to have them understand why a simple cut-wire alarm circuit is designed the way it's designed, and without going "deep" into solid state physics.

• Can you be more specific about what you actually want? Ideas for explaining the role of transistors? Material for it? Experiences with such explanations? Is this going to be a public lecture or something of that kind or part of a longer course? – Joonas Ilmavirta Feb 25 '15 at 16:49
• @JoonasIlmavirta, I've updated the question accordingly. – Sparkler Feb 25 '15 at 17:36
• Are you sure you want to use transistor logic? Or would diodes and resistors be sufficient? – Joel Reyes Noche Feb 26 '15 at 12:40
• You can also explain boolean logic gates using billiard balls. – Joel Reyes Noche Feb 27 '15 at 1:15
• In that case you can inspire yourself from allaboutcircuits.com/vol_3/chpt_5/2.html Alas their page on MOSFET/IGFET is not yet done. – Fizz Feb 27 '15 at 17:32

You can explain them as electrically controllable switches.

• If a voltage is applied to gate and source, this means switched on: Current may flow between drain and source. Or voltage is transferred through the transistor.
• If no voltage is applied, this means switched off: No current may flow resp. voltage is blocked.
• Yeah, but things get complicated depending which type of transistor you're taking about. This is a good explanation for FETs but not for BJTs. I feel this question is not really at home on this site. – Fizz Feb 27 '15 at 10:13

This may be off the mark of your question, but I have explained logic gates composed of (transistor) switches by using water pressure as analogous to electricity. Below, water will only reach the pipe-end $C$ if water pressure comes in both along pipe $A$ AND there is water pressure from $B$ that pushes up and closes the normally open switch, connecting $A$ to $C$.

If either there is no water pressure at $A$, or there is no water pressure at $B$, no water reaches $C$. So this is an AND-gate. Etc.

You could follow the suggestions made in the other posts, but I would start with an even simpler model which you could build and demonstrate easily: levers.

Consider building a contraption (a la Rube Goldberg, but much more boring) of remotely flipping a light switch (where down means light is off and up means light is on). One way to do this is to attach a long rod to the switch as follows: fix one end (use a universal joint as needed) of the rod to the switch, and fix the middle of the rod to a stationary object. You now have a logical inverter (the other end of the rod will be up when the switch is down, and down when the switch is up).

This requires a representation (0 means down and 1 means up) and a tolerance for building more complicated contraptions (concatenate two inverters to get the Boolean identity function, take two inverters and a spring or a lead weight on the end of a third inverter and make a two input AND gate by placing the three levers in a star configuration with the weight in the center; pushing on the ends on one of two of them will cause the third end to fall), but conceptually they are very simple: this only requires people having seen a lever before, and the contraptions aren't that complex. You will need very few levers to make a model of a transistor, and many people should make the analogy between lever position and voltage level easily.

Gerhard "Mixes Water And Electrons Mentally" Paseman, 2015.02.26

• To be fair to your audience, you should mention that the transistor is not a two-state device always, and that the lever model is just to help in describing the Boolean algebra aspect. It is possible to adapt the lever model for multistate devices, but I would not try it for a public lecture. Gerhard "Likes To Please The Audience" Paseman, 2015.02.26 – Gerhard Paseman Feb 26 '15 at 19:33