Is is possible to create modular logic gates from transistors for teaching purposes?

Question

I'd like to work through some basic logic circuits with some youngsters and wanted to build them up from scratch (transistors), not logic gates if possible.

We've done the basics: buffer, NOT, AND, OR, NAND, XOR just using NPN mosfets and resistors. We've even been able to get to D-latches. As an example, the AND gates we're playing with have been build like this:

^{simulate this circuit – Schematic created using CircuitLab}

The problem is that as the circuits have grown in complexity, with transistor emitters feeding into other transistor bases, I've only been able to get things to work by very carefully balancing resistor values to make sure transistors are triggered when they should be.

I would like to be able to build modular logic gates, which are identical and can just be plugged into one another without needing to carefully calculate resistor values.

Another restrictions is that we currently have 200 BC547ATA NPN transistors and a big bunch of resistors, so if its achievable without buying any more components, that would be ideal.

Answer 1

If all you have is NPNs and resistors, you'll want to use some form of RTL. Here, the basic logic element is the NOR gate. The one-transistor gate uses fewer transistors (obviously), but the multi-transistor gate is more robust in several ways.

And there's a teaching opportunity here, to show how the NOR gate is a "universal" logic element — all other functions can be created by combinations of NOR gates (including the degenerate 1-input NOR gate, or inverter).

Even large systems have been built this way. The original Cray-1's logic was entirely implemented using 4- and 5-input ECL NOR gates!

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One implementation strategy would be to build up individual 3-, 4- or 5-input gates on single-inline modules like these:

(source)

These are easy to plug into a breadboard socket, allowing the students to focus on the logic they're building. If you need a large number of them, have a custom PCB made.

And if you're really ambitious, you could plug them into a universal wirewrap card for larger projects and a more permanent implenentation.

Answer 2

Building discrete logic with MOSFETs is much easier than bipolar transistors, because the high gate impedance prevents the issue you describe. NMOS circuits are easier than CMOS (which is more commonly used because it has lower power).

Answer 3

I think you want to follow TTL logic. I haven't done it myself, but the problem with this simple wiring up of transistors that you are showing is prone to fail when you put more of them together.

So here is the TTL way of doing things. This is a NAND gate

what I find a good instruction here -- and you might want to sprinkle in a littler diode-resistor logic too -- is that you are entering on the emitters. This keeps the students appreciating the difference between how the logic bits "flow" as opposed to how currents flow, you know, appreciating the fact that a logic 0 state means the output element needs to sink current (that's the same lesson with the diode AND gate).

And you don't need dual emitter transistors, I have read you can just wire two transistors in parallel, base and collector together and separate emitters.

PS: if you can use my approach (please report back) and you find that you're now in need of more transistors, I will donate another 200 to your project.

The totem-pole output stage will also be interesting to experiment with. The whole nine-yards: open collector, normal always on, and three-state. Cool project.

Answer 4

The modules below are liable to be a better starting point.
Resistor value may need adapting to suit.

I've shown BC337-40 because they are cheap and superb. The BC547 is acceptable also. Smaller base resistor values MAY be needed for lower beta transistors but a high ratio between collector and base resistors minimises loading.

Note how the AND works. The inputs via diodes stop R1 pulling the base high.
So an open input is equivalent to a high input.

The inverter input is undefined without an input. If liable to be left floating add and eg 100k to ground AT the input.

The buffer has a Vbe voltage drop but should drive an LED to ground with series resistor acceptably. Reduce base resistor value for more LED drive.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 5

The simplest BJT gate circuit is the DCTL (Direct Coupled Transistor Logic) 2-input NOR.

^{simulate this circuit – Schematic created using CircuitLab} This is a universal component: any digital system may be reduced to these. To get it to fan out, you need transistors with closely matched Vbe. If your transistors are all from the same manufacturing batch, you're probably OK. Otherwise, you may add a ~100k resistor in series with each base to prevent any input hogging the current from the output it's connected to.