Logic Gates using only conductive material

Question

Is it possible to build logic gates (at least only OR, AND gates) by just using electric wires and resistors with no semi conductors (no diodes, no transistor or any other semi conductor material or device)?

The reason I'm asking this is paper circuits, by using only graphite pencil (graphite pencil lead contains some amount of graphene which is semiconductor but not a detail to focus since its in minimal amounts and don't affect it) and paper (recreating electronic components to some extent, and to try to challenge myself) making capacitors, resistors are simple but what about logic gates.

So I thought something like this, a power source that only supplies a consistent current (even if its shorted) e.g.: bench supply.

• drawing 2 lines connecting them together somewhere before the output.
• making output on end and to have higher resistance by drawing paper faintly, less current can pass due less graphite = less conductive.
• electricity takes shortest path that does have less resistance.

if a: false b:true then current going to rush through a and return to supply no electricity passing also due everything connected it should affect the output, output should be negative after this point.

Will something like this work? Here is thing susposted to drawn to paper.

I know this is short circuit technically but as stated earlier using a source that supplies fixed amount of current. By doing that at least its not going to burn or destroy the supply. I revised it a bit.

Answer 1

No, this is not possible with only resistive (or capacitive/inductive) elements like your graphite tracks. The reason for this is that resistive elements are passive and can only attenuate signals; they can't amplify them. Amplification requires an extra power source that can "inject" additional energy in a way that is controlled by the input signal. You need amplification to build logic circuits - otherwise the signals would only get weaker and weaker with every stage until they're undetectable.

Another way to look at the same problem is this: Given that you only have resistors and voltage/current sources to work with, you can apply the superposition principle to any circuit that you're able to build. This, in turn, means that the output voltage of any circuit that you could possibly build with graphite tracks is just a linear combination of the input voltages. In other words, the only kind of circuit you can build is one that takes its input voltages, makes them weaker by a certain factor, and then adds them up. The output voltage will always be less than the largest input voltage, too. You can't physically achieve any switching behavior.

This is also what the example circuit you drew does: It's a simple voltage divider that outputs the average of the two input voltages.

Current in a resistive circuit doesn't "switch" between paths once one of the paths becomes "full". Instead, the current divides between all of the available paths in proportion to their conductivity. In your case, that means there's less current flowing through the faint track and more flowing through the thick one. The ratio between these two currents will always be the same.

Here's yet another way to think about it: Logic gates are non-linear because they switch abruptly (their output voltage doesn't increase or decrease proportionally to the input voltage). Resistors, capacitors and inductors, however, are linear elements. Any circuit that only contains linear elements will itself be linear, which means it can't exhibit any switching behavior. Therefore it can't contain any logic gates either.

If you want to build logic circuits without semiconductors, you could use electromechanical relays. (Or vacuum tubes - but these are expensive and harder to use.) In general, you need some kind of non-linear amplifying device.

Answer 2

Question: Will it work?

Answer: No

Historically using voltage and switches form primitive logic with ON/OFF to make tandem wired-OR logic with AND in series. Exclusive-OR (XOR) is done in-home stairway switches with 3 wires between two SPDT switches to have each switching toggle the function of the other's position (Up/Down) for On/Off.

In some cases, negative logic was used so that a Normally Open (NO) switch could have been a logic "0" for "Off" with a pullup resistor to V+ and a closed switch state to ground flowing current as "1", although this must be declared by design as we assume normally logic "0" = near 0V.

The 1st (electric) computers used this logic with magnetic Relays, then vacuum tubes, diode logic, then resistor-transistor logic or RTL then TTL then NMOS briefly then CMOS.

But this assumption is false for DC.

"electricity takes shortest path that does have less resistance"

The correct assumption is DC conducts the path of least resistance, regardless of distance.

While high voltage potential (HVDC such as a cloud) may arc across the shortest gap or insulation path with lowest breakdown voltage, which is often the shortest path (or tallest tree or antenna) which then turns the insulation into a conductor of the least resistance.

Thus two traces of graphite are independent of distance but resistance is determined by the amount of conductive material.

Then if the path to the short is assumed to be lossless relative to the short, that short will have the full voltage across it's path with some current and the output will be at the potential in between half-way which is neither 0 or V+=1.

Ground is always defined locally wherever you decide is 0V. But if the traces leading to the short are also graphite, "the short" no longer has 0V on side side with a network of resistance paths which is again ambiguous.

Reason why it won't work

So in logic we define thresholds with margin or V_low(max) or V_hi(min) for both outputs and inputs for each state of lo or hi (0 or 1) and the grey zone in between is undefined.

Your schematic was in between "0" and "1" at best with missing specifications. (Again if you don't understand, be specific and try to find out why with a question then research it, or ask and be specific)