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Project Notes

#109 OR Gate with Diode Logic

Test the basic diode-logic OR gate


Diode logic was used extensively in early computers but is largely obsolete now, since it cannot isolate inputs or outputs without additional circuitry, and there will be at least a 1-diode voltage drop to deal with across the gate.

It is interesting to see how various logic gate may be realised with diodes (and resistors) alone. Here is the basic OR gate.

Output is high given any of the inputs are high:

0 0 0
0 1 1
1 0 1
1 1 1

In this circuit with VCC of 5V and where Vf is the diode forward voltage of approx. 0.7V:

  • Out logical “0” == 0V
  • Out logical “1” == VCC - 1 x Vf ~> 4.3V

Pull-down resistors R2,R3 are used to prevent floating inputs.

The Arduino is not a core part of the circuit. It only does the following:

  • provides +5V power supply (for convenience)
  • measures the input and output voltages for plotting. It uses the PlotNValues sketch for this purpose.

Here’s a sample trace. The upper trace is the output, the lower traces the inputs.

processing trace



The Schematic

The Build

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.