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#112 OR Gate with Resistor-Transistor Logic

Test the basic resistor-transistor logic OR gate.


Resistor–transistor logic (RTL) is a class of digital circuits built using resistors as the input network and bipolar junction transistors (BJTs) as switching devices. RTL is the earliest class of transistorized digital logic circuit and is largely obsolete now.

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

This is a “classic” two-transistor OR gate, often used when describing the fundamentals of digital logic. Given two ideal transistors (NPN) with common collector, we treat them as switches: when either is on, the output should be high:

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

Here’s a sample trace. The lower two traces are the signal inputs, and the upper trace is the output of the gate.

processing trace

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

  • provides +5V power supply (for convenience)
  • automates the toggling of the driving inputs
  • measures the input and output voltages for plotting


The circuit uses a digital output pins to sequence inputs to the gate. Three analog input pins are used to read the two input signals and the resuting output, with the values is echoed to the Arduino serial port for plotting with PlotNValues (a simple Processing sketch).


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.