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Project Notes

#013 NOT Gate with Resistor-Transistor Logic

Test the basic resistor-transistor logic NOT gate (inverter)

Here’s a quick video of the circuit in action:



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 inverter gate.

0 1
1 0

The circuit uses a digital output pin as the input to the inverter. The output is tapped at the collector of the transistor. The output voltage is read with an analog input pin, and the value is echoed to the Arduino serial port.

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 input
  • measures the input and output voltages for plotting

Here’s a sample trace. The lower trace is the signal input, and the upper trace is the output of the inverter.

processing trace

We clearly lose significant voltage at the output because of the current-limiting resistors. There is also noticable noise in the output, mainly because the circuit is so simple it lacks any attempt at smoothing the source voltage or output. These would be significant issues when cascading the inverter output to subsequent logic blocks.



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.