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Project Notes

#113 TTLBufferDriver

Using an OpAmp as a final buffer stage for TTL or similar circuits


TTL devices and other logic circuits often need to chain to other circuits. A buffer is generally needed to ensure there is a good impedence match to the following circuit, and also to ensure a consistent signal is passed on.

OpAmp buffers can be used to provide unity gain and isolate the input and output sides.

This circuit uses an LM324 configured as a comparator in order to “clean up” the output of an imperfect Resistor-Transistor Logic AND Gate.

With a 5V power supply, the basic AND gate drives a 3.8V output when both transistors are on, and 0V when both are off. But when the lower transistor is on, there’s gate-emitter leakage resulting in a semi-on state of about 1.9V.

The voltage divider Rd1/Rd2 set the 0/1 threshold for the OpAmp comparator at half-way (2.5V), so the resulting output is a clean ON or OFF - nothing in between.

Here’s a sample trace. The lower two traces are the inputs to the AND gate. The top trace is the output of the OpAmp, and the lower-top trace is the output of the AND 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 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.