Fork me on GitHub

Project Notes

#025 74LS75 Quad Latch Test

Use an Arduino to demonstrate a basic 74LS75 Quad Latch driver circuit, and plot the results with Processing.

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

74LS75 Quad Latch Test


The 74LS75 Quad Latch is a 4-bit bistable latch from the 7400 series of TTL integrated circuits.

This circuit demonstrates the basic operation. It uses the Arduino to latch a data series to the 4 input pins (D1-D4), while sampling the 4 output bits Q1-4. The 7475 also handily offers inverted output pins, but they are not used here.

Data is transferred from input to output pins when enable pins go HIGH. While enable is kept high, latch output will follow latch input. When enable goes low, previous output state is maintained, regardless of subsequent input changes.

It is possible to separately latch bits 1-2 and 3-4, but for simplicity this circuit latches all bits at once (with pins 4 and 13 in tandem).

Here’s a sample trace, with Q1-4 plots stacked:

processing trace

The trace is generated with LEAP#090 PlotNValues (a simple Processing sketch). It reads the data from the serial port and plots the input and output value over time, with some coloration effects thrown in for good measure. In other words, we’re using Arduino and Processing as a basic oscilloscope! And it kind of works, mainly because the frequency is so low.


The circuit uses a digital output pins to sequence inputs to D1-4 and latch the value. Four analog input pins are used to sample the latched output Q1-4, with the values is echoed to the Arduino serial port.

The circuit diagram uses a custom 74LS75 Fritzing Part available here.

The Breadboard

The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

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.