Story

I have used the original IRQ module for a CPU indicator on my Commodore 64 for quite some time and find the circuitry very versatile.

When I built my own Mechboard64 for my C64, I wasn't very happy with having the Shift-lock LED status under the keys so I reused the IRQ circuitry as a way to handle it but that meant my computer lost the CPU usage indicator in favor of the Shift-Lock.

Which is why I came up with this new design, built on top of the classis IRQ module, to have BOTH.

Purpose

This circuit is meant to handle 2 different input signals and display them on a single RGB led.

I use it on my Commodore 64 to display the IRQ state (CPU usage) and the Shift-Lock of my Mechboard64 - together in a single PCB module.

In reality this very versatile 7404-based design can be used for quite a lot of purpose. I have used its predecessor in the past as a base for many mods - now my new version opens even more possibilities.

Schematics

I have modified KiCad design from Tor-Eirik Bakke Lunde - he himself based his design on an old article based off a Dutch Magazine.

My alterations adds a second signal and third LED color.

The Shift-Lock uses spare inverters on the 7404.

The idea is to have the normal (Idle) state (I use red) and CPU-activity IRQ state (I use greed) coloration modified by the blue LED for Shift, resulting in purple (Shift-lock Idle) and Cyan (Shift-Lock + IRQ)

Of course, any one half of the circuit could also be used separately, and any TTL signals could be fed to the circuit, for infinite possibilities.