Commodore 64 IRQ Bicolor LED

This is a very old project published in a Dutch magazine to capture the state of the IRQ for Commodore 64 (vic-20) with a LED. After many years i have decided to make it.

On the photo you can see the project and the right connections on the cartridge port.


  • 1 x IC 74LS04.
  • 1 x Bicolor LED.
  • 2 x 330 ohm resistors.
  • Tiny wires.

How it work?

When the CPU generates an IRQ the green LED lights up, otherwise is red, if the irq is turned on and off quickly the led is orange.


  1. RandyInLA
    May 31st, 2011 at 00:23 | #1

    The bicolor LED I bought at Radio Shack (part: 276-0012) only had two leads, not three. To make this work with the schematic, simply skip the connection from GND to the middle lead of the LED, as it doesn’t exist.

    Certain c64 applications (eg.GEOS) cause the LED to stay orange (turning on/off very fast). I don’t like the look of the orange, so I wanted a simple way to turn the effect on/off that didn’t involve opening the c64. This can be achieved by either:
    1) Adding a 330Ohm resistor in between the IRQ line and pin 1 of the 7404 and then add an externally mounted SPST switch wired to short pin 1 of the 7404 to GND when closed. Shorting to ground overrides the IRQ signal, leaving the LED red.
    2) Adding an externally mounted SPDT switch, connecting the middle pin of the switch to pin 1 of the 7404 then one of the other switch pins to the IRQ line and the last switch pin to GND. The switch now chooses what is connected to pin 1; IRQ or GND.


  2. August 29th, 2014 at 09:56 | #2

    This is a great mod for the 64 and we have made a small plug in PCB, have a look at -

  3. September 1st, 2014 at 23:32 | #3

    @Tim Harris

    Hi Tim, you have made a nice work!.