This Commodore 64 IRQ LED mod is a fun little hack that I spotted on eBay last year. Since my C64 mainboard has just come back from being re-capped I finally took the opportunity to fit this whilst I had the case open.
It’s a very simple little device that changes the colour of the C64 (or VIC20) power LED according to IRQ activity. When the computer is just idling the LED will glow red as usual. However when the CPU is active and generating interrupt requests (IRQ’s) the colour changes to green. This allows you to instantly see at a glance if your C64 is doing something. Anything that causes rapid IRQ’s will actually make the LED appear to be orange as it flicks rapidly between red and green.
I picked up two of the devices, one for my 64 and the other for my VIC. I should point out that this isn’t a destructive hack. Nothing is permanently altered or damaged in any way and it can easily be reversed if desired. The device itself is really simple and there’s (usually) no soldering required. It consists of a tiny circuit board containing an LED, a couple of resistors and a single chip that detects the IRQ signals and triggers the LED colour changes. Connected to the board are 3 wires that are terminated with IC clips. These clips attach to the cartridge port pins and this is how the device monitors IRQ’s.
For breadbin C64’s and VIC20 computers fitment is extremely simple. You just unplug and remove the existing power LED and replace it with the little circuit board. There’s a small black plastic ring on the inside that needs pulling off and then the LED should push into the case from the outside with a little bit of force.
There’s a spare black plastic collar for mounting the LED supplied in the kit in case you break the existing one. Also supplied is a little double-sided adhesive pad that you can use to fix the board in place. The new LED will need a little pressure to snap it into place and with the help of the adhesive pad it should be held nice and secure.
Now it’s just a matter of wiring the board up. The 3 wires need to be attached to the front row of cartridge port pins using the IC clips as indicated below.
One the VIC20 the green clip goes onto pin 22 (Ground), Red – Pin 21 (+5V) and White – Pin 19 (IRQ).
On the C64 it gets wired up as follows; Green – Pin 1 (Ground), Red – Pin 3 (+5V), White – Pin 4 (IRQ).
Time for the moment of truth – putting the case back together and giving it a test drive. The LED in my kit had been soldered on in reverse so when my VIC was idle it lit up green and when busy it changed to red. I could have solved this by de-soldering it and flipping it round but it really doesn’t bother me so I left it alone. Other than that it works exactly as advertised and I’m really happy with the result.
I hinted earlier that non-breadbin installs aren’t quite so simple. I have a C64C and as supplied the mod will not work with this model. There’s a couple of reasons for this. The most obvious being the C64C has a rectangular LED rather than the usual round one found in Breadbin style machines.
The other problem becomes apparent once you open up the case. As can be seen in the photo below the power LED is at the opposite end of the case to the cartridge port so the supplied wires are too short.
Fortunately these issues are easy to sort. I mentioned about the LED to the seller (Tim Harris who runs Shareware Plus) and he kindly supplied the board without an LED fitted so I could fit a rectangular one myself. These ones here are a good fit: rectangular LED’s on eBay.
In order for the LED to fit in the existing hole I soldered it to three 10 inch lengths of wire and then soldered the wires to the circuit board. I fitted some heat shrink tubing over the joints to insulate them. This allowed me to mount the circuit board close to the cartridge port and also have the LED in the correct place.
I attached the board with a small double-sided sticky pad to hold it in place. I also carefully bent the wires on the LED 90 degrees so the cables would lie flat along the top of the case.
The IC clips were connected to the cartridge port pins as follows:
- Green – Pin 1 (Ground)
- Red – Pin 3 (+5V)
- White – Pin 4 (IRQ)
After giving it all a quick test I put the case back together and had a play around with it. When idle the LED lights up red as normal – a much more vivid red than the photos show. When the CPU is actively generating an IRQ such as when loading off a disk the LED with light green. Rapid IRQ activity that can happen when playing a game makes the LED appear orange.
I’m really impressed with this little mod. It’s one of those things that’s kind of pointless but also completely essential at the same time. I love having a visual indicator that my computer is doing something and during loading or saving operations it functions as a kind of drive activity light.
You can see it working clearly in the video below, taken whilst I was loading a program off a floppy disk.
If you enjoy tinkering and like the idea of having an activity light on your C64 or VIC then I can thoroughly recommend this. Did I mention that it costs less than a tenner too? A real no-brainer for me.