So, what's the difference between the Breville 800ES/B and the Gaggia Classic Pro that's normally used as a the basis of a Gaggiuino conversion? Once you remove the 800ES/B's control electronics, there really isn't a lot of difference. Like all espresso machines, there's really just a boiler and a pressure pump, so wiring these up according to the Gaggiuino documentation is easy.

There is one significant difference - the 800ES/B doesn't have a solenoid valve that controls flow between the boiler and the grouphead. Instead it has a manual valve that you have to twist to the left to brew and to the right for steam. While it might seem like this creates additional steps to brew (open the valve _and_ press the brew switch), happily the valve is set up with two microswitches that tigger when the valve is opened to the left or right. We wire these microswitches up in parallel and use them as the brew switch.

The other difference is how we have to mount the thermistor to the boiler. Unlike the Gaggia, there's no convenient M4 threaded hole that we can screw the thermistor into. Instead, we'll have to use a thermistor with a ring mount, and attach this to the boiler using one of the screw holes that previously held the thermal switches. It's important to use one that is ungrounded. I'm not sure exactly why, but I know that the first thermistor I tried was grounded and had huge issues with electrical interference. Here's the correct one I used: https://www.aliexpress.com/item/1005003664282170.html?spm=a2g0o.order_list.order_list_main.37.21ef1802TWlqg9

Above is a picture of the boiler. On the left is the thermistor with the metal braided cable. In the middle is the thermal cutout fuse with the brown wires. On the right is one of the original thermal switches. This is not used by the Gaggiuino system and is completely disconnected.

One final warning - the thermal cutout fuse needs to remain wired into the circuit in such a way that if it blows the boiler will not be powered.