I currently don't have a laptop that can be used without being connected to a power supply.
When I had one, I tested if my wall socket is interfering with the signal and I think it is, because the score line became more stable after pulling the plug.
So could it be that other wall sockets in my house are not a problem? Or do I definitely need a laptop to use HEGduino?
The ratio looks good when you are on battery/laptop, so it should be working. And yeah this thing is a good test of how well grounded your house is haha. Does the score change when you wiggle your forehead? The handband should be snug but not uncomfortable, you want it laid down like a piece of tape so light doesn't bleed in our out. You can also pull the black tape on the photosensor open a bit to get more light in but that shouldn't be a big issue. It should be a pretty gradual change in the score as it is only meant to multiply the changes in the ratio to make it more obvious. Don't worry about the score number just the change in it as you exert yourself. Deep breathing exercise should increase it. V2 won't have the noise issues nearly as bad thankfully due to a better analog front end but your kit should be working fine by your account.