After a lot of jumping through hoops, I finally found the schematic for the GY-511 magnetometer and accellerometer module. I wanted to see what voltages I could run it at.

Many sellers have links to a Dropbox file that is blocked.

I found some links to the Chinese site Baidu which I ran via Google Translate. But they seemed to require a registered user to download.

Registering at Baidu appeared to require a mainland China phone number. But then I found a site that said that you can register from abroad using this link:

https://passport.baidu.com/v2/?reg&u=http://pan.baidu.com®Type=1&tpl=netdisk&overseas=1

And sure enough, I got the SMS code to my Swedish phone embedded in an SMS full of Chinese characters, and was able to register. The second file I tried to download actually had the schematic:

It looks like I could power it through the 3.3V output with a voltage that the LSM303DLHC chip accepts (down to 2.16 V). It draw so little current, that I should be able to power it from an I/O pin on the nRF51822, to turn it on and off.