HMC5883L may not activate by default. Must run this command to activate first i2cset -y 1 0x68 0x37 0x02 i2cset -y 1 0x68 0x6a 0x00 i2cset -y 1 0x68 0x6b 0x00 then set continuous measurement mode i2cset -y 1 0x68 0x02 0x00