Dear Quim,

The red flash means the unit has less than 10% power. The winter sunlight may not be very strong. 

Could you please place the sensor under the sunlight for another 2-3 days to charge it up? Also, Do you have an incandescent lamp at home? If so, you can try to put the sensor under the incandescent lamp for 24-48 hours and it may come back to work.

Thanks,

Wency

Netro Support