Nokia TWS-521 Guide De Démarrage Rapide page 6

3. Using your earbuds
3.1. Touch control
4. Additional controls
4.1 Use single earbud:
You can use either left or right bud as a mono Bluetooth headset after initial pairing.
4.2 Reset to the original settings:
i) Put both buds into charging case.
ii) Press each of the buds 4 times rapidly.
iii) The indicator will turn purple and it is now reset.
3 secs
5 secs
1 secs
5