It helps me to use a fine thread in the bobbin (Aurifil 50wt 2-ply or polyester embroidery bobbin thread, which is 60wt). The bobbin holds a lot more thread when the thread is fine. I check the bobbin before I start to give me an idea of how much is left. For me, if I have a full bobbin wound with one of these threads, I know that I will run out of steam for a marathon sewing session long before the bobbin runs out of thread. I have found that checking the bobbin *before* I start sewing helps. Nowadays, if the bobbin is low, I will change it out rather than risk forgetting about it and sewing a mile with no bobbin thread.

Sometimes my machine will fail to indicate low bobbin, and it's always due to a film of dust/lint over the sensor light, which is a tiny bubble light on the inside of the door. The lint prevents the light from shining through the bobbin and hitting the light sensor behind the bobbin, thus no alarm. Happens with lintier threads. Just wipe it off with your finger or a Q-tip from time to time.

I have a Bernina and there is a slight difference in the soft hum. Someone told me and I appreciated that helpful hint!

:DAhh, yes the dreaded "air sewing". I check before I start and project and my bobbin sounds a little noisier when it is getting empty but it still surprises me occasionally.

Ive done the 'air' sewing too !! but i do have a low bobbin indicator... it lets me know when im low but i still keep going till its completely out... Usually ill sew leaders and enders till i reach the tail end - if im going to need a new bobbin for a marathon stitching why not use it for lil pieces.. Why waste that thread right ?!?!?!

I don't. I just keep going, and all of a sudden something doesn't look right. Usually when I'm in the middle of what I'm sewing together.

I have a singer 301. It doesn't tell me anything. :)

I can see when it is running low. The needle plate has a clear window.

Just last night I was chain piecing blocks and sewed about ten blocks with no bobbin thread! I usually notice before that long though.

My machine will beep when it's getting low and text comes on the screen that tells me that there might not be enough thread to complete my project. But if i stopped then, It would waste so much thread. I can sew for a long time with the amount still on there so i do. I inevitably still run out in the middle of something. LOL

Mine has a little red light that flashes when the bobbin is getting low. It's really handy!