How can I float a small ring magnet?

I am creating a fluid level indicator for my automated drink mixer and need either an alternate solution or an item that can float a small ring magnet.

Currently, my idea is to sink a reed switch into a straw, with a ring magnet around the straw.  Once the magnet floats down to the reed switch, it lights up the indicator that the alcohol is (almost) gone.

Unfortunately, I cannot find a durable non-toxic way to make the magnet float.  I've done several searches and the closest matches I've found are fishing tackle, which I'm not sure would work all that well.  I've found manufactured devices, but $30 per bottle is a bit much to spend.  I'd like to keep it to under $5 per bottle.

Burf6 years ago
Make a cylinder of cork or expanded polystyrene foam to whatever dimensions are sufficient to float the magnet. Epoxy the magnet to one end of the cylinder and slip the combination over the straw, magnet on top.
Affix the reed switch at point within the straw so the magnet activates the switch when the bottle is near empty.
kidmosey (author)  Burf6 years ago

I knew there was something simple my brain was missing. thanks!
rickharris6 years ago

Ultra sonic measurement of surface from top of vessel

A mechanical dipper to detect how far down the surface is (this can be automated)

A float switch

Weigh the vessel

Weigh/measure the liquid used to assess how much is left

lemonie6 years ago

You might prefer to use an optical-switch.
If you've got e.g. an IR transmitter/receiver, or one of those paper-detecting components from a printer/fax paper-path - you may get that to work.
I mention this because I've seen ink cartridges which indicate "empty" optically. OK they've got a black plastic float in 'em like a cistern ball-cock too but they work.