After the not-success of the previous tubular bell system, I went searching for a better electromagnet and a better chime. Thank you eBay !
I received two chimes, both made in China but with different quality and different notes. I think I'll use both :-) One for user signals and one for system errors.
They are shorter, with a higher pitch than the tubes. The more expensive one sounds better but the sustain might be a bit too long :-DI might use it for the system errors.
The operate horizontally, suspended by strings. They work even better upside-down ! This is fortunate because the the electromagnets I have found have no spring to hold the hammer back. But gravity is enough (when the coil is totally de-energised) so there is no spring to add. It's a very fortunate combination of mechanical parameters.
The electronic circuit is pretty simple : a charge capacitor is being held at +12V (through a current-limiting resistor) and a SPDT switch (either mechanical or a relay) discharges it through the 30 ohms coil. 1000µF is a good compromise, I tried 470µ and 2200µ and both have issues...
I didn't implement it but I should add a diode (top of the diagram): this would dissipate the energy in the coil for a faster return to standby position.
You can hear the sound in this obligatory video: