After refilling one of the Canon inkjets with the newer chipped carts (ip4200, etc), one loses the ability to monitor the ink levels using the printer's cirquitry and software. This is unfortunate.
It occurred to me that it would be quite easy to make a low-cost add-on ink level warning device - comprising a sensor that detects when the rear portion of the cartridge (the chamber with no sponge inside) runs dry. This could be achieved with a two-terminal sensor that detects increased electrical resistance when there is no longer any ink between the terminals. The terminals could project from the end of a rod that extends down into the cartridge from the top. It could be inserted after drilling a hole - one for each cart. The sensors could cause an LED to light up (one for each cart). It could also set off a beeper.
If anyone ends up building one, please send me one - or at least the instructions and component list. I'd do it myself but my knowledge of electronics is too scant.