I recently upgraded my front brake from rim to hydraulic disc. That meant the hidden wire brake sensor couldn't work, so I thought I'd post two different solutions I've tried.
1) add a cable to the hydraulic brake lever
This was pretty quick and easy and worked well- I drilled a small hole for the cable to exit and cross drilled and wired through the nipple
2) use a magnet and a hall sensor
This was also pretty easy, but you need a few tools. I liked the l.e.d. of the original hidden wire sensor, so I added one with a 1K resistor across the 5V to signal pins, so it comes on when the magnet is detected. To attach the magnet to the alloy brake lever, I stuck it to a steel bolt and stuck this into the inside of the lever with silicone. It works really well and can be adjusted by moving closer or further from the neodymium magnet- it switches when the lever comes within about 8mm of the sensor.
1) add a cable to the hydraulic brake lever
This was pretty quick and easy and worked well- I drilled a small hole for the cable to exit and cross drilled and wired through the nipple
2) use a magnet and a hall sensor
This was also pretty easy, but you need a few tools. I liked the l.e.d. of the original hidden wire sensor, so I added one with a 1K resistor across the 5V to signal pins, so it comes on when the magnet is detected. To attach the magnet to the alloy brake lever, I stuck it to a steel bolt and stuck this into the inside of the lever with silicone. It works really well and can be adjusted by moving closer or further from the neodymium magnet- it switches when the lever comes within about 8mm of the sensor.