Magene Gemini 210 Dual-Mode Sensor

The Magene 2019 Gemini 210 Dual-Mode Sensor is a tiny device that straps to the inside of your crankarm (weighing less than nine grams it won't intrude) and offers cadence and speed information, available via ANT+ or Bluetooth. It's compatible with everything: head units, training platforms, apps. Using geomagnetic sensing, no magnet is required; with its 500 hour battery life, it may be one of the least intrusive gadgets a rider in training will have.
The Gemini is a reliable, inexpensive, precise addition to your training kit.