PS-SBS/01RO  Futaba FASSTest Telemetry Optical RPM Sensor 

This is the Futaba SBS-01RO Optical RPM Sensor. Getting the performance information you want has never been easier, lighter or more convenient!

This sensor plugs into the S.Bus2 slot on telemetry-enabled receivers, and transmits the data directly to FASSTest systems. 

Specifications: 

Displays: revolutions per minute. 
Range: 360-300,000 
Weight: 0.17 oz (4.9 g)