One of many sensors from Hitec, the Hitec Telemetry Sensor Optical RPM provides optical RPM information when used in conjunction with the Hitec Telemetric AFHSS System.
The optical RPM sensor has been made for use with larger, outdoor helicopters. Installation is simple. Attach it to the boom and connect it to the sensor station. It can read from 0 to 50,000 RPM. Since weather conditions may influence the readout, to insure the most accurate reading, make sure you install the sensor as far away from the fly bar paddles as possible.

Maximum Operational Distance: 150mm
Measurement Range: 0-50,000 RPM
Dimensions: 36mm x 26mm x 12mm
Wire Length: 500mm

Includes:
(1) Hitec Telemetry Sensor Optical RPM
(1) Double Sided Foam Tape
(2) 1.5 x 9mm Screws
(2) Zip Ties