recorders, motion detectors, "slow-speed" sensors (to detect pedestrian and traffic movement up to 85 km/h (50 mph)), traffic signal controllers, automatic door openers, automatic traffic gates, process control equipment to monitor throughput, burglar alarms and equipment to detect trespassers, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors. Radio amateur use By virtue of their low voltage operation, Gunn diodes can serve as microwave frequency generators for very low powered (few-milliwatt) microwave transceivers called Gunnplexers. They were first used by British radio amateurs in the late 1970s, and many Gunnplexer designs have been published in journals. They typically consist of an approximately 3 inch waveguide into which the diode is mounted. A low voltage (less than 12 volt) direct current power supply, that can be modulated appropriately, is used to drive the diode. The waveguide is blocked at one end to form a resonant cavity and the other end usually feeds a horn antenna. An additional "mixer diode" is inserted into the waveguide, and it is often connected to a modified FM broadcast receiver to enable listening of other amateur stations. Gunnplexers are most commonly used in the 10 GHz and 24 GHz ham bands and sometimes 22 GHz security alarms are modified as the diode(s) can be put in a slightly detuned cavity with layers of copper or aluminium foil on opposite edges for moving to the licensed amateur band. Typically, the mixer diode if intact is reused in its existing waveguide and these parts are well known for being extremely static sensitive. On most commercial units this part is protected with a parallel resistor and other components and a variant is used in some Rb atomic clocks. The mixer diode is useful for lower frequency applications even if the Gunn diode is weakened from use, and some amateur radio enthusiasts have used them in conjunction with an external oscillator or n/2 wavelength Gunn diode for satellite finding and other applications. Radio astronomy Gunn oscillators are used as local oscillators for millimeter-wave and submillimeter-wave radio astronomy receivers. The Gunn diode is mounted in a cavity tuned to resonate at twice the fundamental frequency of the diode. The cavity length is changed by a micrometer adjustment. Gunn oscillators capable of generating over 50 mW over a 50% tuning range (one waveguide band) are available. The Gunn oscillator frequency is multiplied by a diode frequency