AiM MyChron3 XG Log - Manual (Page 12)

The LAP transmitter (Beacon)
The optical Beacon Transmitter has to be placed on the trackside to mark laps.
Ensure the infrared receiver eye faces the side of the track where the Beacon
has been placed, otherwise the system will not record lap time.
The Beacon transmitter is powered using 8 AA batteries or an external 12V power
cable. If you are using 8 AA batteries, unscrew the back cover of the Beacon
transmitter and place the battery pack into the transmitter casing.
The transmitter has two operating modes: Low power and High power. The Low
Power Mode has to be used when the track is less than 10 meters wide, while
the High Power mode has to be used when the track is wider than 10 meters.
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To activate the High/Low Power function, please open the Beacon transmitter
with a corkscrew and place the clip (located directly below where the battery
pack is attached to the beacon transmitter board) either over one of the two
connectors (for low power mode) or over the two connectors (for high power
mode).