Typically requires 2x AA batteries for the outdoor transmitter and 2x or 3x AA/AAA batteries for the indoor base station.
Always insert batteries into the outdoor sensor first , then the base station. This ensures the base "catches" the sensor's initial sync pulse. wireless weather station h10515 dcf manual link
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If the signal isn't found, you can often manually set the time by holding the or MODE key for 3 seconds until the digits flash. Typically requires 2x AA batteries for the outdoor