How do I wake up the screen on the Honeywell T1 Pro Thermostat?
Press any of the three buttons to wake up the screen. It will stay lit for 45 seconds if powered by the C wire, or 8 seconds with battery power only. Depending on the installation configuration, the screen light may always be on.
From the official Honeywell T1 Pro Thermostat manual, page 2
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