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Author Topic: Making Thermostat LCD display always-on  (Read 1686 times)
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I'm wondering if anyone can help me to hack a Honeywell RTH8500D Thermostat to have an always-on LCD screen. I have it hooked up using a C-wire; so it does not need batteries to operate, but there must be a way to bypass the need for having to touch the screen to bring only 5 seconds of power to the LCD.
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I think the easiest way to do this is to bypass the built in backlight power source and supply your own 5V (or appropriate) source to the backlight constantly.

You should be able to use a multimeter to test points on the honeywell board, when the multimeter reads positive voltage with the backlight on and 0 with it off, you've found it. Solder a jumper on and supply the same voltage.
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You can change it in the settings...

Steps from Manual:
1 Press SYSTEM. (This being the SYSTEM Box that has Heat/Off/Cool)
 You’ll see several blank buttons on the bottom of the display.
2 Press and hold the center blank button until
the screen changes (approximately 5 seconds).
3 Cycle through the functions, press st next to
the four digit number on the left.
4 As needed, change options for any function by
pressing st next to the number on the right.
5 When you have made all changes, press DONE
to save and exit.

Scroll through the numbers on the Left till you get to the following:
0280 Backlight

Then change the setting on the Right to:
0 = Backlight off, then on for approximately 8 seconds after keypress.
1 = Backlight always on low intensity, full bright after keypress.
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