i designed one. try to see if this is alright.
i used a 220 ohm resistor. the input at the right most is from the main power. i used a zener diode to regulate the voltage (at about 8.2V ) supply from the main. when there is a power failure, the back-up battery-- 9V alkaline battery -- will supply the MCU. this happens when the main drops below the back-up power supply. the system will be fed to the MAX883 voltage regulator which need a minimum input of 6V to regulate.
is this alright?
thanks guys.
-jay
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