Indoor/outdoor thermometers are handy, but the outdoor transmitter can consume too many batteries, depending on the unit. I decided to power the outdoor transmitter with an old phone charger. The voltage of the phone charger needs to be dropped about two volts to match the requirements of the transmitter. This Instructable is really about altering the voltage output of an old 5 volt switch mode phone charger to fit special power needs you might have.
I powered up the transmitter for this thermometer with some partially used AAA batteries. Notice the battery with a diagonal line through it on the upper half of the display. This is a weak battery symbol.