I am wanting to add a switch to the schematic to control the power, essentially limit power consumption by turning the device "on" and "off". Any suggestions on how to go about doing this?
Is the device the Arduino+LCD+Sensor? Can you not add a switch into the 9V power source? Hopefully not a 9V PP3 battery as they don't last long at all.
Yes, Arduino+LCD+Sensor. Changed 9V battery to 6 AA batteries in series to increase current. If you add the switch to the battery pack, would you need to add a PUR or just connect straight to the + terminal of the battery pack and then GND the other side of the switch?
If your concern is power consumption, you do not power the Arduino UNO via "Vin" or the "Barrel jack".
The UNO runs on 5 V. Now if you are using a "1602" (or "2004") display with a "backpack", you do need a reasonably regulated 5 V (otherwise four NiMH "AA" batteries would power it directly, but the display contrast may differ as the batteries run down) so perhaps you would use a switchmode "buck" converter from your 9 V battery to power the board at a regulated 5 V.
As far as the display goes, if you command the backpack to shut off the backlight, the current drain reduces from 25 mA to about 2 mA which is far less than the UNO draws anyway. And if you can correct a wiring mistake on the backpack, you save nearly another 1 mA.
But of course, the UNO is the wrong board for battery operation where you should use a Pro Mini to save on the current drawn by the USB interface. Then you could put it in "sleep" mode and draw only a few milliamps overall.
Whatever is a "PUR"? 
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