bperrybap:
Time
Depending on the skills and knowledge of the person, it may be much quicker and easier to get a simple switch up and working.
exiledyorkie,
you may want to take a look at jacks button library: GitHub - JChristensen/JC_Button: Arduino library to debounce button switches, detect presses, releases, and long presses to handle the button input
and for the timeout, you could use jacks timer library: GitHub - JChristensen/Timer: A fork of Simon Monk's Arduino Timer library
It could be something as simple as when a button is pressed, you turn the backlight on, then use the after() method to call a function after the desired amount of time that simply turns off the backlight.
For a couple of bucks, it might be cool if you used something like a rcwl-0516 microwave/radar sensor instead of a button to sense when you walked up to the device. The display backlight would just light up when you walked up to it.
--- bill
bperrybap:
The hd44780 library includes several different i/o class for different interfaces.
You can use it for both direct pin control, hd44780_pinIO, as well as i2c control using a PCF8574 i/o expander based backpack, hd44780_I2Cexp.
The API is the same regardless of which h/w interface you have.
The only difference when using different h/w is you use a different i/o class and the lcd object constructor definition will be different.
The sketch code to drive the lcd is the same.
--- bill
Hi Bill
Thank once again, those links look very interesting. I'd have never come across those. I have a screen without the I2C adapter, so I may have a go with that first, see if I can get it working.
The sensor sounds like a great idea. Not sure if I'll use it for this but its something to paly around with.
Paul__B:
OK, battery power. So you may be considering putting the Arduino to sleep as well to substantially reduce current draw.
If so, and battery current draw is critical, then there is a modification you want to make to the I2C backpack if at all possible. There was a major mistake early on in the application of the HD44780 LCD driver chip where a designer copied something from a test circuit in the chip documentation where a potentiometer was used to set the contrast voltage. Once made, this blunder became ingrained in almost all following designs and "instructional" literature, since few designers actually understood the magical workings of a multi-level multiplexed LCD - or cared to spend the time to do so.
So what? The contrast-setting potentiometer should not be connected to Vcc - 5 V. If you can cut this connection on the backpack without interrupting any other connection on the same trick, you will save 500 µA which it useful as the contrast ladder itself already draws about 500 µA and the HD44780 chip (or clone) itself about 400 µA. This is much less than the backlight of course, but is significant when battery powered (unless perhaps you are talking about a car battery). This correction will also make the contrast setting easier.
bperrybap:
If you using battery power and have the extra pins, then don't use the i2c interface with a backpack. Direct pin control will use less power than using an i2c backpack.
I do plan to play around with sleep mode. I've read the Nick Gammon article but just not had chance to play around yet. Just in the process of finishing uni. So I've done a bunch of reading/video watching.
I thought about running from mains but I'd rather use batteries so will play around before it is built. I will only use it during the evenings, so I wouldn't have thought it'd consume so much power. I plan to have the temp sensor elsewhere and use rf24 to transmit data. Although I'm yet to receive those too.
Great advice about the pot on the board, I had not heard of that problem but it's definitely something to consider. Maybe I'll use the display alone. Or just order an OLED 
Thanks again guys. Amazing info there!
