Ultrasonic Rainwater Tank Capacity Meter

If you are anything like me and have a little bit of an environmental conscience (or are just skinflints eager to save a few bucks - which is also me…), you may have a rainwater tank. I have a tank to harvest the rather infrequent rain we get in Australia - but boy oh boy, when it does rain here, it REALLY rains! My tank stands about 1.5m high and is on a plinth, meaning I need to get out steps to check the water level (or - because I am so lazy, balance precariously on top of an old gas bottle from the BBQ that has now taken up permanent residence as a ‘step’ beside the tank).
I wanted some way to be able to check the water level in the tank, without all the climbing and hanging onto the drainpipe with one hand (while worrying about what spiders might be behind it - you’ve heard about Australian spiders - right?)… So, with a renewed late in life interest in electronics, and cheap Arduino clones from China on ebay, I decided to have a go at building a ‘widget’ to do the job for me.
Now, my ‘dream’ widget was to be permanently installed in the tank, use a solar charged power source, with a remote readout in my garage, or maybe a wireless transmitter using Bluetooth that I could check from my phone, or perhaps even an ESP type device hosting an automatically updated webpage, so that I could check the level of water in my tank from anywhere in the world over the internet… but really - why do I need all that? So I dialled my grand ideals back a bit (well, quite considerably), and did away with the wirelessness of the solution, the permanently installedness, the solar charging, and the ability to check my tank’s level from the back end of beyond (always assuming the back end of beyond has WiFi available, that is…)
The resulting project was downgraded into the hand-held unit seen above, that can be simply held over the opening of the tank and activated by a push button, with a digital readout, that can be read from ground level - far more practical.”