It seems I can’t just leave the central heating alone. When we got the heating in (after 2 years of zero heating here, cooold) the tank was fitted with a wireless level monitor which measured the level of the tank using an ultrasonic sensor and sent the data back to a base station plugged into a wall socket using its combined battery pack/aerial. This worked fine for a couple of months until the battery pack went dead. A new battery pack worked for a couple of days and it never worked again – exactly the same as the unit fitted to my parents tank , an absolute waste of money. I switched to dipping a stick into the tank which proved far more reliable but slightly inconvenient over the last two years.
A few weeks back i was browsing ebay, as i do, and found some ultrasonic distance sensors for about €4 delivered. (I have no direct link as the auction has expired but searching for SR04 will bring up any amount of similar units.) I had no plan for them immediately but I bought some just to have anyway. It was only after they came that it struck me that a way of remotely monitoring the oil level would be a nice project.
The units are very simple to use. There is 4 pins: 5v, GND, Trigger and Echo. By applying a 10uS pulse to the trigger pin you activate the sensor to take a distance reading. The sensor replies on the echo pin with a pulse that is as long as the time taken for the sound wave to travel to the distant object and return to the sensor. Knowing the speed of sound we can then calculate the distance.