My 91 gallon reef aquarium requires RO water for top-up as the saltwater evaporates. Usually you get a container, fill it with RO/DI water and have an auto top-up setup to keep topping up the tank as the water evaporates.\

The problem is that if the reservoir runs empty, you risk damaging the top-up pump (mine has a safety shutoff), but worse - you run the risk that the tank's sump runs dry and you burn out your main return pump.

I initially tried to use Neptune Apex' LLS-15 sensor but it turned out to be horrible. It would measure accurately down to about 12cm, then it would spike up to 18cm and stay there - while the reservoir runs empty. Recalibration did not help.

So I decided to build my own. The basic principle of operation is based on an ultrasonic distance sensor that measures the distance to the top of the water level in the container (water reflects sound waves). Laser rangers would not work as water is translucent to light. This sensor sends its data to a controller that alerts me when the level falls below a threshold.

Here is the parts list:

1. DFRobot Arduino Leonardo (DFR0221)
2. DFRobot RS485 Shield (DFR0259)
3. Industrial Ultrasonic Distance Sensor with 1mm accuracy and 10 - 150cm range (URM14)
4. XL6009E1 DC-DC step up converter
5. NRF24L01 RF transmitter

Below is a (terrible) schematic-ish diagram - I had trouble finding the correct components in KiCad and I wanted to indicate how I wired it, so this is really more a wiring diagram than a schematic.

Principle of operation:

• The Leonardo uses the RS485 shield to communicate with the URM14 ultrasonic sensor.
• The system is powered via a small power supply and a USB cable connected to the micro USB port of the Leonardo - thus 5V DC
• The sensor needs at least 7V, so I had to add a DC-DC boost converter to supply 12V DC to the sensor
• To get data out of the system, I added an NRF24L01 RF transceiver. I am using this for other IoT devices in my house so this just sent data to my main controller every 30 seconds.
• Nice thing about that ultrasonic probe is that it also measures temperature so I am sending those measurements over to the controller as well.
• I have calibrated it and the Leonardo will send an alarm if the water level drops below 5cm.
• I added a little feature where the LED will blink slowly when it is below 5cm, then faster until it is solid on at 2cm. This LED is underneath the tank so I can easily see it if I walk past.
• The controller will send a push notification to my phone on alarm condition, and it also logs the data to influx for Grafana to display.
• Finally, I added a toggle switch to allow me to stop transmission of data for when I need to refill the reservoir and need to take out the probe, to not get spurious readings.

The sensor is IP65 rated and built like a tank. I verified the 1mm accuracy and it responds instantaneously. I also verified that with its extremely narrow beam, it can easily measure 50cm in the can without hitting the walls.

Here is a graph showing how nicely it tracks the water level (the dip on 11/08 was before I had the toggle button and I disconnected the probe for a case upgrade):