Saturday, November 7, 2009

Improved level sensor for the DIY aquarium top-off project

In my
original post regarding an automatic water top-off system for aquariums, I designed a sensor head that consisted of a plastic rod with a set of pocket holes drilled at the tip. The holes were diametrically opposed and angled so that they intersected at a point about 1cm in front of the plastic rod face. I inserted a plastic fiberoptic into each of the holes, and the system would allow sensing a liquid level surface by measuring the amount of light reflected off the surface. If the liquid covered the two probes, all of the light would be scattered off into the liquid and the signal would be almost zero. When the level fell below the fiber ends, the reflected light would trigger the top-off pump.

This system worked very well until nearly flooding my house a couple weeks ago. By extreme luck, I happened to be sitting near the aquarium when an air bubble got trapped between the two fiberoptic ends. This caused the top-off pump to run even after the water level had risen higher than the sensor head. I heard the tank dripping water and quickly shut off the pump. Following this event, I decided to improve the reliability of the sensor.

I did some searching for commercially-built liquid level sensors and found that many of them operate by submerging a prism and measuring the amount of total internal reflection. When the prism is submerged in liquid, the light will pass out of the prism and into the liquid. When dry, the prism will reflect most of the light internally. By positioning the fiberoptics symmetrically, the light signal will be drastically changed by the liquid surrounding the prism.

I machined and polished a piece of acrylic into a point. Then I drilled two holes that would snugly hold the fiberoptics. I mounted the whole thing on a Delrin rod.

This sensor should be much less sensitive to air bubbles, snails, dirt, etc than the previous model. For the next week, I'll be monitoring the top-off system and manually controlling the pump. If it looks good, I will connect the pump and let everyone know how it works.


  1. Ben,

    I'm still baffled by your work!

    Would it be possible at all to make a second version of this sensor? I really like the system, but I lack the skills ;)

    Best regards,


  2. Jan, thanks for the comment. I'm a little worried about building and selling the top-off controllers since they are still in "beta" and need more testing. I have not had any flooding problems since upgrading the sensor, but it does have a slight problem with "drift." The water level will drop 5mm (or so) over the course of each month since the plastic prism's transparency is changing. I can compensate for this by adjusting the fiberoptic light receiver, so it just a minor inconvenience. I am not sure if this problems is caused by invisible algae on the sensor or the plastic itself is degrading slightly.

    Good luck, let me know if you decide to build any aquarium stuff yourself.