Liquid level sensor and controller (auto top-off for aquarium)

I built a liquid level controller that senses the level of water in my aquarium and powers a water pump when necessary to compensate for evaporated water. This is known as an "automatic top-off" system. The controller is based around a Keyence FS-V11, which sends light out a plastic fiberoptic, and measures the amount received via another fiberoptic. The optical probe makes use of total internal reflection to control how much light is returned to the Keyence based on how far the probe is immersed in the water. It is also completely passive, non-metallic, and has no moving parts for high reliability. The Keyence triggers a solid state relay that supplies power to a standard AC outlet. The pump is then plugged into the outlet, and moves water from a reservoir into the aquarium on command.

Even more detail for aquarium DIY top-off system

In this post, I'll describe the machines and methods that I used to make the optical sensor head for the aquarium top-off system.

Step 1: Cut off 2" of 1/2" diameter black Delrin rod. Delrin is a brand name for a specific type of acetal copolymer plastic. You can get it at www.mcmaster.com


Step 2: Square-off the end of the rod on the lathe.


Step 3: Drill two holes at 30* to the rod's major axis so that they intersect a few mm in front of the rod's face. You may need to use a punch or dremel to prevent the drill bit from wandering on the sloped face of rod. I like to use the vise-within-a-vise method for drilling angled holes on the drill press.

Step 4: Taper the end of the rod with a coarse file.

Step 5: Use a smooth file to tidy everything up.

I found that the optic fibers should be cut square, and then recessed into the sensor head so that nothing sticks out. This allows air bubbles to form harmlessly in each of the spaces created by recessed fiber ends. If the fibers stick out too much, an air bubble might become trapped between them, and the system will have a "false positive".

More detail for the DIY auto top-off system for aquariums

I've had a few requests for plans of the auto top-off system for aquariums. This project is fairly straight-forward, and I would imagine the most difficult part is machining the sensor head that holds the two optical fibers. This will require a drill bit that is sized just right, and the angles need to be pretty accurate. The wiring and electrical side of this project is very easy, and only requires minimal soldering. Here's some rough instructions and part list that I emailed to an interested DIYer:

This project involves wiring a household electrical outlet. As you know, saltwater is a great conductor and the wiring must be protected from drips, etc. Always disconnect the power when working on the circuit, and use good wiring practices.

I'd start by getting a Keyence fiberoptic sensor FS-V11 (or similar) ebay like this:

http://cgi.ebay.com/Keyence-FS-V11-FSV11-Fiber-Optic-Sensor_W0QQitemZ390020341450QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item390020341450&_trksid=p3286.m20.l1116

Next, you need a wall-wart power supply (AC adapter) to provide low voltage to the Keyence. You'll need 12V DC at almost any current rating (mA rating). Be sure the AC adapter is a linear voltage supply, not a "switching" supply -- it should be relatively heavy.

You'll need some 1mm jacketed plastic (PMMA) fibertoptics. I didn't find any at a great price on eBay. You'll have to hunt around a little. Edmund Optics sells it per foot, so you might be able to just order a small amount.

Get the Aqualifter AW20 pump ($10 or $15)

Get a solid state relay eg Kyotto KB20C02A (Jameco #175214) $6.55

You can wire it all together in a standard electrical box with an electrical outlet from a home improvement store.

The basic idea is that the Keyence device gets power from the 12V DC supply, then controls the solid state relay with its output. The solid state relay controls power to the electrical outlet, which is where the AW20 pump is connected.

I don't know if I would recommend this project to someone who has never done any electrical/electronics projects before. It involves wiring a household electrical outlet, and this really has the potential to cause a lethal shock -- especially around saltwater aquariums.

Another problem is that a malfunction might flood your house! I noted problems about air bubbles. If the fiberoptic sensor head traps a large air bubble, the top-off pump will continue running even after the aquarium overflows. For the first week or two, I used the top-off system as just an indicator -- not risking any floods. You should do the same if you attempt this project. I modified the sensor head, so that I doubt air bubbles will cause any more problems, but you never know.

Feel free to give me more feedback in the comments...