The least expensive aquarium top off I could find was approx. $100, these depended on small float style sensors or optics that were in the grow tub or mounted on the outside and sensed through the tub.
Having saltwater tanks in the past, neither solution was fool proof and in the air bubble, nutrient, algae growing environment like a grow tub I could not see it working.
Having used something in the past that kept drain traps filled with water to prevent smells coming back I modified the circuits boards intended target usage.
Liquid Water Level Detection Sensor Module, Maluokasa 12VDC/AC Relay Controller Switch Automation Detection Pump Tank Water Level
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This is a 12v detector and relay combination, will switch 12v without a problem and is (at least has been) reliable, the KEY to this sensor is that requires only three wires.
The detector will also switch 115, but will need 12v to operate the “Sensor†wires. I have a lot of spare parts, so sticking with 12v for both eh detector/sensor and the motor was a no brainer. It would be simple to wire a 115v pump into this, but 115v and water tend to find each other.
I used this for the power, I can run up to three systems on his power brick.
12V 6A 72W AC DC Power Supply Adapter (Input 110V-220V, Output 12 Volt 6 Amp 72 Watt) for CCTV Security Camera DVR, ZUEXT DC Converter LED...
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I used a solar hot water pump, they are rugged, submergible, 12v and cheap
DC 12V Solar Hot Water Heater Circulation Pump Low Noise 3M Discharge Head 2.1GPM
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Once you have power run to both the motor and the board, connect the telephone line (Only need three of the four). It was hit or miss for me on determine how to adjust the length of the sensor wire that ends up in the tub. You end up with a common, high wire (Stops pump) and low wire (Starts pump)
I heated the hose and attached it to the pump, mounted the pump on a piece of nylon board and used a ½ threaded pvc elbow to get the very bottom of the tank.
Used schedule 40 ½ pvc to contain both the end of the hose and the “Sensorâ€, you need to drill a small hole in the 3/8 pvc line to prevent it from syphoning back after the pump stops.
I had most of the connectors and wire, adapted and overcame on a couple of things. It works exactly how I wanted it to. Working on the correct mixture of nutrients for a top off but should work with a 1/4 solution.
Its portable, just drill a hole in the top of your container, and plug in the hose/sensor combo. As long as the depth of the tub is the same, and I'm not positive that is critical.
Fluids are kept at a 1.5 inch ban, you can adjust by adjusting the distance between the three sensors.
******Note****** I only use a auto top off system when there is no other choice, in this case my fishing partner is also my growing partner. There is no way (That I know of) you can premix nutrients that will adjust PPM correctly for the plant. At best, I'm using a safe bet in about a 1/3 mixture.