Most auto topoff systems are used in a sump and have a float switch (or 2 or 3) placed into the sump to trigger the pump via a relay controlled switch and/or external aquarium controller. The sump isn't a necessity, but helps in hiding the float switch from view.
The simplest system is 1 float switch with a relay controlled outlet, but this has two problems:
1) No backup float switch so if it sticks on, you are SOL. If it sticks 'off', you are SOL.
2) Depending on the sensitivity of the float switch it may result in high on/off cycles for your pump.
The next step up from there is using 2 float switches with a latching relay system (turn the pump on when low float is triggered, turn the pump off when the high float is triggered).
But the better solution is to use a programmable aquarium controller. You can program it to turn on for X time when triggered, set an alarm if the low float doesn't 'unset' in that time, etc. With enough programming and float switches you can make it safe enough to almost sleep at night.
The simplest system is 1 float switch with a relay controlled outlet, but this has two problems:
1) No backup float switch so if it sticks on, you are SOL. If it sticks 'off', you are SOL.
2) Depending on the sensitivity of the float switch it may result in high on/off cycles for your pump.
The next step up from there is using 2 float switches with a latching relay system (turn the pump on when low float is triggered, turn the pump off when the high float is triggered).
But the better solution is to use a programmable aquarium controller. You can program it to turn on for X time when triggered, set an alarm if the low float doesn't 'unset' in that time, etc. With enough programming and float switches you can make it safe enough to almost sleep at night.
