Finally, a top-off control sensor that is mounted to your sump with a magnet rather than a clunky bracket!  Driven by a pressure-sensitive switch located in the relay box, a strong magnetic field won't affect its operation.  
 
The sensor tube can't get clogged with calcium deposits or activated by wayward snails as there are no moving parts.  This top-off system is capable of maintaining the sump water level within +/- 1/4" tolerance (1/2" overall) and features a built-in range buffer or hysteresis effect, which prevents rapid on-off switching that sometimes occurs when cheapo float switches bob up and down in small waves.
 
The magnet bracket is capable of holding on up to 1/2" thick glass.  The slim design relay box is unobtrusive and can handle up to a 3 amp pump at 240v AC.  The green LED light indicates when the circuit is active and the pump is running.

The magnetic bracket also contains both an adjustable 1/4" to 1/2" fitting, useful for holding the tube from your top-off pump in place.  This fitting can be switched out for a backup mechanical float valve, which will physically close and stop the flow from your pump in the event of an electrical or switch issue.  
 
We do not recommend the use of the backup float when connected to a peristaltic pump.
 
Switch and all components are made in the USA.

Technical Data:
Water level sensitivity: 1/2" 
Magnet strength: up to 1/2" glass
Magnet bracket dimensions: 1-3/4" front to back x 2-1/4" wide (excluding external magnet half, 1/2" thick)
Switch box dimensions: 4" x 2.25" 
Input power: AC 240v
Relay box capacity: 3 amps
Switch rating: 10+ million cycles
Warranty: 1 year
 
Click HERE for Standard outlet box ATO installation manual