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12 Volt Toilet Tank Refiller

12 Volt Toilet Tank Refiller

This circuit controls a small 12 Volt pump that is used to fill a toilet tank from an external rainwater collection cistern. The Panasonic hall effect sensor is the heart of the system.
The hall effect sensor’s output pulls to ground when in the presence of a relatively strong magnetic field. When the magnet is pulled away from the hall effect sensor, the output goes high via the 10K pull up resistor. This turns on the IRFZ34N MOSFET transistor, which pulls the negative lead of the 12V pump to ground, turning on the pump. The LED is also turned on.


The 1N4004 diode snubs out potential spikes from the motor, and the 1000uF capacitor across the motor eliminates motor brush noise from the DC wiring. The 100uF capacitor filters noise from the 12V power source. As with all electrical circuitry, it is important to have a fuse and a switch in series with the power source, that is not shown in the schematic.

The 10 amp silicon diode is used in a crowbar circuit. Its purpose is to protect the circuit from reverse DC polarity on the power terminals. Reverse polarity causes the fuse to blow.

This circuit will work with most momentary action (non-latching) hall effect sensors, you may want to experiment with the sensor and magnet to find the most sensitive orientation for both parts.

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