12 Volt Toilet Tank Refiller

The 12V pump on the side of the cistern

The circuit in action

The hand-wired circuit board

The schematic

Introduction

A flushing toilet is something that most city dwellers take for granted, but it can be a luxury for those who live away from utility water and electricity. This circuit provides on-off pump switching for a small 12 Volt pump that is used to fill a toilet tank from an external rain water collection cistern. The 12 Volt power comes from a small solar power system. The cistern is located at ground level, so the pump is required to move the water up to the toilet tank.

Specifications

Nominal operating voltage: 12V
Idle current: < 1 ma
Pump Current: < 5A

Theory

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.

Careful observers will observe a second diode on the board, it is a crowbar diode who's job is to blow the fuse if the power is ever connected with reverse polarity. The crowbar diode is not shown in the schematic.

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.

Construction

The circuit board is hand wired on a piece of prototype PC board. The board is connected to the side of the toilet tank with a piece of bent plumbing strap metal. The magnet is connected to the toilet tank float with duct tape, a better magnet mount could certainly be fashioned. Sometimes it is necessary to cheat when working in the field. If the magnet has a gap around the side, fill it with silicone sealant.

Hind sight is 20-20, it would be best to mount all of the circuitry in a dry area outside of the toilet tank. The hall effect sensor and magnet are the only components that need to be near the water. The sensor could be soldered to the end of a fairly short three conductor wire and coated with epoxy or silicone sealant for water proofing. A little water does not harm the circuit, but it may cause it to get stuck in the "on" mode until it dries out.

The pump inlet is connected to a valve and reducer assembly at the bottom of the cistern tank. The pump outlet travels through a short section of conduit, into the building, and into the toilet tank via a piece of bent copper tubing. The copper tubing output should be above the highest water level to prevent back-siphoning.

The pump output tube and wiring were built with connectors so that the pump could be removed when we are away from the building. A more permanent version could be made by mounting the pump in a metal box on the side of the building.

Alignment

Bend the metal strap that holds the PC board, so that the hall effect sensor is directly above the magnet. Adjust the height of the PC board so that the circuit shuts off at the desired water level.

Use

Flush the toilet, the red LED will light up, and the pump will move water into the toilet tank. As soon as the water level reaches the top, the pump will shut off. Just like in the city. If you want to use this device in an area where the temperature gets below freezing, the water supply, pump, and water tubing will need to be mounted in an area that does not freeze, such as underground.