A recent project at work required that I drain water from two tanks periodically. Since both of the tank drains are located below the level of all the drains in the room, I would fill up buckets and transfer the water to the drains manually. Soon I realized that I could simply put a pump in the bucket to automatically pump the water to the drain whenever the tanks were drained. This is the story of how my brother and I accomplished this task.
Step 1: Designing the System
For a pump I chose a very small fountain pump. These pumps work great, but they lack a control system to turn them on when the water level rises, and more importantly, shut them off when the water is pumped out of the bucket. Since the bucket we were using was quite small (2-3 gallons), most commercially available float switches were too large for the system. However, on amazon.com I found some small stainless steel float switches and ordered one. We connected the switch to the pump and tried it out. It did turn the pump on when water was added to the bucket and it also turned the pump off when the water level fell low enough. However, when the pump shut off, the water in the tube would flow back into the bucket and raise the float, turning the pump on again. The pump would constantly cycle on and off, which would destroy it very quickly.
I did a bit of digging online and found the relatively simple pump controller circuit seen above. This system utilizes two levels of float switches, a 12V relay and a 120V relay to drive the pump. 12V DC is supplied to the float switches, which are normally open when not being floated. As the water level rises it lifts the bottom float (Float 1) and closes it. This sends current to the common (COM) pin of the 12V relay. Since the control wire to the 120V relay is connected to the normally open (NO) pin of the relay, current does not pass through the relay and onto the 120V relay (The pump remains off). When the water level rises further and closes the top float switch (Float 2), current is supplied to the coil of the 12V relay, which closes the connection between the COM and NO pins. Current is now free to flow to the 120V relay, which energizes the pump. At this point, the pump would shut off as soon as the water level fell to the point where the top float switch would open. However, a feedback loop is added between the NO pin and the + side of the relay coil. As the water level falls and the top float switch opens, current continues to flow through the bottom float switch, through the COM and NO pins and back to the relay coil, keeping the relay energized and the pump on. When the water level falls low enough to open the bottom float switch, this circuit is interrupted and the pump shuts off. Since the two floats are located at different levels, the water in the tube does not turn the pump on when it drains back into the tank, even if the bottom float switch closes.
Step 2: What We Used
We used the following items for this build:
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1 circuit board with 4 threaded standoffs and 8 screws
Some zip ties
Two 1/4" bolts with nuts and washers
4 lengths of wire (16 gauge will work)
Step 3: Assembling the Circuit Board
The circuit board is the heart of this system. Before doing anything to the circuit board, the four standoffs are attached to it.
Begin by drilling a small hole directly between four of the existing holes in the circuit board (somewhere near the center of the board). This hole needs to be large enough to accommodate the COM pin of the 12V relay.
Next, the diode needs to be bent, cut, and placed into the circuit board such that it connects across the relay's coil pins. We found that the coil pins for our relay were located on the end of the relay with the COM pin closest to it. After pushing the diode into the board, the wires can be bend over behind the board such that they nearly touch the coil pins. This will help with soldering everything together.
Finally, the four screw terminals can be placed into the board around the relay. The locations of these terminals aren't critical. We chose the locations shown as they make the connections on the back of the board as neat as possible.
Step 4: Soldering the Board
With everything assembled on the front of the circuit board, the entire board can be carefully flipped over and the circuits soldered onto the board. The easiest method for creating the solder "lines" is to add a small drop of solder to each connection point along the "line" and then connect them together to form the circuit.
Step 5: Preparing the Enclosure
The enclosure needs to be prepared to house all of the electronics. First, a rotary tool can be used to connect four holes and form a square hole in the side of the box, through which the power supply cord passes. We also drilled eight holes in both ends of the enclosure. These holes can be connected using a cutoff disk to form ventilation slots in the enclosure. These slots are to prevent the enclosure from overheating as it will house both the 12V DC power supply and the 120V relay.
Step 6: Secure the Power Supply in Enclosure
The 12V power supply is attached to the enclosure by passing zip ties around it and through holes strategically placed in the bottom of the box.
Step 7: Provide Power to the Circuit Board
The cord leaving the 12V power supply (the 12V DC end) is cut (about 6" from the power supply box) with the two wires being exposed, stripped back, and attached to their respective screw terminals on the circuit board. At this point, the circuit board can be attached to the enclosure by threading 4 more screws through holes in the enclosure and into the bottoms of the standoffs.
Step 8: Adding the Solid State Relay to Enclosure
The solid state relay (120V relay) is secured to the enclosure using two 1/4" bolts (1" long), which pass through the bottom of the enclosure and are affixed with nuts and washers.
Step 9: Supplying Power to the System
Power for the 12V power supply will be taken from the fountain pump cord to allow the entire system to utilize a single power cord. Around 1.5" of insulation is stripped back on the pump cord around 1 foot from the pump, exposing the three wires inside. The white wire is cut as this will be switched by the solid state relay. A small section of black wire should be stripped back. (Note that I also stripped the green wire, but didn't need to do this and had to tape it back up). I also cut the cord leading to the 12V power supply (the 120V end of the power supply) at around 1 foot from where it would attach to the power supply. The two black wires inside this cord are separated and stripped.
As seen in the second picture, the one black wire leading to the power supply is soldered to the exposed section of the pump cord's black wire. The second black wire is wrapped around the cut white wire on the end of the pump cord away from the pump (the side where it can directly receive power from the outlet). Leave this wire unsoldered for the moment.
Two 16 gauge wires around 1 foot in length are cut, stripped, and attached to the two cut white wires of the pump cord. These wires will run to the 120V side of the solid state relay. All of these connections can now be soldered with everything being taped up as nicely as possible. I like to use a rubberized electrical tape on the outside of connections like this as it creates a very nice weather-tight seal that looks better than regular electrical tape.
Step 10: Connect the Solid State Relay
The connections to the 120V solid state relay can now be made. The two wires from the power cord are connected to the 120V AC side of the relay, where either cord can run to either connection point. Two additional wires are connected between the circuit board and the 120V relay, with the polarity of these connections being important.
Step 11: Installing the Floats & Attaching Their Wires to the Circuit Board
The floats are installed into the bottom of the bucket through a 3/8" hole, which is sealed using the o-ring that came with the floats. The four wires from the floats connect to the four terminal screws on the circuit board. You may need to experiment a bit to determine which float wires go to which float. We found that the two black wires were for the bottom float, while the red ones were for the top float.
Step 12: Installing Pump & Testing System
After the pump is plugged in and placed into the bottom of the tank, the system can be tested by lifting up on the floats. When the bottom float is lifted, the pump should remain off, but when both the bottom and top floats are lifted the pump should begin to run. When the top float is released, the pump should continue to run until the bottom float is also released. Be sure to conduct this test quickly as the pump is not designed to operate without water in the tank.
Step 13: Conclusion
The completed pump controller was assembled in less than a day and operates as expected. A similar setup could be used for any sump-type drainage system.