Battery powered Aquarium Air Pumps are common and can be easily bought from Local Fish Stores. But the downside to such pumps are that you need to replace the batteries each time they run out, or recharge the batteries with a separate charger. You also need to be there to switch it on in case of a power loss in your house.

In this instructable I will show you how to modify such an air pump to recharge and start up automatically in case of a power loss.

List of Necessary Parts:

LM2596 Buck (Step-Down) Converter - 1
5V Relay - 1
TP4056 Charging Module with DW-01 Protector - 1
Copper Dot Board - 1
18650 Rechargeable Battery (Li-Ion) - 1
Terminal Connectors (Optional) - 2
PCB Spacers (Optional) - 2
Double-Sided Tape - 1
Copper Wires
Diodes (1N4007) – 3
Micro USB Cable - 1
USB Charger - 1
Battery Powered Air Pump (the main ingredient - doesnt matter which brand or company, as long as it works) - 1

These parts are readily available online and can be found in local electronics stores as well. I would like to give you links to these parts but I myself bought these locally so I would not want to recommend you sellers which I do not have any experience with.

Step 1: Step-Down the Voltage With a Buck Converter

First I tried to run off the pump with a normal power bank. Note that the power bank outputs 5 volts, but the air pump only needs 1.5 volts considering the type of battery used (in my case it used two D batteries in parallel arrangement which would still output 1.5 volts to the motor).

I used an LM2596 buck converter to step down the voltage from 5 volts to 1.5 volts. Note that going over voltage can kill the motor or wear out the pump quite fast. Trust me, you do not want that happening to your pump.

I connected the power bank to the input marked in the buck converter temporarily. Make sure of the polarity and not to short out the contacts. The output from the buck converter goes to the motor of the pump.

Don’t turn on the switch for the pump till we use a voltmeter to check the output from the buck converter. The small potentiometer in the buck converter should be turned counter clockwise in order to reduce the voltage, and vise versa. You can use your finger nail or a small screwdriver to adjust the potentiometer.

If you would like to speed up the pump, you can slowly turn the potentiometer clockwise to increase the voltage by a small amount.

Step 2: Building the Control Circuit

Now its time to build the control circuit which will turn ON the air pump when there is a power loss and charge up the battery when there is power. I will go step by step with the necessary components for the circuit.

1. 5V Relay

We need a 5 Volt relay in order to turn the pump on and off automatically.

I drilled a small hole in the dot board so that all the contacts of the relay can be fixed into the dot board. (The contact or pin which is between the coil contacts of the relay are located in such a way that it does not fit to the dot board, so I had to drill a hole at that exact spot so that all the contacts of the relay can be fixed to the board).

I added some flux to help solder the contacts to the board. I highly recommend using flux when soldering since it helps mate the parts together. Even the generic flux would not cost much, and would still do the trick.

2. TP4056 Charge Module with DW-01 Protector

We also need a TP4056 Battery Charge Circuit which has the DW-01 Battery Protector already built-in. If you are only able to find the TP4056 without the DW-01 you will need to buy a DW-01 Battery Protector Circuit so that your Battery will not go over discharged.

BEWARE eventhough most generic 18650 batteries claims it has over discharge protection on the cover, thorough testing has proved that it does not. Without a proper discharge protection circuit, the battery will be over-discharged and will become useless.

I will connect the Charge Controller to the Dot Board with copper wire, which makes a conductive connection while holding the Controller to the Board in place. There are six contacts in the Charge Controller, all of which should be connected to the Dot Board

3. Fly-back Diode

I soldered in a Diode to act as a Fly Back Diode for the Relay. A fly back diode is required when you are dealing with coils, and one is used here since a relay is mainly a coil.

4. Terminal Connectors (Optional)

I used terminal connectors to connect the battery and the buck converter(which is in turn connected to the pump motor) to the dot board, but you could just solder in them directly if you so wish. I used a permanent marker to mark in the polarity of the connection, so that I do not connect the battery or the buck converter in the wrong way.

BEWARE connecting it with the wrong polarity can damage the circuit.

5. Connecting the PCB components

I used copper wire to complete the connections with the components. The wiring diagram has been attached with this instructable and I have made it as simple as possible. Make sure you connect the diodes in the correct polarity as stated in the wiring diagram. If you need any assistance, I will be happy to help.

Step 3: Connecting the Rechargeable Battery

I thereafter connected wires to the 18650 battery by soldering it in place.

This method is NOT recommended since it heats up the battery. The recommended method is using a battery spot welder, but since I’ll be using only one battery, I'll just use solder.

I used a piece of sandpaper to clean up the terminals (of the battery) and added a bit of flux which makes it easier to solder the wire and the battery together.

I used electrical tape to secure the wire more firmly and to cover up the battery terminals.

Step 4: Fixing the Issue With the Relay Not Turning OFF

I connected the battery and an LED to the circuit to test it out. But I came across a small issue.

The LED stayed ON till I plugged in the power, but the LED did not turn back on after I disconnected the power. It is suppose to start back up once again when there is a loss in the power.

I used the voltmeter to diagnose the issue, and when I connected it to the contacts of the Coil (of the relay) there was around 1 Volt still powering the relay (making the relay stay ON - the relay should turn OFF in order for the pump to turn ON - since I connected the Buck Converter to the Normally Closed contact of the Relay).

So I added a Diode between the positive of the TP4056 input and the Coil Contact (of the Relay).

After I tested it out again, everything worked as planned.

Step 5: Installing the Components to the Pump Casing

Now its time to fix all these components into the air pump.

I desoldered the battery connectors which were meant for the D size battery, so that I can solder the wires from the motor to the output of the buck converter.

I also added another diode as a fly back diode for the motor since motors are comprised of coils as well. This is not compulsory since buck converters are cheap but I added it anyway.

I added spacers to the buck converter board so that I can mount it to the casing of the air pump. I used two pieces of double sided tape to stick the control circuit to the casing since I did not have enough room to use spacers and bolts. The battery was the perfect size to have a grip onto the casing without needing any glue, but you can use hot glue or double sided tape to stick it in place.

I connected the buck converter and the battery to the control circuit and stuck it down to the casing while aligning it to the opening I previously cut to fit a Micro USB cable.

Step 6: Turning ON the Pump

Always use a check value when you are going to connect your air pump to the air stone or airline in the aquarium. If any water cycles back into the pump, it will destroy your motor and other components.

Once I plug in the power, the pump stops working and charges the battery, but after its disconnected or there is a power loss to your house, the pump starts back up, ensuring that your aquarium stays aerated.

I hope this instructable benefited you in any way. If you have any questions, I'm happy to help :)