Get your aquarium to send you a text message alarm when your aquarium leaks/overflows!
This sensor only takes a few hours to build and you can use it to detect the presence of even small amounts of water on the floor. Connect it to a buzzer to provide an audible alarm, or if you own a Neptune Systems Apex controller, you can have it alert you with an email or text message.

How it works: The idea behind this sensor is to make a simple switch that closes when it comes into contact with water. Two wires in a speaker cable serve as the two sides of the switch. The cable is cut at regular intervals and reconnected with uninsulated butt-splice connectors. These connectors conduct electrical signals from the copper wires inside the speaker cable to the outside environment, and therefore each pair of connectors is a switch that "senses" the presence of water. When a puddle of water contacts two crimp connectors, current can now flow between the two wires of the speaker cable, closing the switch. This signal can be used either to turn on an electronic buzzer or be fed into your aquarium controller to do things such as switch on devices or send a message.

Pretty much everything necessary for this project other than the aquarium controller (if you choose to use one) can be bought at a hardware store.

Materials Needed:
14-gauge speaker wire - Get as much as the intended length of the sensor. I chose a thicker wire because I wanted to minimize voltage loss across the 20 feet that I bought. If you plan on having a shorter sensor, you can use a thinner gauge.
Uninsulated butt-splice wire connectors - Buy 2x the number of points where you want the sensor to check for water - I got 40.
Wire clips - Buy one for every pair of connectors (20 in my case).If you're going to be laying these down on the floor, you should probably look for adhesive-backed clips.

Tools Needed:
Knife - for cutting apart the two strands of speaker wire.
Crimping tool
Wire stripper - you can use the knife for this if you don't own a wire stripper.
Pen
Measuring tape

Alarm Circuit:
Electronic chime(such as Radioshack item 273-071), 9V battery and some way of connecting wires (either solder and soldering iron or insulated crimp connectors)
OR
Neptune Systems Aquacontroller Apex and I/O Breakout Box (breakout box is not necessary but convenient)

Step 1: Making the Sensor Probe

1. Decide where you want the sensor probe to go. I put mine in a ring around my main aquarium tank and the sump. Then choose at what point in the speaker cable you would like to start detecting water leaks.
2. Separate the two wires in the speaker cable using a knife. You want to separate the wires across a length of about 1.5 inches. Be careful to cut between the wires so that the plastic sleeve around each wire remains intact.
3. Cut both wires with the wire strippers. It's really important to stagger the cuts so that when you install the crimp connectors they can't contact each other directly. I had about 1 inch separating the cut in one wire from the cut in the other. This will result in you making four wire ends, two short and two long (see photo).
4. Use the wire strippers to take about 6mm of the plastic sleeve off of each wire end.
5. Reconnect the speaker wire ends using the crimp connectors. This step is easiest if you crimp the connectors onto the short leads first and then connect the short leads to the long leads.
6. Mark where you want the next set of crimp connectors to be. I put mine 6 inches down the wire.
7. Repeat until you have as many detection points as you want.
8. One end of the speaker wire is going to connect to the Apex controller or buzzer circuit. Choose an end for this and strip both ends of the wire. For the other end, I decided to put in another pair of crimp connectors, making sure to stagger them 1 inch apart as with the others.
9. Once the probe is ready, attach it to the floor using the adhesive wire clips. It's good to place one wire clip next to each pair of crimp connectors to make sure that the connectors are held close to the floor.

** Note: Once the sensor is plugged in to either the Apex controller or a buzzer circuit in the next step, the crimp connectors will be live. The Apex controller outputs a 5V signal which is completely safe even if you should grab on to a crimp connector in each hand. On the other hand, if you are going to hook the sensor into a buzzer circuit, make sure not to power the buzzer with any voltage higher than 9V.

Step 2: Connect to Apex Controller

If you don't have an Apex controller and intend to use a buzzer, skip to the next step.

Connecting the sensor to the Apex controller:
1. Connect the Breakout Box to the Apex base module using the i/o cable.
2. Push the stripped ends of the speaker wire into the ports labeled "GND" and "I6". It doesn't matter which wire goes into which port. Use a small flathead screwdriver to tighten the connection.
3. If you don't have a Breakout Box you will need to solder the ends to the socket labeled "I/O" on the Apex base module. The equivalent pins in the socket are shown in the diagram labeled "Figure 6".

Programming an alert:
1. Go to the Apex web interface and click on configure - outlets. You should see a screen like the one below. In the Outlet dropdown menu navigate to EmailAlm_I5. There will be a default program loaded already, probably having to do with temperature setpoints. The important part is that the program should always begin with the line Set OFF. Type If Switch6 CLOSED then ONâ to set up the email alert. If you want to also have the Apex controller make an audible noise, you can go to the outlet SndWrn_I7 and type in the same line. You might want to change the warning sound (go to configure - misc) as the default sound is really short and a little difficult to notice.
2. You will also have to make sure that the Apex controller is set up to send emails. Go to configure - network. Under the Ethernet heading you should check to make sure that the DNS address is correct (you can usually find this number by typing your router's IP address into your browser). Under the email heading you should set Email enable to Enabled, and type in your email server information. The server information is specific to your email provider and some providers are more complicated to interface with Apex. The example screenshot I put up is for a gmx email account, for other accounts you should read this forum to find the appropriate settings for your email provider. Once you are done you can test that your email is working by pressing Email test and then Update Email Settings. If it doesn't work, try changing the SMTP port from 25 (the default port) to another port supported by your email carrier.
3. If you want the controller to alert you with a text message instead of an email, you need to find out how your cell carrier handles email-to-text conversion. Most carriers will have an email address that can be used to send text messages to a phone. For AT&T for example, the email address is Your-10-Digit-Number@txt.att.net. A pretty comprehensive list of these addresses can be found on this page.
4. Test out the sensor by shorting two crimp connectors using either a small piece of metal or a puddle of water. If nothing happens, check the connections - one of the crimp connectors on the probe might have come loose and may require re-crimping.

You're DONE! Now you can relax and go on vacation knowing that if disaster strikes in your aquarium room while you're away, your very own alarm system will allow you to order someone who stayed home to handle the emergency!

Step 3: Connect to Buzzer Circuit

1. The circuit you need to put together couldn't be simpler! It has only three components: the electronic chime, a 9V battery, and the sensor probe. Use a battery clip for the 9V battery and connect the positive lead to the chime, and connect the negative side of the chime to either of the two wires in the sensor probe. Connect the other wire in the sensor probe to the negative lead of the battery clip and you're done! You could either solder the wires together or use crimp-on (insulated) connectors for a less permanent connection.
2. To test the circuit, use either a puddle of water or a small piece of metal to short one of the crimp connector pairs on the probe. If nothing happens, check the connections - one of the crimp connectors on the probe might have come loose and may require re-crimping.
You're DONE! Go relax in your back yard comforted by the knowledge that should disaster strike and your aquarium spring a leak, your very own alarm system will waste no time in springing you into action!