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.
     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)
     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.

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