I designed and built this simple little conductivity tester for my AP chem teacher to replace her makeshift version made from jumper wires and a partially dissected Christmas tree light that was constantly in danger of either blowing out the Christmas light or melting whenever a Bunsen burner was too close to it. This version fits entirely into the stopper except for the battery, which is velcroed onto the arm of the test tube stand. Whenever you need a new battery just add some Velcro tape to one of its sides and snap it in place and if the copper electrodes get stuck in something or become corroded you can remove them and make new ones by simply attaching uninsulated copper wire to new 22-16 AWG Male Connectors.

Parts:
1 Rubber Stopper with 2 holes in it
1 9V Battery Snap
1 LED (any color)
1 1.5K ohm Resistor
2 22-16 AWG Female Connectors 1/4"
2 22-16 AWG Male Connectors 1/4"
22 to 16 AWG uninsulated Copper Wire
Velcro tape

Tools / Supply's:
Lead-free Solder
Hot Glue sticks
Soldering Iron
Hot Glue Gun
Wire Strippers
Needle-Nose Pliers
(Optional) Set of Helping Hands

NOTES:

Be sure to test your LED and resistor before assembling this!!! There is nothing more annoying then getting part or all of the way through a project only to realize you have to stop and undo the work you have done because of a bad LED or other component.

If the Electrodes are having troubles staying attached, then you can "tighten" the female connectors with a pair of pliers to help them clamp down on the electrodes better.

Sorry about the lack of pictures. I did not think to do a how-to for this until partway through, and so do not have pictures for some of it.

Step 1: Resistor

The first step is to bend one of the wire leads on the resister 90 degrees, so the body of the resister can fit in one of the holes in the stopper, with the straight lead poking out the bottom of the stopper, and the bent lead poking out the top.

Step 2: LED

Next we add the LED, start by bending the negative leg/lead on the LED (the one the is closest to the flat side/edge on the LED) straight up around the LED body. Then insert the LED into the empty hole in the rubber stopper, so the positive lead sticks into the hole, and the negative lead sticks straight up, away from the stopper's top. This is a lot easier if the outer diameter of the LED is the same, or only slightly smaller then, the inner diameter of the holes in the stopper, because that way the LED will fit snugly inside the hole and be much more secure.

Step 3: Electrode Connectors

Now we will attach the two 22-16 AWG 1/4" female connectors. First, if either of the leads sticking through the holes in the rubber stopper extend out of the holes and past the bottom of the stopper, then cut off the extra length with the wire cutters on your wire stripper (if your wire stripper has one, if it does not then use a pair of side-cutters). Then, use the crimper on your wire strippers to crimp a connector onto each lead.

For the resister it is easiest if, after cutting the lead to length, you just pull the resister out of the hole and then crimp it separately. Then you can thread the resister back through the hole. However the LED is too big to allow for this, so you have to kind of push the LED into the stopper a bit to get its lead to stick out a bit farther so you can crimp the connector onto it. Once the connectors are crimped onto the leads, then add some hot glue to their bases and push them into their respective holes on the bottom of the stopper (the bases of the connectors should be able to fit snugly in the holes in the rubber stopper).

Step 4: Soldering

Now you should have the female connectors in the holes in the rubber stopper and they should be connected to the positive lead of the LED, and one of the leads on the resistor. The negative lead of the LED and the other lead of the resister should be coming out of the top of the stopper, and the LED should be visible, with only a little bit of its base stuck into the hole in the stopper.
Now take the 9V battery snap and strip the ends of its lead wires. Twist the negative lead of the LED and the negative lead wire (normally the black wire) of the battery snap together close to the stopper (cut any excess length off the LED lead as needed), and then repeat this process with the resister lead and the positive lead wire (normally the red wire) of the battery snap. Before soldering test the conductivity tester by connecting a 9V battery to the battery snap and bridging the two female connectors sticking out of the bottom of the stopper with a wire. The LED should light up.

If the LED lit up then remove the 9V battery and bridging wire, and solder both of twisted wire connections we just made. If it did not light up then start troubleshooting and go over the instructions again to make sure you did not miss anything. Once the solder has cooled, test the conductivity tester once more, if it works then you are good to go.

Step 5: Final Touches

Now that we know everything works we can go back and attach the resister and LED more securely with additional hot glue. Also, wrap the soldered connections with electrical tape (or if you have some on hand you could use shrink wrap/heat shrink tubing). If you have some non-reactive silicone sealant on hand you could probably seal the bottom of the conductivity tester with it, which would be useful if you need a more airtight seal. Also I have no idea how reactive hot glue is (based off the little bit of research I have done on it I assume its not very reactive), but the sealant would help prevent it mixing with any chemicals. The Velcro tape is for attaching the battery, you can stick one side of a bit of Velcro tape to the 9V battery you use to power the conductivity tester, and the other side onto a ring stand or in some other convenient location to hold the battery in place.

To make the electrodes just take an appropriate length of insulated 22-16 AWG copper wire and crimp a 22-16 AWG 1/4" male connector onto one end. The male connector will allow the electrodes to easily attach and detach from the conductivity tester as needed. So if your electrodes get old and gunky, or need to be replaced for some other reason its easy and cheap, just pop the old electrodes off of the conductivity tester, and make new ones with some more uninsulated copper wire and male connectors. Just try to keep the electrodes straight so they don't touch each other when the conductivity tester is in use and potentially give a false positive.