This project is an 'empty water detector' to tell you when the water is gone from a vessel - originally, I designed it for a christmas tree, but it will work for your dog's water bowl or just about anything else.

Parts list

220k Resistor
Small Perfboard
2N3906 Transistor
2x or 3x AA Battery holder
3mm Red LED
Extra wire for the probes

You can order the kit from Gadget Gangster & grab a PDF version of these instructions, here. You can also share your own projects on Gadget Gangster, too.

Here's a video demonstration

Step 1: Gather Materials

First, gather all your materials. If you've ordered the kit from Gadget Gangster, your project will come with a halfboard - put the halfboard in your vice, as shown in the picture. You'll also need a soldering iron and 2 AA batteries.

Step 2: Add the Resistor

Add the resistor from M3 to N7.
Bend out the leads on the other side of the board, flip the board over and solder the resistor down, and trim off the excess wire.

Step 3: Add the Transistor

Flip the board back and add the transistor from O6, O7, and M8. The flat side of the transistor should point to the edge of the board.
Spread apart the leads, flip the board over, solder & trim the excess wire, and flip back over.

Step 4: Add the LED

Add the LED from P6 to Q6. The longer lead goes into P6, shorter lead into Q6. Spread apart the leads, flip the board over, solder the LED down and trim the excess.

Step 5: Add the Probes

The probes are the wires you'll put in the water;

Take the red wire, cut in half, and strip the ends of each wire, & tin the ends (lightly twist the ends of the wire together, heat with your soldering iron, and add a bit of solder.
Once the wires are tinned, put on wire in P7, the other wire in Q8. These wires are the probes that you'll put into the water. You can tie the other ends of the wire in a knot, with one end a little shorter than the other.

Step 6: Connect the Batteries

Lastly, connect the battery pack. Thread the black and red wires through a few of the bigger holes on the bottom of the board for stress relief, and connect the red wire to Q23, Black wire to M22.
That's it! Add 2xAA batteries and insert the leads into the water you'd like to test. When the water's gone, the light will illuminate and you'll know it's time to add water!