Rudolf the Red-Nosed Christmas Tree Water Level Indicator

After forgetting to water the Christmas tree for a couple days, I thought to myself "gee, it would be nice to have a simple indicator like an LED to know when the tree needs to be watered." Then I thought "well, it would be look kind of ugly to have a bare circuit and LED hanging on the tree, I should make it into an ornament!" And of course, what could be more appropriate than a reindeer ornament with a red LED for a nose? Then I searched Instructables and quickly found out I was not the first person to have this idea - so I hope this will provide a beginner-friendly spin on an existing concept. If you're the type whose Christmas tree is shedding dead needles left and right because you forgot to water it for a week, then this project is for you.

Step 1: Materials

The directions I'll provide here are specific to the Rudolf ornament I used, but obviously you can modify this project for an ornament of your choiceor use a different color LED.

Parts

• 47Ω resistor (current-limiting resistor for the LED, exact value does not matter)
• 100kΩ resistor (pull-up resistor, exact value does not matter)
• MOSFET (N-channel; needs low threshold voltage since circuit is only powered by 3V)
• red LED
• 2xAAA battery holder
• AAA batteries (2)
• Hookup wire (solid or stranded will work)
• Heat shrink tubing (optional)
• Stuffed reindeer Christmas tree ornament. I found mine for $3 at Target.
• Drinking straw
• Scotch tape

Tools

• Wire strippers
• Soldering iron
• Xacto knife
• Tweezers
• Hot glue gun
• Needle & thread

Step 2: Build the Circuit

The Fritzing diagrams above show the circuit. It has so few components that I decided to solder the leads together directly, but you could also use a small piece of protoboard. A few important notes (see the figures for additional comments):

• The circuit just uses two hookup wires as a probe - no fancy moisture sensor needed! See next step for how to build the probe.
• I always like to test circuits on a solderless breadboard before soldering them. Make sure your circuit works before you spend the time to solder it!
• Depending on the ornament you choose, you may want to solder the LED leads to the rest of the circuit after you attach the LED to the ornament. I pushed the LED through the back of the ornament (as opposed to pushing the leads in from the front), so I could solder the whole thing first.
• The length of the probe wires will depend on where you want to put the ornament on the tree. I decided to hang it at the bottom so I could keep the wires pretty short.
• I used some hot glue to tack the MOSFET to the back of the battery pack, making the whole circuit compact so I can hide it behind the ornament.
• I'll be stuffing the LED inside an ornament, so I used heat shrink tubing to cover the leads. I didn't want to accidentally pinch them together and cause a short circuit.
• Test your circuit after you finish soldering it - I accidentally burned out the LED on my first run, probably from getting it too close to the soldering iron.

How does the circuit work? There is a large pull-up resistor connected to the MOSFET's gate. Normally, this will pull the gate up to a "high" voltage, turning the MOSFET on and causing the LED to light up. However, one of the probe wires is also connected to the MOSFET's gate and the other is connected to ground. When both probe wires touch water, the resistance between them is low, so this pulls the MOSFET's gate pin down to ground and turns the LED off. Set up this way, the LED will only turn on when the water level drops below the probe tip.

Step 3: Build the Probe

To make the probe, just stick the two probe wires through a drinking straw. Pull them out the bottom, bend them upwards, and tape them to the straw so the metal ends are exposed but not touching. Now, test your circuit with a glass of water. When the water level is below the probe tip, the LED should turn on. When the water level reaches the probe tip, the LED should turn off.

Step 4: Rudolf Surgery Time

No reindeer were harmed in the making of this Instructable.

Now for the hard part. For me at least. Crafty people, try not to cringe when you look at the pictures here. I know, I should really sign up for that hand-sewing class they're offering.

The goal is to replace the reindeer's nose (in this case, a patch of black fabric) with the LED. I decided to do this by cutting a small hole to remove the nose and pushing the LED out from the inside (as opposed to threading the LED wires in from the front - I thought it would look better with the LED only sticking out a little bit). This required making an incision along Rudolf's neck to insert the LED and then sewing him back up. I had to use tweezers and a combination of pushing the LED in from the rear incision and pulling it through the nose incision to get it through (is reading this making you cringe?). Ultimately, it worked well enough for the ornament to just have a "good" side, with the stitches and wires hidden in the back.

Step 5: Hang the Ornament!

Thread the probe into your Christmas tree stand's base and attach it (nothing high-tech required, duct tape should be fine) so the tip is at the water level where you want the LED to come on. Hang Rudolf in a prominent spot to make sure you won't miss the LED (don't want to go through all that work for nothing). Depending on the size of your ornament, you may be able to hide the battery pack directly behind it, or just stuff it farther back into the branches out of sight. Now keep an eye on Rudolf's nose and he'll help you keep your Christmas tree alive!

As always comments, questions, and suggestions are welcome. Thanks for reading and Happy Holidays!