This little Xmas tree moisture sensor is a perfect give away for family and friends. I soldered about 10 of them this year to give them away and the feedback was just awesome. This little helper prevents your Xmas tree to dry out and loose pine needles early. This usage is quite easy, hang the ball into your Xmas tree and place the two electrodes into the water reservoir. If the reservoir is filled, the led is off, if it’s on, the reservoir is empty and you have to water your tree.

Parts list

For this project we will use the following components

• A transparent Xmas ball
• A 2N2222 NPN transistor
• A 220 Ohm resistor
• A CR2032 battery holder + battery
• A red led
• About 2m of 26 AWG wire (2x 1m)
• Wire ferrules
• Hot glue

Solder it up

Bend the legs of the transistor and the led, then solder the collector of the transistor to the cathode (short leg) of the led. Because the transistor has no markings for the pins ensure that flat side of the transistor faces downside. In this orientation the pinout is, (f.l.t.r) collector, base, emitter.

Now take the CR2032 battery holder and flip it and solder the led transistor package to. Keep an eye on the polarity, the led must be soldered to the positive side of the batter holder.

Tipp: Pre solder the legs and the pins on the battery holder.

In the next step we will solder the risitor between the anode of the led and the base of the transistor. This will ensure that we have matching current on the base if the circuit is closed.

With the resistor in place, the circuit is complete and we can now have a look at the wires. So grab the wires and pre solder the two ends. This way it becomes much easier to solder the wire to board later.

After that solder the wires to the base and the emitter of the transistor and cover the everything with hot glue.

Prepare the Xmas ball

Until the hot glue gets cold, we can focus on the transparent Xmas ball. To get the wires through we have to drill a hole inside the ball. I use a 5mm metal drill and place the hole about 2cm away from the edge of the half ball. If you drill the hole more to the edge it might crack the plastic.

Now take the ends of the wires and put them through the drilled hole. Then grab the ferrules and applie them to the end of the wires. Don’t add the ferrules first, because then they wont fit trough the hole.

Last not least, put add a CR2032 battery into the battery holder and add both halfs of the ball together. The led should light up beacuse there is no contact between the probes.

Now, hold the probes into water. The led should turn off because the contact is closed.

Tipp: To hold the probes of the Xmas tree moisture sensor near to each other, you could use an insulating screw joint.

Sum up

And that’s it. You have successfully build your own Xmas tree moisture sensor 😃 So, start over again and build some more of these guys for sharing them with your family and friends.

If you like this project, feel free to share the article or an image of your Xmas ball 😉