Introduction: Automatic Flower Wateter

About: Electrical Engineering Student. Love electronics and learning something new everyday. E-Mail me @ razeksk [at]

A simple project that will automatically water your flowers when the soil gets dry. Only using two Simple Circuits and some supplies from your local hardware store, you’ll be sitting back enjoying lemonade while your plants water themselves in no time!

All done with Simple Circuits.  Check out our Kickstarter!

Inspired by the famous blog post Best DIY Cheap Soil Moisture Sensor, we wanted to make an analog version of the automatic flower pot watering system. This project can be done with 2 SimpleCircuits.

Step 1: Make the Moisture Probe

Shawn Verrall, the brilliant mind behind DIY cheap moisture probe did such a great job explaining how to make your own moisture probe for this project that we’re not going to try and repeat it. Visit the link above for instructions on how to make your own soil moisture probe using materials available at Walmart.
This tutorial starts after you’ve made your probe and ready to electrify it without writing any software code.

Here’s what you need from your SimpleCircuit test kit (

N-Channel MOSFET

Step 2: Get to the Store!

What you need from the store:

A 12-24V power source that can supply at least 350mA or 750mA respectively. I’m using a wall transformer from an old router for this example. If you aren’t sure if your 12-24V wall transformer is correct, post a picture of the electrical details sticker and we’ll help you out.
An irrigation Water Solenoid (Home Depot has plenty of these. If you aren’t familiar with irrigation solenoids then ask an employee for an “irrigation water solenoid” or “sprinkler valve”). FYI: A standard garden hose is 3/4″.
A moisture probe from Best DIY Cheap Soil Moisture Sensor
A voltmeter
A resistor between 10k and 1MEG. (The exact value is not important because your varying the turn-on threshold using the potentiometer on the comparator circuit board). FYI: If you do not want to use a resistor, you can use a second moisture probe as a replacement. Store the second moisture probe in a dry place next to the circuitry. We tested this and it works but we’re not sure how well it works over time, temperature, and humidity.

Step 3: Initial Testing

Find Power and GND:

Cut the barrel connector off your wall wart power supply and use your Voltmeter to help you determine which wire is positive and which one is ground.

Test your 12-24V supply:

If you’re using the same water solenoid as me, polarity on the wires doesn’t matter. Take the two wires and touch them directly to the solenoid wires — if the solenoid clicks on then your good. Unplug your wall wart and set it aside. If your solenoid did not turn on then it may be too small.

Step 4: Connect It Up!

Follow the schematic. 

Step 5: Try It Out!

Stick the moisture probe in the soil and set the turn pot on the comparator to the middle. Adjsut the comparator until the ciruict behaves the way you want (i.e. the pump turns on when it’s supposed to). The best way to adjust how long the water flows for is to move the probe closer or further from the water source.