A circuit with a special switch turns on an LED to let you know when it’s time to water your plant. Green thumbs for everyone!
Setup and Supplies
Putting together a Thirsty Plant Kit is easy if you have a pre-cut template to help keep wires in place. You can use a laser cutter to make acrylic templates, use small pieces of perfboard, or cut out templates yourself from other material. Collect the supplies below or order a pre-make kit from Technology Will Save Us.
Supplies for each Thirsty Plant Kit:
- Laser-cut acrylic cutout for holding wires in place based on this template
- One diffused blue 10mm LED (or another style of LED)
- One 47k resistor
- One MOSFET transistor
- Two pieces of wire with ends stripped, one red wire and one black wire (about 6″ each)
- One AA battery holder
- Two AA batteries
- One potted plant!
For a workshop with 25 students, you can expect to spend about $130 on supplies.
How to Assemble a Thirsty Plant Kit
Assembling a Thirsty Plant Kit consists mostly of twisting wires together in the right order. The goal is to create a MOSFET-switch circuit that acts one way (LED off) when the bottom of the unit is in moist soil, and another way (LED on) when the bottom of the unit is in dry soil.
Step 1: Put the LED into the cutout
Insert the LED leads through the two holes in front center circle of the cutout. The longer (positive) LED lead should go through the round hole, and the shorter (negative) LED lead should go through the square hole. If you have trouble pushing the leads through the cutout holes, then check to see if there are any stray pieces of plastic in the holes; if so, push the pieces out with some other tool first.
Step 2: Seat the LED securely
Push the LED leads all the way through the cutout holes so that the front of the LED is flush with the front of the cutout. Then spread the two leads slightly apart so that the LED does not fall back out.
Step 3: Bend and insert the resistor
Bend down the two legs of the resistor at 90-degree angles and then insert the legs into the two small holes in the cutout to the left of the LED. There is no difference between the two legs, so you can insert the resistor in any orientation.
Step 4: Secure the resistor
Push the two resistor legs apart just enough so that the resistor sits securely in the cutout and does not fall out.
Step 5: Insert the red wire from the battery holder
Thread the red (positive) wire from the battery holder through the large circle marked with “+” on the cutout. Note that although it is not clear from the picture, in this step you should be using the red wire that is attached to the battery holder, not the extra strand of red wire.
Step 6: Twist the positive wires with a resistor leg
Use your fingers to twist together the red battery-holder wire, the long positive lead of the LED, and the top leg of the resistor. Then, if you like, use a pair of pliers to make your twist tighter and neater.
Step 7: Flatten the twisted wires to the back
Push up the wires you just twisted so that they lie flat long the back of the acrylic cutout, pointing upwards.
Step 8: Insert the MOSFET
Push the MOSFET transistor through the front of the cutout, positioned so that its flat end is up, near the LED bulb. You may not be able to push the MOSFET all the way in, and that’s okay.
Step 9: Twist a resistor leg around the MOSFET source
Turn the cutout over and push the right (“source”) leg of the MOSFET down to the right (looking from the front this would be the left side of the MOSFET). Twist the free leg of the resistor around that MOSFET end and then push the twisted wires flat against the back of the cutout.
Step 10: Twist the negative LED lead around the MOSFET gate
Now bend the center leg of the MOSFET up and twist the negative lead of the LED around that leg. Push the twisted wires flat against the back of the cutout to keep them secure.
Step 11: Wrap the loose wires around the cutout fork
Thread the loose red (positive) wire through the left fork of the cutout base and wrap the exposed wire around the fork. Do the same with the loose black (negative) wire on the right fork. You may have to strip more of the plastic off the ends of the wires so that you have enough exposed wire to wrap around the fork ends.
Step 12: Feed the loose wires through the top holes
Following the curved lines on the front of the cutout, lie the loose red and black wires flat onto the front of the cutout and feed their ends through the holes at the tops of those curved lines.
Step 13: Insert the negative wire from the battery holder
Thread the black (negative) wire from the battery holder through the hole marked “-” to the left of and below the MOSFET.
Step 14: Twist together the two negative wires
From the back of the cutout, use your hands to twist together the black (negative) wire from the battery holder and the loose black (negative) wire.
Step 15: Twist the negative wires around the MOSFET drain
While looking at the back of the cutout, twist the twisted black wires around the left (“drain”) leg of the MOSFET. This step can be tricky; you may need to fold down the negative wires in a curve along the back of the model to keep things looking nice on the front. You may need to use needlenose pliers to help wrap the twisted black wires more tightly around the MOSFET leg, and press the leg down to keep things secure.
Step 16: Twist the positive wire around the MOSFET source
Again while looking at the back of the cutout, wrap the loose red wire around the rightmost (“source”) leg of the MOSFET. As in the previous step, you may have to fold down and arrange the red wire, and use needlenose pliers to make your wrap tight and to flatten the leg down to the back of the cutout.
Step 17: Insert the batteries and test the LED-on circuit
Insert the two AA batteries into the battery holder; the LED should light up. Right now there is no connection between the wires wrapped around the bottom ends of the cutout fork, so the MOSFET allows the circuit between the battery pack and the LED to close.
Step 18: Dip into water and test the LED-off circuit
Now dip the ends of the cutout fork into a glass of water; the LED light should go off. The water makes a connection between the wires wrapped around the bottom ends of the cutout fork, so the MOSFET allows the circuit from the battery pack to bypass the LED.
Keep Plants Alive with a Thirsty Plant Kit
The Thirsty Plant Kit can help you keep your plants hydrated and happy without overwatering! The LED light will tell you when you should water you plant based on the level of moisture in the soil around the plant.
Step 1: Put the bottom fork into the soil around your plant
Push the assembled Thirsty Plant Kit into the soil of a potted plant. If the soil doesn’t have enough water in it then the soil will not complete the bottom circuit and the LED light will remain on. Time to give your plant some water!
Step 2: Water and wait…
Now water your plant. The water in the soil will make a connection between the two wires wound around the base of the cutout fork. This causes the LED circuit to be bypassed, which in turn makes the LED light turn off. Now wait and watch; when your plant needs water again, the Thirsty Plant Kit will tell you by turning on the light!
Understand How the Thirsty Plant Kit Works
Want to know how the wiring in the Thirsty Plant Kit works? Check out the two diagrams below to understand what happens in the circuit when your plant is thirsty, and what happens when there is enough water to complete the circuit between the wires at the bottom of the the cutout forks.
Circuit through the water means LED is off
When the Thirsty Plant Kit is in moist soil, your plant is happy and the circuit is easy. The presence of the water makes a low-resistance connection between the wire ends at the bottom of the cutout, and the current from the battery flows all the way down to the bottom of the diagram, through the water, and back up again, without ever involving the LED. This means that the LED won’t light up and you will know that your plant is thirsty!
Circuit through the MOSFET means LED is on
When the soil around your plant is too dry, the resistance for moving through that soil is higher than that of the resistor leading into the MOSFET. This has the effect of “pressing a button” at the MOSFET source that switches the MOSFET so that the gate and drain are connected. Now the path of least resistance is for current from the battery to flow down through the LED to the MOSFET gate, over to the MOSFET drain, and back up to the battery. Since the LED is in this circuit, the Thirsty Plant Kit light is on; it’s time for you to water your plant.
Done with your Thirsty Plant Kit but want to take things a step further? Try decorating your cutout. Or go even further: use the Thirsty Plant Kit to detect something other than soil moisture, convert to solar power, or find a way to make your plants talk to you…
If you’re interested in extending your Thirsty Plant Kit, check out these links:
- You can use the circuit in the Thirsty Plant Kit to test for other types of connections besides water; for example, try making a cupboard door sensor that uses something else to connect the wires around the fork ends of the cutout when a door is closed.
- The DIY Thirsty Plant Kit from Technology Will Save Us uses solar power instead of a battery pack. Can you convert your battery-powered circuit to one that uses a solar cell?