I'm helping my son with a science project, and we're building this Soil Moisture Tester. That portion is pretty straightforward, but an adjustment my son would like to make is to add a way to automatically water the plant when the moister alarm triggers (this will be an additional experiment he'll perform)


After a bunch of googling, it seems that this solenoid should work with a gravity based water source. However, I'm unclear how to adjust the voltage from the experiment to power the solenoid. Perhaps I could use a 9V battery like this, but do I need to change around the various resistors?


Also, to be clear, I'm not planning on doing the experiments for him...and he will build the breadboard with my supervision Much of the point of the experiment is to test his hypothesis and measure plant growth daily, but I do want him to push himself beyond what is required for his grade level...so for this portion of the experiment, he'll need my help, and his teacher is ok with that.

 

You need a voltage comparator - you can buy these blocks as a complete module

https://www.amazon.com/ELECTRONICS-SALON-Voltage-Comparator-Module-Relay/dp/B014H0U5IA

 

Welcome to AAC!

we're building this Soil Moisture Tester.

That circuit was a let down...

it seems that this solenoid should work with a gravity based water source. However, I'm unclear how to adjust the voltage from the experiment to power the solenoid. Perhaps I could use a 9V battery like this, but do I need to change around the various resistors?

You can't operate a 12V solenoid from a 9V PP3-type battery. It depends on the pick-up voltage for the solenoid coil.

This random schematic from the internet is what the moisture detectors I have look like:

The moisture probe is the top gray object.

The circuit is a bit simplistic. The 10k pot lets you violate the common mode input range of the comparator. I'd operate from 5V because with 3.3V the guaranteed input voltage range is quite small.

You could operate at 9V, or higher, if you made the LED current limiting resistor larger (because the comparator can only sink 5-6mA).

Elementary/middle school science experiments are fun. I did one with my son about which substance conducted electricity better. Even his 4th grade teacher was excited to see the results. I did an LED brightness experiment with my daughter.

 

You need a voltage comparator - you can buy these blocks as a complete module

https://www.amazon.com/ELECTRONICS-SALON-Voltage-Comparator-Module-Relay/dp/B014H0U5IA

Thanks for the response, but I'll admit I'm not very familiar with this all...would a relay not be something similar?

 

would a relay not be something similar

No. A voltage comparator is designed to compare two voltages and generate an appropriate output.

In the circuit I posted, the moister the soil is, the closer the voltage on pin 2 of the connector will be to ground. By adjusting the voltage on the inverting input of the comparator, you determine when the comparator will switch.

When the voltage on the non-inverting input (pin 5) is greater than the voltage on the inverting input (pin 6), the output of the comparator will be HIGH (a comparator output is either HIGH or LOW).

The output from the circuit would drive a transistor that would control the solenoid. The comparator alone can't sink enough current to energize the solenoid coil.