Add DC pump to Moisture indicator project

[Texan4u]

For my kids 6th grade science project we are trying to modify the project "Green Technology: Build an Electronic Soil Moisture Sensor to Conserve Water " by adding a small DC pump
While the output of the 4011 chip used in this project is strong enough to drive an LED directly, it is not powerful enough to drive a pump. We are working on how to use a transistor and an external power supply to drive a small DC pump by controlling the transistor with the output of your circuit.
Attached is a rough circuit, how do I pick the transistor type and size for this DC pump ?

[bfinio]

Hi - a couple things:

1) In case you haven't seen it, we have an Arduino-controlled version of this project with a more advanced soil moisture sensor and a pump. https://www.sciencebuddies.org/science- ... t-watering

2) If you want to stick with the non-Arduino version, the type of transistor you want is called an N-channel MOSFET. You can get them on Amazon but they are usually sold in bulk:

https://www.amazon.com/gp/product/B07CTF1JVD/

Hobby electronics sites will sell them individually, but usually don't have free shipping:

https://www.adafruit.com/product/355

The voltage and current ratings of these MOSFETs are typically way higher than what you'd need for small hobby projects anyway - e.g. the one from Adafruit is rated for 60V/30A, and your pump is only 3V/1.2A, so that is not an issue.

Note that in your circuit diagram you would need to remove resistor R4 and the LED from the output, and just drive the MOSFET directly with the output of the 4011 chip.

Finally, we have a video about how to control a pump with an Arduino that I would recommend watching: https://youtu.be/To3DKP99-1U?si=Nwc_oAOH3WIuje-o. It will show you how to use the MOSFET and how to power the pump (in your case you just have the 4011 chip providing the high/low logic signal instead of an Arduino). Since your pump is only rated for 3V you won't want to power it directly with the 9V battery.

Hope that helps!

[Texan4u]

Thanks a lot Dr Finio for prompt quick response ! We will definitely try the N-Channel MOSFET option without Arduino controller for controlling the pump.
I also noted your suggestion to remove R4 and LED .
Can we add LED somewhere in the circuit where it can light up and indicate there is no moisture and turns off as soon as the pump adds water to make the soil wet? Can I add it in between 4011 and Gate of MOSFET ?

[bfinio]

Hi,

You could potentially put the LED and its resistor in *parallel* with the connection to the MOSFET gate, not in series. Meaning, you would keep with the existing circuit with the LED and resistor connected from the output of the 4011 chip and ground. When you add the MOSFET, add another jumper wire connecting the output of the 4011 chip to the MOSFET's gate pin, but do not remove the LED. I have not tried this myself but I believe it will work. If you put the LED and resistor in *series* between the 4011 output and the MOSFET, which is how you had it drawn in your original diagram, the circuit will not work as intended.

[Texan4u]

Thanks Dr Finio for the suggestions, my kid is waiting for the hardware parts to arrive. He is excited to try this project. We will definitely post some pictures once completed.
Thanks once again !

[bfinio]

Great, let us know how it goes!

[Texan4u]

Hello Dr Finio
We got the materials and assembled the kit. The MOSFET and pump does not work. The project works as stand alone, so no issues with components and connection in the original moisture detection set up.
When connected with Mosfet and Pump, the LED display works as intended , but the pump does not turn on.
Can you please review the attached circuit and let me know what the issue is. M stands for motor in the attached picture and the MOSFET legs are Source , Drain and Gate from top to bottom vertically

userimage.png (305.27 KiB) Viewed 6418 times

[Administrator note: I replaced the pdf with an inline image.]

[bfinio]

Hi - in the circuit diagram you provided, the MOSFET is wired correctly, but you are powering the pump directly from the 9V battery. As we discussed above, it looks like your pump is only rated for 3V, so there's a risk this may have burned out the pump. To power the pump, you would need a separate 3V battery pack (e.g. 2xAA), which shares a common ground with the breadboard and the rest of the circuit, and then connect the positive wire from the pump to the positive wire from the 2xAA battery pack. Do NOT connect the positive from the 2xAA pack to the positive from the 9V battery because this will create a short circuit.

You can confirm that your pump still works by just directly connecting its positive and negative wires to the corresponding wires from a 2xAA battery pack, without worrying about the rest of the circuit. As with other parts you can buy these battery packs in bulk on Amazon or get them individually from companies like SparkFun/Adafruit: https://www.amazon.com/s?k=2xAA+battery ... ith+switch

If the pump doesn't work when you try that, then unfortunately it is probably burned out and you will need a new pump.