Question about Soil Moisture Detector circuit

[acircuit]

Here is the link to the project I am talking about: https://www.sciencebuddies.org/science- ... erve-water

I have tweaked this circuit and reconstructed it several times. It worked originally, but now the LED light never fully turns off like it did before. Instead, it dims when moisture is detected, but does not turn off fully. I triple checked my circuit every time and replaced each of the parts. I just cannot get it to turn off.

[bfinio]

Hi - did you purchase the Science Buddies Electronic Sensors Kit for this project or buy your own parts separately? There is a chance your NAND gate could be damaged, but the chip used in this project actually contains four NAND gates, so you could switch to a different one. Look at the picture in the top right on the first page of this datasheet:

https://www.ti.com/lit/ds/symlink/cd401 ... e.com%252F

The pins on the chip are labeled counter-clockwise from the top left. Pins 1 and 2 are the inputs to the first NAND gate and pin 3 is the output.

To try using a different gate, you could rearrange the circuit on your breadboard such that:

- everything currently connected to row 1 on the breadboard (also pin 1 on the NAND gate) moves to row 5/pin 5
- everything currently connected to row 2/pin 2 moves to row 6/pin 6
- everything currently connected to row 3/pin 3 moves to row 4/pin 4

Leave the jumper wires connected to pins 7 and 14 in place, you don't need to move those.

Let us know if you are able to try that and get it working.

-Ben

[acircuit]

Thank you for your suggestion. I reconnected the circuit as instructed and I still have the same issue. The LED turns on, but when the 100k Ohm resistors touch water, the LED dims but does not turn off. If you have any other ideas, please let me know. Otherwise, thank you so much!

[bfinio]

Hmm - I should have asked this before, but what happens if you touch the two resistor leads directly to each other instead of putting them in water?

[acircuit]

The same thing happens. I just tested it again after not using the circuit for awhile, and the LED nearly went out. After i keep testing it, it only dims, though. I think there is just something wrong with the nand because the circuit itself works for the most part except the LED completely going out.

[bfinio]

Ok - I see that you have also been in touch with support via email - at this point I would recommend that they send you a new NAND gate. If you have already tested a second gate on the chip and that didn't work either, it seems likely that the entire chip is damaged. While waiting for the new one, you could test the third and fourth gates (see the diagram on the datasheet I sent earlier) just in case.