Soil Moisture Sensor Parts

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Roheka15
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Soil Moisture Sensor Parts

Post by Roheka15 »

Hello,

I was wondering if I can substitute a 74HC595 IC for a CD4011 NAND gate IC (which is part of the kit)? Also does a 470 MΩ resistor work better than a 330 MΩ; does it matter if I don't use those exact types of resistors? What are the differences and will the sensor still work?
bfinio
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Re: Soil Moisture Sensor Parts

Post by bfinio »

Hi - are you sure you got the right part number for the 74HC595? When I search for that part it is described as a shift register, which is not the same thing as a NAND gate:

https://www.jameco.com/z/74HC595-Major- ... 46105.html

The CD4011 is a quad 2-input NAND gate:

https://www.jameco.com/z/CD4011B-Major- ... 12634.html

meaning if you look at the circuit diagram on the datasheet, you will see that the chip has four NAND gates, with two inputs each:

https://www.jameco.com/Jameco/Products/ ... 2634TI.pdf

There should be a 7400 series equivalent, but I don't think it's the part number you mentioned. For example this:

https://www.jameco.com/z/7400-Major-Bra ... 48979.html

however you have to be careful, because if you look at the datasheet it is NOT an exact pin-for-pin replacement for the CD4011, some of the gates are arranged differently:

https://www.jameco.com/Jameco/Products/ ... 979FSC.pdf

for example on the CD4011, pin 4 is the output of one of the NAND gates; in the 7400 pin 4 is one of the inputs. The orientation of the gate is reversed.

For your second question - the 470Ω resistor (that's 470 ohms, not 470MΩ or mega-ohms) is a current-limiting resistor for the LED in this kit. Roughly, the resistor size is calculated with

Resistor size = (Battery voltage - LED voltage) / (desired current)

In this case the battery voltage is 9V, the LED voltage is about 2V, and the desired current is about 20 millamps (0.02 amps). When you plug in those numbers you get 350 ohms. So a 470 ohm resistor is a little bigger and errs on the side of caution with not having too much current through the LED, but you should be OK with a 330 ohm resistor.

Hope that helps! Please write back if you have more questions.

Ben
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