Moisture circuit for Senior Design Project

SgtWookie

Joined Jul 17, 2007
22,210
If you change your supply to 5v, the IRF510 MOSFET will not be able to turn on.

Keep it at 12v. If you are going to use the MOSFET, then use ~2.4k for R4. No point in loading the output of the comparator unnecessarily.

If you go to Radio Shack, you might want to pick up an LM339. That is a quad comparator; it also needs pull-ups on the outputs. I don't like to sink more than 5mA from their outputs. However, I'm not really certain how a real LM319 will perform; the simulation shows a very long transition time for the difference in input voltages. A '339 would switch much more quickly.

You might also pick up a 50k to 200k pot to replace R1/R2 with, making your circuit easily adjustable.
 
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wayneh

Joined Sep 9, 2010
16,128
If you change your supply to 12v, the IRF510 MOSFET will not be able to turn on.

Keep it at 12v.
Assuming you meant 5V in the first sentence, but I'm not so sure it wouldn't work anyway. Yes, the MOSFET would not be saturated, but it's so much overkill for this application that I suspect it would work fine anyway.

But agreed that keeping it at 12V is best.
 

SgtWookie

Joined Jul 17, 2007
22,210
Assuming you meant 5V in the first sentence,
Yes, I did - thanks. I've corrected it.
but I'm not so sure it wouldn't work anyway. Yes, the MOSFET would not be saturated, but it's so much overkill for this application that I suspect it would work fine anyway.
I like to make sure that our budding engineers go by the specifications.

I feel much much more confident saying "It will work with a 12v supply" than "it might work with a 5v supply."

It's so much easier to get it working in the beginning by following the specifications. After it's working, then they can experiment with it. At least they will know that it CAN work, and how to get it working again.

Where I run into problems on here with that is when I make very specific recommendations as to how to proceed, and then the OP is still having problems, and I eventually find out 20 or 30 posts later that they failed to follow some critical recommendations. I had that happen with some fellow from India; I told him he needed to replace his opamps with low-power, low noise, rail-to-rail versions, and gave him three recommendations for different opamps to purchase that would work.

He didn't do that; he used something else because it was a few cents cheaper. However, his project wouldn't work. The cheaper opamps were not rail to rail, induced more noise into the system, but worse yet, caused him to exceed his power budget - and adding another supply or replacing the existing supply with a higher wattage version would cost more than the original opamps that I recommended.

He would up having to purchase a set of one of the opamp models I recommended, as they used so much less power that he had power to spare.

I got off topic here - but that episode is just another one of the reasons why I'm a stickler for following the specs. Even if you are well within the specs, a project still might not work the 1st time you try it - but since you've followed all of the specifications, there are just a few things that could be wrong - incorrect wiring, poor soldering, or a bad part.
 

wayneh

Joined Sep 9, 2010
16,128
Even if you are well within the specs, a project still might not work the 1st time you try it...
True enough, and I agree that experimenting at the fringes of the specs is not how most folks here want to spend their time. They're too busy with their over-unity machines! ;) (Not funny, too soon?)
 
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