newb: I've got my 1st circuit working, but it can be better..

OK, I made a simple and far-from-perfect moisture sensing circuit,
basically I've broken the base feed to my transistor into two leads
that I stick on the dirt of a potted plant, the wetter the dirt, the
brighter my LED is.

I'm learning.

I would now like to modify the circuit so that the LED is either ON or
OFF, but I'm not sure how to accomplish this... I need to fully
saturate a transistor I'm thinking, but don't know how to do this
correctly.

I've been reading up on http://www.AllAboutCircuits.com as well as
http://www.kpsec.freeuk.com/index.htm but haven't found the answer yet.

If anyone has a couple seconds and could give me some suggestions or
maybe a link, that would be very much appreciated.

Thanks!
Steve

 

If your intention is to get something working easily, a
comparator with open collector output would be your
best bet for this. Just setup a pair of voltage dividers
on the input that set the trip level at the conductivity
that you want the LED to be barely on or barely off.

 

Thanks for the reply Larry!

Uhmm... I don't follow you too well on your response, I'm sure it's
simple to most everyone here, but it lost me a bit.

I found this site: http://home.cogeco.ca/~rpaisley4/Comparators.html
and will be reading it to try and grasp this whole thing.

Thanks again!

 

That looks like a good introduction to comparitors.
The circuits shown in the 3rd schematic block from
the top closely resemble what you will need. With
your LED in series with RL and your soil probe
substituted for R1 or R2, it's the right topology.

The important thing is to understand what the
comparitor actually does. Then, seeing how to use
it for your application will be easier.

You're welcome.

 

Study that until you understand, its a very commonly used device and you
will be able to design many circuits around it.
Lots of luck with your new interest !!