I am building a project that indicates moisture in the soil and when it detects the low resistance in the dry soil turns on an LED. (I already have a circuit for this.) (Was gonna use something like http://www.redcircuits.com/Page18.htm)

I was wondering if anyone had any good ideas how I could use a simple sensor to pick up that light off the LED to signal a pump to activate until the light turns off, in which case the pump turns off.

Anything would be helpful, I am not quite sure how to really go about it, or what components I would need, but it doesn't sound too hard to accomplish. (Examples are always nice.)

-Thanks

 

Don't bother sensing the light, just take the signal that's lighting the LED and use that to control a relay or a MOSFET.

Show us your existing circuit and we can go from there.

 

That looks very similar to THIS THREAD.

You can use pin3 or pin 10 of IC1 in the circuit you posted to drive transistor, a MOSFET preferably, that drives a relay for your pump.

 

If you wish to use the LED light, you can use a phototranssitor to sense it.

Be careful that the LED's wavelenth is within the transisor's wavelength range.

 

Alright, thank you very much, so pin 10 or 3 to power a transistor which drives a relay? So why doesn't it matter which one you go off of? I am just learning this stuff, so what exactly does MOSFET do and according to this circuit, what kind of relay would go with an applicable MOSFET for this circuit?

 

Go the other way. Choose a relay that safely switches your load. For safety and longevity, it should be rated for well over your expected voltage and current. It will require a certain amount of current to be activated.

Then choose a MOSFET that can handle the current needed by the relay. This shouldn't be too hard, since MOSFETs can handle surprisingly large currents for their size. I'd choose one that's rated for at least about 4X the expected current. There's almost no penalty for going large. I use IRF540N (N-channel MOSFET) for all sorts of general purpose switching.

When choosing the MOSFET, be aware that you'll need either a logic-level or "normal" MOSFET. The former will be full on with 5V on it's gate, the latter needs at least 10v. You want the MOSFET to be fully on or fully off - a switch - so that it's not generating heat.

The IC is a multiple Schmitt trigger; each trigger converts a messy analog signal into a digital on or off. Outputs 3 and 10 are just the outputs of two different triggers on the same device.

 

The IC is a multiple Schmitt trigger; each trigger converts a messy analog signal into a digital on or off. Outputs 3 and 10 are just the outputs of two different triggers on the same device.

Are not pins 3 and 10 logical inversions, and if so, won't the relay's operation be inverted depending upon which pin drives the MOSFET's gate?

 

As wired in the linked circuit, yes. The final logic needed by the OP will depend on what type of relay he gets, NO or NC.

 

Alright, sounds good, thank you guys very much!

This week I am gonna order the components, is there any suggestions (assuming I follow the circuit I posted) as to the specifics of the other components I need and their ratings? As far as like standardized components... I was thinking that the pump doesn't need to be anything to elaborate, maybe like a small pump used for aquarium filters or?

Once again thank you, you got me excited to actually hook this stuff up and give it a whirl

 

When choosing the MOSFET, be aware that you'll need either a logic-level or "normal" MOSFET. The former will be full on with 5V on it's gate, the latter needs at least 10v. You want the MOSFET to be fully on or fully off - a switch - so that it's not generating heat.

So if I look at the circuit again I see that it is a 3V circuit, and I have to meet at least the 5 volts for the MOSFET... Won't I have some problems having the MOSFET fully charged? Do I need to change the voltage of the circuit? If so, what else needs to change?