I'm beginning in electronics and have found a moisture sensor that appears to be cheaper but similar to this one
http://www.redcircuits.com/Page18.htm
Which was suggested in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=3645&highlight=moisture+sensor
The sensor circuit can be found here:
http://nurne.blogspot.com/2008/01/water-me-man.html
In the subsequent blog entries someone suggested that instead of a 2n2222 they use a MOSFET tied to a high ohm pull down resistor:
Great concept. Love the idea of a dirt-cheap, dirt-sensing circuit. As someone who could can plead guilty to involuntary plant-slaughter, I can see the utility.
Unfortunately, the circuit as drawn is going to consume a little over 7mA in the idle "off" state, which is bound to be a battery-killer. Dead battery could mean dead plant. Consider using a p-channel MOSFET with a multi-megaohm pull-down resistor. That will take close to zero current (pA) when off and still keep the part count very low. You'll have to pay a little more for the MOSFET than the 2N2222, but what's the cost of a new plant nowadays?
My question is assuming he is looking to replace the 2n2222 with the mosfet, where would I place the mega-ohm resistor for the pull-down?
I've sent the blogger an email asking for clarification based on that reply but haven't received a response so now I turn to you all, the giant pulsing brain of the interweb. This circuit was interesting to me because of the apparent cheap cost and with the MOSFET the long lasting nature of the battery (especially when tied to a blinking led).
P.S. I found that when I measured the resistance of freshly watered soil, I still got resistances of 6-8k ohms, in his original design it appears to me that it would fire when the resistance got above 1k - which didn't seem right from what I was measuring. If anyone else finds this interesting I would be greatful for an outside opinion as to the general resistances measured for your various flora. Thanks!
http://www.redcircuits.com/Page18.htm
Which was suggested in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=3645&highlight=moisture+sensor
The sensor circuit can be found here:
http://nurne.blogspot.com/2008/01/water-me-man.html
In the subsequent blog entries someone suggested that instead of a 2n2222 they use a MOSFET tied to a high ohm pull down resistor:
Great concept. Love the idea of a dirt-cheap, dirt-sensing circuit. As someone who could can plead guilty to involuntary plant-slaughter, I can see the utility.
Unfortunately, the circuit as drawn is going to consume a little over 7mA in the idle "off" state, which is bound to be a battery-killer. Dead battery could mean dead plant. Consider using a p-channel MOSFET with a multi-megaohm pull-down resistor. That will take close to zero current (pA) when off and still keep the part count very low. You'll have to pay a little more for the MOSFET than the 2N2222, but what's the cost of a new plant nowadays?
My question is assuming he is looking to replace the 2n2222 with the mosfet, where would I place the mega-ohm resistor for the pull-down?
I've sent the blogger an email asking for clarification based on that reply but haven't received a response so now I turn to you all, the giant pulsing brain of the interweb. This circuit was interesting to me because of the apparent cheap cost and with the MOSFET the long lasting nature of the battery (especially when tied to a blinking led).
P.S. I found that when I measured the resistance of freshly watered soil, I still got resistances of 6-8k ohms, in his original design it appears to me that it would fire when the resistance got above 1k - which didn't seem right from what I was measuring. If anyone else finds this interesting I would be greatful for an outside opinion as to the general resistances measured for your various flora. Thanks!