I am attempting to build an automatic plant watering device. I am using a preexisting circuit built around four #4093 Quad 2 input Schmitt NAND Gate IC's. This circuit generates a 2kHz pulse for 0.1 sec. that is intended to light an LED. I am replacing the LED with a solenoid that will drive a valve connected to a reservoir of water. I need to turn turn the 2kHz pulse into a single pulse so that the solenoid can be actuated.

 

Simplify. Put a diode on the output of IC1B to put the negative pulses into a capacitor and use the voltage on the cap to drive R6.

I am assuming the solenoid can not switch on in a tenth of a second, and you would want the water to run for more than .1 second. If you use this modification, whenever "dry" happens, IC1B will start producing negative pulses at 2 Khz. As long as IC1B is flip-flopping, the solenoid will have power, and that will persist until "wet" happens plus a few tenths of a second as R6 drains the capacitor.

 

I'm sorry I haven't gotten back to this post as I've been in the hospital. So what you are saying is to get rid of IC1C and D altogether? Should I maybe use a 0.01uF cap to drive R6 (I'm not sure my math is right)?