Componants :
- R1,R4________470K 1/4W Resistors
- R2____________47K 1/2W Trimmer Cermet or Carbon
- R3___________100K 1/4W Resistor
- R5_____________3K3 1/4W Resistor
- R6____________15K 1/4W Resistor
- R7___________100R 1/4W Resistor
- C1_____________1nF 63V Polyester Capacitor
- C2___________330nF 63V Polyester Capacitor
- C3,C4_________10µF 25V Electrolytic Capacitors
- D1__________1N4148 75V 150mA Diode
- D2_____________5mm. Red LED
- IC1___________4093 Quad 2 input Schmitt NAND Gate IC
- Q1___________BC557 45V 100mA PNP Transistor
- P1,P2_______Probes (See Notes)
- B1______________3V Battery (2xAA, N or AAA 1.5V Cells in series)
Device purpose
This circuit i.e. water watching in flower-pot or plant watering project is intended to some signal when the plant needs water. Water watching circuit will glow a LED when the ground of any plant (or flower-pot) becomes too dry and when there is some conductivity between probes which are inserted in that ground the LED will automatically turn off. Adjusting R2 will allow the user to adapt the sensitivity of the circuit for different grounds, pots, and probe types.
Circuit Operation
The circuit operation of water watching indicator can be explained as follows: IC1A and related components R1 and C1 form a 2 KHz square wave oscillator feeding one gate input of IC1B through the voltage divider R2/R3 made variable by adjusting the Trimmer R2. If the resistance across the probes is low (as when there is a sufficient amount of water into the pot) C2 diverts the square wave to ground, IC1B is blocked and its output will go steady height. IC1C inverts the high status to low, thus keeping IC1D blocked: the LED is off.
When the ground in the flower-pot is becoming too dry the resistance across the probes will increase (and their will be no conductivity between two ends of probes) and C2 will be no longer able to divert the square wave to ground. Therefore, IC1B output begins to transfer the 2 kHz signal to IC1C which, in turn, passes it to the oscillator built around IC1D. No longer disabled by a low level on its input, the IC1D oscillator slowly pulses Q1 base low causing the LED to flash, signaling the necessity to water the plant. The short low pulse driving the base of Q1 is actually a burst of 2kHz pulses and therefore the LED flickers about 2,000 times per second – appearing to the human eye as if the LED was steadily on for the entire duration of the pulse.
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