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The circuit is used to indicate the presence and absence of water in the tank and also the water levels. The figure shown is at the time when the tank is dry. The circuit consists of two humidity probe A, B, multi-harmonic oscillator, and a light-emitting diode LED. One oscillator is framed by 555 and R5, R6, C, etc., whose oscillation frequency f = 1.44 / (R5 +2 R6) C, gives a frequency of about 1Hz.
When the tank is not dry, then, the resistance between A, B will be smaller, BG1 gate G to the circuit, ground voltage close to 0V, so that N-channel JFET conduction BG1, BG2 corresponding cut-off, its radio most potential of 0V, 555 while the pin is low due to in a non-triggered state.
When the stores, the resistance betweenA, B increases, BG1 gate G potential due to negative pressure close to the cut-off, BG2 conduction, the corresponding increase in BG2 of the emitter potential. When the potential to rise equal to potential of 555 pin potential is high (> 0.4V) then the Oscillator Start-up by the output of the oscillation signal, drives light-emitting diode LED flash 1Hz issued to advise the watering.