If you are one of those people who would like to spend their well earned summer vacations in the comforts of a hill resort but worried about your potted companions back home, then a plant tender is what you need.
As the soil dries up, its resistance increases causing the voltage at pin 2 of IC555 to fall below 1/3 Vcc. This activates the trigger comparator and the output goes high, switching on the pump. The output remains high for the time interval set by VR1 and is given by
Tminutes = 1.1 R.C/60*106 where R is in ohms ans C is in microfarad
The pump switches on again only soil dries up.
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 1 KΩ
VR1, VR2 = 1 MΩ
C1 = 470 µF/16V
C2 = 0.01 µF
IC1 = NE555
T1 = SL100
D1 = 1N4001
RL1 = 12V, 100Ω Relay