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

T_minutes  = 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.

PARTS LIST
Resistors (all ¼-watt, ± 5% Carbon)

R₁ = 1 KΩ

VR₁, VR₂ = 1 MΩ

Capacitors

C₁ = 470 µF/16V

C₂ = 0.01 µF

Semiconductors

IC₁ = NE555

T₁ = SL100

D₁ = 1N4001

Miscellaneous

RL₁ = 12V, 100Ω Relay