This is the easiest and cheap plant watering system you can make.i didn't use any microcontroller.it is basically a transistor switch.you need to add some resistance between collector and base, to prevent transistor from getting defacted.(dont use without resistance if you do you will witness holly smoke)
Step 1: Parts You Need
1. Pcb - 1
2. Transistor - 1 (bc 547)
3. Vr - 1 (1-2m ohm)
4. Resistor - 1 (1k)
5. Relay switch - (6v)
6. Metal probes
Take a pcb and fix and solder all components according to schematic.
This is the circuit diagram.as i told yoh this is a transistor switch.
Before connecting pump make sure circuit is working properly. For that i have connected an led with battery. After testing it will replace led and battery with pump.
Now after that testing i connected a small water pump.whenever soil gets dry,the resistance between two probes increases that makes pump on.after soil gets water the resistance between two probes gets low that makes pump off.