The figure below shows a schematic of a simple automatic plant watering / irrigation control circuit for automatically watering your plants in lawn, garden, farm etc. We often cannot arrange time for watering our plants due to which the plants become weak and dry. For proper care we have to water them whenever the soil gets dry
or whenever the soil have not enough moisture contents.
The circuit shown here will work as a plant watering watcher or dry soil detector. The two probes shown in the diagram will detect the absence of moisture content in the soil and switch ON the relay switch at the preset value of dryness. You can connect any AC or DC pump or sprinkler system with the relay to switch ON automatically to water the plants.
Working of the circuit is simple, the circuit is using two transistors, which are working as switches here. When the probes will not detect moisture in the soil then the resistance between the two probes will increase due to which the transistor Q1 will become switch ON. When the Q1 will switch ON it will provide the required voltage at the base of the transistor Q2 making it switch ON too and hence the relay will become activated.
The 50K variable resistor is used to adjust the circuit to switch on the relay on the required level of dryness. The circuit can be operated with 6, 9 or 12 volt DC. It is important to use the same voltage relay when operating with 6 volts. A 12 volt relay will not work on 6 volts. For transistors Q1 and Q2, you can also use other similar transistors like 2N3904 or 2N2222 etc.
For probes you can use any wire, steel nail or similar object. But make sure it is of steel because the iron things will become rusted in the moisture and the circuit will not work properly. For best performance put the probes few inches under the soil and make few feets distance between them or experiment with other procedures and use the one that works best.