The article explains a 10 stage soil moisture sensor meter circuit with an integrated automatic water sprayer mechanism for restoring  the critical condition of the soil. The idea was requested by Mr. Remy.

 Technical Specifications


I come today asking for help making an automatic watering circuit to water my tomatoes for me.

I am requesting the use of a soil moisture meter(cheap eBay style) to sense the moisture of the soil.

Then the moisture value is compared to a set value from a potentiometer maybe. If level is too low then a relay is turned on for a settable amount of time. After a shower the soil is measured again.

Rise repeat.

The ability to daisy chain many together would be a great help.

For wow factor I was thinking that having a few(3) leds light up as a scale to indicate the current moisture level would work well. Thank you for your time and experience.

I have learned much from you and hope to add this to my knowledge.

Remy

Circuit Diagram



The Design


Referring to the given schematic we see a simple yet highly accurate soil moisture sensor meter with an automatic preset water shower system for restoring the soil moisture level to optimum points.

The design is based on a single voltage sensor/LED driver IC LM3914 or a LM3915. The shown sensor pins which are basically two brass rods are configured as voltage sensors across the critical soil area where it may be inserted.

The voltage across these pins depend upon the level of moisture present across that particular soil area. This sensed voltage proportionate to the soil moisture level is applied across pin5 of the IC for the required comparison with an in-built reference voltage level.

The threshold level at which the shower pump is supposed to be switched ON is set by P1.

Depending upon this setting, the IC internal circuitry senses the soil moisture and produces a shifting sequential low logic across the shown 10 outputs starting from pin1 to pin10.

This sensed output across the relevant IC outputs are indicated by 10 respective LEDs which light up in sequence in response to the rising or depleting soil moisture levels.

Selecting Bar Mode and Dot Mode


The LED illumination sequencing style could be selected to simulate a bar mode or a dot mode by appropriately positioning the pin9 switch of the IC to either ON or OFF.

The stage comprising BC547 and BC557 constitute the relay driver stage for controlling the motor pump switching as per the user preference.

The base of the PNP transistor is appropriately integrated with any of the output pins of the IC depending at what moisture threshold the user wants the motor to be started or stopped.

For example suppose pin15 determines a particular moisture threshold level of the soil and the user feels it to be the unsafe level at which the motor needs to be started in order to restore the soil moisture, then this pinout could be chosen and hooked up with the base of the BC557 transistor for the discussed motor switching.

Once the motor is switched ON the soil is showered until its moisture level is restored to the desired level and this prompts the IC to revert its sequence from pin15 to pin14 and toward pin10, switching OFF the motor and the shower.

The above process keeps repeating making sure that the soil moisture level never goes down below the undesired parched condition.

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