SOIL MOISTURE & HUMIDITY SENSOR - IRRIGATION AUTOMATION

Circuit description:

The soil humidity is measured with the sensor according to the principle of electric resistance. The more humid the soil, the lower the measured resistance value. The reaction point (i.e. when the relay switches the pump on) is adjusted with the trimming potentiometer.

Use as directed:

Control of the soil humidity in gardens, fields and grasslands. Switching on of a spray-irrigation system when the soil becomes too dry.

Assembly instructions:

Assemble the device in a dry and weatherproof place. Put the humidity sensor with the metallic heads into the soil (the metallic heads must be put completely into the soil, the black handle juts out of the grass). You have to measure at the spot, which is also reached by the lawn spray-irrigation system. So the sensor measures in a depth of approx. 30...40 mm. If you have a field and it is important to you that the soil will be kept humid in deeper layers, then you may also bury the complete sensor including the handle deeper. The measurement then takes place at the spot where the metallic feeler pins of the sensor are. The cable towards the sensor may be prolonged up to 20 m (normal bipolar cable). The basic device requires an operating voltage of 12 V stabilized direct current voltage. Commercial stabilized plug power supplies 12 V, which have a 3.5 mm jack plug as output and where the positive pole is to the fore of the point of the jack plug may be employed. Most of the power supply units available in specialized trade include multi-plugs and the polarity is switchable. The jack plug is inserted into the jack of the basic device. An LED at the device lights up. If you want to water your garden only at certain times of the day or certain weekdays, you will need a timer. Such a timer to be inserted between the socket and plug power supply is available as daily or weekly timer in a specialist shop. Then this timer has to be programmed in such a manner that the clock only switches on at the times desired by you and then sets the soil humidity sensor into operation via the connected plug power supply. The soil humidity sensor switches on the connected load (pump, magnetic valve) via a relay. This relay contact in the module has been approved by us for voltages up to max. 25 V (AC or DC), switching current max. 3 A. The relay itself may also switch voltages up to 230 V AC 3A. But for this purpose additional safety precautions such as protection against accidental contact, interposed fuses, etc. are required. If you want to switch voltages > 25 V, this should be done by an approved electrician who will also adhere to the safety regulations of the VDE (Association of German Electricians) and will install the module safely in accordance with the valid legal safety regulations.

After switching-on it is measured whether the soil has to be irrigated. If yes, the built-in relay turns on the connected pump for about 18...30 minutes. If no, the module waits for about 18...30 minutes and then measures the soil humidity again. This continues as infinite loop until the operating voltage is switched off. If the small sliding switch is slid downwards, the time delay of approx. 18...30 minutes is switched off and the device measures the soil humidity continuously and turns on the irrigation pump immediately and without time delay in case of need. You should, however, use this adjustment for your first tests only. The sensitivity is adjusted with the trimming potentiometer (i.e. as from which soil humidity the irrigation shall start), to the left: low soil humidity, to the right: high soil humidity.