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ELECTRONICS: PLEASE HELP - Plant moisture sensor not working - or is it me??? Answered


Can anybody PLEASE help, I am in the UK and eager to grow melons next spring. I need to know when the roots of the tub need water so decided to make a moisture sensor. I found a ‘plan’ online, bought about £20 of components (I need to make  two sensors) and have spent all afternoon TRYING to get the online schematic to work. One attempt was connected via a circuit board and the other in air connections. Neither attempt works, the LED comes on on both attempts but does not go out when the terminal probes are put in water, as they would when the plant roots are moist.

i enclose the original internet schematic and a (poor) picture of my ‘‘air connection’ version....should this plan work? And if so, why isn’t it?

thanks in anticipation

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Jonathanrjpereira (author)2017-12-21

Calibrating your circuit to sense the correct moisture levels be a pain. You will need to select the correct resistor based on the threshold level of moisture which you will have to decide upon.

Instead, buy a soil moisture sensor and an Arduino Nano. You will be able to add several other sensors which can measure temperature and humidity too. Arduino is a great platform to start learning about microcontrollers and electronics. You will also find several online tutorials for your project. Instructables has Arduino and Electronics Classes which will help you get started.

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dhaprt (author)2017-12-19

This circuit will not work as is. Here is how to do it. Get rid of the 220k and replace it with 2.2k. Connect the LED directly from the battery + to the collector. Connect the sensor directly from the 2.2k to the base. Do not make any other connection to the base. Connect 150 ohms from the emitter to battery negative. Make sure your LED is connected with polarity correct. Test the circuit by touching across the sensor with wet fingers. The 150 ohms controls the brightness of the LED by limiting the current through it. You can raise or lower the 150 ohms to change the brightness, but don't lower it too much or you will fry the LED.

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Downunder35m (author)2017-11-25

If you want to use such a simple sensor you might as well use your finger - or get a self watering pot until they go in the wild.

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TN1946 (author)2017-11-25

Here are the pictures - internet plan and air construction -

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