Olihou

Hi Audioguru Of course not as, any pure water getting into contact with the dirty soil will become not pure. Minerals in the soil will certainly be dissolved in water. I wonder if there is any "soil" without minerals. If there is, then it is not suitable for plants -- plants need nutritions like N, P and K. They are normally in the form of soluable minerals in soils. I would very much like to hear any encounter of situation that the circuit fails. ::) Thanks. Oli

Hi Audioguru In my place I am given to know that commercial "distilled water" is not trully fully distilled -- there are still minerals such NaCl as ions which makes water conductive. Even if it is 100% distilled and free of minerals, there are still likely minerals in the soil. So there is little chance that out circuits would not work -- unless you do use 100% distilled water AND the synthetic soil you have mentioned (What actually is it? I do not know if there is anything like this in my place?) ;D Oli

Dear Audioguru Thanks for your comments. I hope you do not mind that I have not follow your design faithfully. I have just used parts readily available. Hence the electrical performance is different from what you have built. :( However functionally I trust it is as good as yours. :) As a matter of fact, I do have R8 in the circuit. I have used 56 kohm instead of 47 kohm. I do not have time yet to do what I intend to do: to study and hence to design the probes so that the equivalent soil resistance would fit the range of the circuit best, and so that the change of brightness of LED would reflect closely the condition of soil. I will report to you and all interested in due course. There is one more thing I intend to do: following this "watering watcher" project, I would like to build an automatic "watering" device. In fact this is what I have dreamed of before the very fortunate encounter with you guys at this wonderful fourm ! :D Best regards Oli

Dear Audioguru I have obtained the following measurement results: Ohm between probes On Pulse of LED Battery Current ---------------------------- -------------------- ------------------- 47 Nil on scope 0.6 mA 100 0.3 uS 0.6 mA 1 K 1.5 uS 0.7 mA 10 K 14 uS 0.8 mA 100 K 100 uS 1.0 mA Opne circuit 600 uS 2.0 mA Note: WIth R1=560K and C1=.002 uF, the operation frequency is around 800 Hz, i.e. T=1.25 mS R3=12 Mohm, R4=100K, giving a pulse of 30 mS at 1 pulse/2 seconds My observation is that for 47 ohm and below, no visible light from LED 100, a little bit light 1 kohm, easily visible 10 kohm, brighter 100 kohm, even brighter open circuit, very bright My feeling is that if, in actual use in soil, the soil resistance falls in similar range as above, then we can tell from the brightness the degree of dryness quite confidently. However, if the range is only from 100 kohm and above, i.e. the LED is seen quite brigt to very bright all the time, then it would be a bit difficult to tell the degree of dryness. Of course this will very much depend on the separation distance of the probes, the sizes and shapes of the probes, the chemical composition of the soil ...... This should be my next steps of experimenting with this wonderful toy ! Comments are welcome ! Thanks and best regards, Oli

Dear Audioguru Thanks for your comments on the LED. Mine is really one with a beam of very narrow angle and I will look for a replacement. I think that current consumption might not be a problem for the resistance-to-flash-rate option, as we could control by design a low flash rate to reduce the average current. For example, from 1 pulse per 5 second for very wet to 2 pulse per second for very dry (this is a range of 10 to 1). Do you agree? Thanks. Oli

Dear Audioguru Thanks for your prompt reply. I will check the performance of my LED. It is one with unknown part number, just picked up from the shelf. I agree that as you said "It would be fairly easy to make it flash very slowly when the resistance is high (dry soil) and flash quickly when the resistance is low (wet soil), but that operation is backwards and it would be too complicated to reverse it.", or in academic terms, it is easy to build a "resistance-to-delay" circuit but not a "conductance-to- delay" circuit". Having said that I am still dreaming for a not too completed circuit for the latter -- am I too much a perfectionist? Nevertheless I would think that the present circuit is already good enough for the practical use. After all one's main concern is when to water the plant, and a rough idea of the soil dryness is just sufficient. Thank you. May I come back to you after further experimenting. Oli

Dear Audioguru I have succeeded in building the unit according to your design. It is working exactly as you have described. I am completely satisfied with the circuit, except that the brightness of LED does not seem to give a clear indication of the level of dryness. The LED I have used is of ultrabright type, the brightness of which changes very little when the soil resistance (simulated by a trim-pot) varies from 1 kohm to 100 kohm and to infinity. :-[ I have hence realized the beauty of the design you have proposed im May: "My new idea is to have a flashing LED indicate soil dryness. When the soil is wet then the LED is off. As the soil dries, the LED begins flashing slowly. As the soil dries even more then the flashing-rate increases. When the soil is very dry then the flashing-rate will be so fast that the LED will appear to be on steadily." Are you developing a circuit like this, or will you? :D Thanks and best regards, Oli

Dear Audioguru I have read through the conversations in one breath. The subject is of much interest to me as a electronics amateur and plant lover. You have really done a good job! I will certainly try out your circuit shortly. Thank you very much!