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When using a soil moisture sensor in a plant to measure water indoor, how can we avoid dangerous water and electrical interaction.

  1. In general, when there's water in the plant
  2. When watering the plant, and accidentally dropping water on the wire connection

This is the sensor of interest so far (open for other recommendation). It seems kinda odd to be powered on 35mA!

http://www.robotmesh.com/soil-moisture-sensor?gclid=CMvW6bSz9bcCFWJqMgod9hYAfA

What I have in mind is to add a 50mA fuse between the power supply and the power input to the sensor itself

Power Source: Possibly 4 AA batteries

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  • \$\begingroup\$ Dangerous water and electrical interaction - what danger do you see? Like any modern household appliance, the danger is "removed" by making sure what reaches the outside world (soil, pets, fish, hands etc.) is safe by limiting the voltage and current by recognized proven methods. I've got a koi pond and to keep blanket weed under control (supposedly) we use a device that drives current into the water through a copper electrode - this is perfectly safe for the fish but not so safe for the spyrogyra algea. It's all relative. \$\endgroup\$ – Andy aka Jun 21 '13 at 17:42
  • \$\begingroup\$ @Andyaka Bottom line I want to avoid any explosion or fire indoor. \$\endgroup\$ – Iancovici Jun 21 '13 at 17:44
  • \$\begingroup\$ How are you going to wire it up? What power supply are you going to use? These are the issues that relate to burning your house. The sensor could be immersed in water or dipped into petrol and would not cause a problem. The ignition temperature of petrol is over 250ºC. Hydrogen, which is the most volatile of "fuels" on ignition energy needs optimally 19uJ and that is equivalent to 1A flowing through a 38.5uH inductor then going open circuit. Check your power supply not the sensor. Anyway spilling water on it makes it safer. \$\endgroup\$ – Andy aka Jun 21 '13 at 18:02
  • \$\begingroup\$ @Andyaka edited \$\endgroup\$ – Iancovici Jun 21 '13 at 18:08
  • \$\begingroup\$ And what are you going to use to monitor the current flow or terminal voltage? \$\endgroup\$ – Andy aka Jun 21 '13 at 18:42
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As discussed in comments, providing that the power source is capable of low amounts of power and there are no components that are likely to store energy (that can generate a spark) there will be no ignition or surface temperature side-effects that could ignite even the most volatile of gases such as hydrogen (19uJ) to ignite in the perfect gas-oxygen mix.

The cable feeding the sensor will have inductance but it is likely to be less than a ten microhenries BUT what is the chance that hydogen will be around? It would need a minimum of 40uH with 1A flowing from the battery pack to be potentially unsafe. This circuit would then need to be broken to produce a spark. No chance. If hydrogen is present then what sort of plants are you cultivating?

You might have some petrol around but you'd need a lot bigger energy spark than for hydrogen (the most volatile of the gases to sparks) but you could ignite petrol with a surface temperature of 250ºC however, this is unlikely to be produced by the sensor circuit and the 4AA batteries.

If you can't ignite the most volatile of gases and fluids with energy or hot surfaces you won't ignite anything known to man.

If you don't believe look up Intrinsic Safety techniques and look out for iginition temperatures of the most volatile gases and also check for hot surface temperatures.

Here is a simple site that should convey the basic principles

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A fuse never hurts! But if you are also interested in automating the watering process what I did is ran tubing with holes drilled through it underneath the soil and attached that to a gallon jug I could fill whenever it got low. The same controller that can tell you when the soil is dry can open a valve on the gallon jug to let water flow into the tube, and into your soil while never worrying about water shorting out your electronics.

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