I would like to build a water sensor power switch. A device which completes the circuit of an outlet adapter when the water sensor has detected water.

I have a very simple idea about this little project. I will modify this plug in mechanical timer(PIMT).

photo of mechanical timer

photo of water sensor

I will disable the timer within the PIMT, and replace the timer with the water sensor to complete the circuit. Do you guys see any flaws on this modification? (sorry can't find the circuit diagram for the PIMT)

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    \$\begingroup\$ It's hard to know without more details on the components, but I'd guess the mechanical timer switches the mains directly using mechnical contacts and the water sensor probably isn't rated / safe for use at mains voltages. \$\endgroup\$
    – PeterJ
    Mar 18, 2014 at 9:10
  • \$\begingroup\$ Hi Peter Thank you for you reply. Do you think i should make one by myself instead? like using a relay to connect to the main voltage? \$\endgroup\$ Mar 18, 2014 at 9:21
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    \$\begingroup\$ These kind water level float switch (at least the ones I see all over ebay) are rated for 220v mains but for just 0.5A (and highish resistance) so it's not a good idea to be used directly with a load but can drive a relay or other control circuit. Your timer seems to me a mechanic one (the output switch is controlled mechanically rather than electrically) so I'm not sure how you intend to use the sensor signal to activate it. \$\endgroup\$
    – alexan_e
    Mar 18, 2014 at 9:25
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    \$\begingroup\$ @mathwannabe What you want can be done with a relay and any small wall wart psu. The float switch will control the supply of the relay and the motor will run on/off. I'm not sure what to make of your last comment, does it mean that you don't want to make your own switch? If not then maybe you should look at ebay to find a ready one that does what you want. \$\endgroup\$
    – alexan_e
    Mar 18, 2014 at 11:13
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    \$\begingroup\$ What has the timer got to do with anything? State what you want to achieve (and if it involves a timer functionally then fine) but don't cloud your question with stuff that is irrelevant to your actual desired solution. What's your budget? How much hysteresis can you tolerate in water level differences? \$\endgroup\$
    – Andy aka
    Mar 18, 2014 at 11:14

2 Answers 2


The simplest way, from a do-it-yourself perspective of someone that has to ask for circuit advice here, to detect water is a float switch. These will be available in most hardware stores around where they sell sump pumps. Complete sump pumps come with a float switch, but they are often available by themselves as replacement parts.

To do this electronically is more complicated. I've had to detect water water before, and a good method is to have two sensors connected to pins of a microcontroller. However, you don't just check for resistance between the two electrodes. There are several issues with two electrodes you have to consider:

  1. Electrolisys. You want to avoid passing any large current and any net DC over time.

  2. Corrosion. This changes the resistance, can cause assymetry, and a battery effect.

  3. Common mode noise.

The way I deal with these issues is to take four separate measurements for each over all reading that is intended to indicate the presence of water. Each electrode is driven strongly high and low. At the same time the other is driven weakly the opposite direction while its voltage is measured. This comes out to a total of 4 measurements that result in no net DC voltage. Do the math and you will see this also cancels out common mode noise, bias caused by the battery effect, and assymetric resistance caused by corrosion.

One set of readings only takes a few 10s or 100s of µs to make. If you only need to know the presence of water every 1 second, for example, then the electrode can be off most of the time. This saves power and reduces corrosion due to electrolosys.

The microcontroller then decides whether water is present or not, and drives a relay accordingly.

  • \$\begingroup\$ Thank you Olin.Yes i know about sump pump.However, sump pump is not a great match for my project. I will follow your guide and do some experiments on it. =) \$\endgroup\$ Mar 19, 2014 at 9:04
  • \$\begingroup\$ @mathw: Then you should explain what your project really is. It's rather annoying to the volunteers here to write answers only to find parts aren't applicable because you gave incomplete information. You also may get solutions you haven't even considered. Present the real problem, not your imagined solution to a piece of it. \$\endgroup\$ Mar 19, 2014 at 12:41
  • \$\begingroup\$ I am sorry for being so ambiguous about my problem. But i have found a product that solves my problem. plumbingsupply.com/electronic-utility-pump-switch.html \$\endgroup\$ Mar 20, 2014 at 12:18
  • \$\begingroup\$ The four electrodes are for a Kelvin connection type setup to avoid the voltage sensing electrodes ever passing a high current to damage them? \$\endgroup\$ Mar 31 at 7:26

Here's a serious problem: those mechanical timers are based on mechanical switches. You can know this by the loud "thunk" they make when they reach the set time and turn the load on or off. Your water sensor has no way to interface to this mechanical device.

A more subtle problem is that you seem to have very little understanding of electricity, but you are playing with a device that can easily start fires or even injure or kill you or other people. Moreover, you are mixing it with water, which is a very likely way for someone or something to accidentally come in contact with the hazards of your device. Most building codes have special requirements for electrical circuits even near water (bathrooms, kitchens) and even more stringent requirements for devices in water. From your description, you seem to be ignorant of all such regulations, which exist to make such devices safe. I'd advise an abundance of caution.

If you want a safer way to accomplish something like this, I'd suggest investigating generic remote-controllable outlets, such as z-wave outlets sold for home automation. Then your water sensor can control this outlet, via the home automation network, without need to interface with mains voltage. Of course, you might as well buy a z-wave water sensor, and then you don't really need to make anything.

  • \$\begingroup\$ Thank you for suggesting the z-wave water sensor Phil. Yes, i am ignorant and that is why i am here to learn =) \$\endgroup\$ Mar 19, 2014 at 9:04

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