I recently bought a really cheap remote control boat. It only turns on when it is placed in water. I would like to know how it determines if it is placed in water. I could not find any sensors that detected fluid in the boat. How does a device know if it is in water and should turn on? How would it go about it?

The boat has 2 motors, each has 3 wires going to it. There is positive and negative terminals connected to the motors, and a third wire soldered directly to chassis of the engine.

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    \$\begingroup\$ Any pictures? Otherwise, we can entertain ourselves with guessing all day. \$\endgroup\$ – Nick Alexeev Jul 8 '15 at 18:37
  • \$\begingroup\$ @NickAlexeev I disassembled the boat and I found that there are 3 wires connected to the motors. I added that to the question. \$\endgroup\$ – HSchmale Jul 8 '15 at 18:42
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    \$\begingroup\$ If it's cheap, chances are there are two metal plates on the chassis somewhere that are designed to sit in the water. The water (assuming it's not purified/distilled) will conduct enough current to switch a transistor and allow it to turn on. That would be my guess. Without pictures of the boat or any of its information, I don't see how anyone can help you further \$\endgroup\$ – DerStrom8 Jul 8 '15 at 18:46
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    \$\begingroup\$ It may be using the motor shafts as the electrodes in the water and connecting to them through the sintered brass bearings typical of such little motors. \$\endgroup\$ – KalleMP Jul 8 '15 at 20:16

Water conduct the electrodes. Out of safety consideration, two electrode are usually placed on the bottom and near the propeller accordingly, to ensure motor turns on only when the boat is in the water. There may have two additional electrode, connected in parallel, on the top of the boat to deal with the case when boat is occasionally up side down.



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  • \$\begingroup\$ Both links seem to be defunct. \$\endgroup\$ – Nick Alexeev Jan 29 '16 at 6:50

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