This article shows you how to make a boat that turns on when placed in the water.

We all know that water is a good conductor that turns on the transistor (by supplying the current to its base terminal) in this circuit that drives the propeller with the motor to move this boat forward.

Supplies

electronic parts: Darlington pair transistor, electrical tape, wires, Darlington pair BJT NPN power transistor (I used TIP122), 1 kohm resistor - 1, 10 ohm resistor high power - 1, 100 kohm resistor - 1, heat sink, heat transfer paste, low current motor, 9 V battery, 9 V harness, heat sink, heat transfer paste, bolt, nut, washer.

mechanical parts: masking tape, packing foam or block of wood, piece of cardboard (to make the propeller), blue tack or plasticine.

tools: wire stripper, scissors.

optional tools: soldering iron, multimeter, voltmeter, for heat sink (cheap diamond drill or hammer nail, electric drill).

Step 1: Design the Circuit

D1 diode is used to stop motor discharging currents from damaging transistor and power source.

D1 and D2 diodes are needed to prevent generated currents from the motor from damaging the transistor when the power source is disconnected.

Rc resistor prevents damage to power transistors when the motor movement is impeded and the motor is effectively a short circuit.

There was an option for me to use the BJT circuit shown in this article:

https://www.instructables.com/id/Motor-Drivers/

This BJT circuit shown in the link above would allow full transistor saturation that Darlington pair transistors cannot do. However, it is my personal opinion that this full saturation is not possible because of the very low resistance of the motor and Rc resistor (I was thinking of reducing the Rc value to increase the speed of the motor).

Another circuit is shown in this article:

https://www.instructables.com/id/Motor-Drivers/

the MOSFET circuit. MOSFET is a good alternative to BJT transistors because they have high gain, high input resistance and we do not need BJT transistor linear characteristics for this circuit. However, MOSFETs cost more money and are rarer than BJT transistors. In my opinion fault analysis with MOSFETs is also more complicated because they are either ON or OFF. MOSFETs cannot be biased at the midpoint and might not turn off due to external interference. If you are going to implement this circuit with MOSFET then you must not exceed the maximum gate-source voltage.

Step 2: Build the Motor Stand and Attach the Propeller

I made the propeller from cardboard.

I used a stripped piece of high power wire and blue tack to attach the propeller.

Step 3: Connect the Motor

You can see that I attached the diode and wires to the motor without a soldering iron.

Step 4: Make the Circuit

I used an old CPU fan. I tried drilling an old in an old CPU fan with a cheap diamond drill from China. I spend half an hour with no progress. Then I inserted a hammer nail into my drill and was able to create a hole in just a few minutes.

I used the soldering iron to connect wires to the power transistor and resistors.

I used masking tape instead of electrical tape to seal the wires and prevent any short circuits that can damage the transistor or might cause the battery to explode.

Step 5: Make the Boat Attach Circuit to Boat

I will not waste your time explaining how I creating the boat from foam packaging material and masking tape. Make sure you use a bag and thus do not create a mess in your room because it is hard to clean up without a vacuum cleaner.

I attached the circuit with insulated high power wires.