The light bulb of this boat turns on when the boat is placed in the water. The boat has a BJT transistor water sensor that activates the light bulb.

However, a more simpler and cheaper circuit is published here:

https://www.instructables.com/id/LED-Water-Beacon/

This sensor can be also implemented with a MOSFET transistor:

https://www.instructables.com/id/MOSFET-Touch-Lamp/

You can see my boat working here:

Step 1: Design The Circuit

The circuit is designed with a simple power NPN BJT transistor fixed bias circuit and drawn with https://easyeda.com online software:

I used an old 12 V light bulb that was made for motorcycle signalling or back lights. At the rated 12 V light bulb supply voltage, the average DC current for my light bulb was 0.1 A. This current value is dependent on light bulb design and production tolerances. At smaller supply voltages the light source current will a lot less. However, modelling the equivalent resistance of the light bulb or a bright LED is beyond the scope of this article. You can use an ammeter to check how much current your light sources is consuming.

Rc1 used for short circuit protection and might not be needed. If the light bulb current is 0.1 A then the voltage across the Rc1 resistor will be:

Rc1 = Ibulb * Rc1 = 0.1 A ** 10 ohms = 1 V

Thus the 10 ohm Rc1 resistor value could be too high and not necessary. You can try using 1 ohm instead or two 2.2 ohm resistors in parallel (to reduce the power dissipation for each resistor). The power across the Rc1 resistor will equal to:

Prc1 = Irc1 * Vrc1 = Irc1*Irc1*Rc1

Rc1 = 10 ohms: Prc1 = 0.1 A * 0.1 A * 10 ohms = 0.1 W = 100 mW

Rc1 = 1 ohms: Prc1 = 0.1 A * 0.1 A * 1 ohms = 0.01 W = 10 mW

The bright LED voltage will be 2 V. Thus the Rc2 current will equal to:

Rc2 = (Vs - Vled) / Rc2 = (9 V - 2 V) / 1000 ohms = 7 mA

A typical bright LED needs 10 mA. However, I chosen 1000 ohms if the supply voltage is raised to 18 V (if you connect two 9 V batteries in series), Iled will equal to 14 mA or 12 V, if you are using a car battery (Iled will equal to 10 mA).

We also need to calculate the minimum transistor base current need turn ON the LED or light.

Maximum collector current for light bulb:

IcMax = 100 mA

Minimum base current for light bulb:

IbMin = Ic / Beta = 100 mA / 100 = 1 mA

The maximum equivalent resistance of water will equal to:

Rw = (Vs - Vbe) / IbMin = (9 V - 0.7 V) / 1 mA

= 8.3 V / 1 mA = 8300 ohms = 8.3 kohms

Maximum collector current for a typical bright LED:

IcMax = 10 mA

Minimum base current for a typical bright LED:

IbMin = Ic / Beta = 10 mA / 100 = 0.1 mA = 100 uA

The maximum equivalent resistance of water will equal to:

Rw = (Vs - Vbe) / IbMin = (9 V - 0.7 V) / 100 uA

 = 8.3 V / 100 uA = 83000 ohms = 83 kohms

Step 2: Build The Circuit

Note: The power transistor is placed face down on the photo below. You do not need a power transistor and heat sink if you are using low current bright LEDs.

Insulate the wires with electrical tape:

The red and the white cable will be placed in the water. They are connected to the bottom of the boat.

Step 3: Attach the Circuit to Boat

You can use any packaging material that floats on water or a piece of wood.

Step 4: Drill Hole for Lights

Drill a hole for the light bulb with scissors.

Step 5: Attach the Cabin

Attach boat parts with ropes and rubber band.

Step 6: Testing

I attached the 9 V battery to boat with a rubber band because my boat capsized a few times due to loose battery (the heaviest part of the boat), thus affecting the boats centre of mass.

I tried placing the boat in hot water:

I also tried increasing the Rc resistor value to 10 ohms:

Try this yourself.