AIS Transmitter

All ships; and many boats; are fitted with an Automatic Identification System (AIS) which consists of a Transmitter and Receiver.  The Transmitter broadcasts data about the vessel and the Receiver obviously receives and displays the data from other vessels.  The data consists of important information such as the vessels course, speed, location, size, name, etc.

Receiver are relatively cheap to make. I did this with the boat chart plotter I recently made for our eldest grandson's yacht.  Transmitters are considerably more expensive to purchase!

Knowing the position, direction and speed of vessels near you has obvious safety advantages.  If you don't have a Transmitter then you are aware of your own vessel's position and can take any necessary action  to avoid a potential collision.  Receiving the other vessels name also allows you to contact it by radio and make them aware of you own location and intentions. 

However there are obvious advantages if you can automatically transmit your vessel's data.

I started thinking about how to make of a cheap AIS Transmitter.  I stumbled upon a boating video where the crew had mention whilst sailing at night they had received numerous AIS signals; yet they couldn't see any vessels.  At dawn they realised the transmissions were coming from fishing net locator buoys set by local fishermen.

This led me to research digital net locators. Specifically, their cost and whether I could modify one for use on a yacht as an AIS Transmitter.

I bought one on Aliexpress for a fraction of the cost of a commercial AIS Transmitter

It's waterproof.  Under the clear plastic end cap is an ON/OFF switch and two sockets.  One socket is a 9.2V charging port and the other a data socket.

When the device is turned on different coloured LEDs illuminate to describe the status of the device

   

The "insides" consists of a printed circuit board, two 18650 lithium batteries and an antenna

The device requires 9.2V to charge the batteries but only 7.4V to power it (2x 18650 batteries @ 3.7V ea).

The higher the device is mounted; the greater the range of the transmissions.  At sea level the manufacturer claims a range of 10km.  I'd take that with a pinch of salt.  The manufacturer also claims the device will transmit once every three minutes for 15 days before the batteries go flat.  I would want the device to transmit more frequently, which means battery life will be significantly shorter.  The stated recharge time is six hours.  If I reconfigure the device to transmit every 30 seconds then the batteries will probably last 48 hours.

I wouldn't want to be sailing for two days before having to stop the device and recharge it for six hours.  It would be better if the device was permanently connected to a power supply.

The obvious solution is to mount the device as high as possible on the mast, thus greatly increasing the range of the transmissions.  This would require a power supply up the mast.  The yacht has a 12V battery bank which mans I would need to convert 12V to 9.2V.  However I also need to do something about the switch on the device.  It only needs to be turned on when the yacht is at sea.  Climbing the mast to turn it one or off isn't satisfactory.  Activating the transmitter remotely is the solution.  Fitting a switch into the 12V supply to the transmitter isn't a solution as the transmitter batteries would continue to power the device if the 12V supply was turned off.  One option could be to have two wires for the 12V supply and another two for the remote switch.  However I prefer another option.  This is to remove the batteries in the transmitter.  Disconnecting the 12V power supply would turn off the transmitter.

To achieve this will require a 12V to 7.4V step down converter.  Fitting it where the current device batteries are seems a good option.  

The next step is to source a cheap 12V to 7.4V step down converter.