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Anchor Alarm - The Sequel
A Stand-Alone Portable GPS-Based Anchor Alarm Refined

by Rick
February 23, 2009

Step-by-Step:

Introduction

After using our prototype anchor alarm (see A Stand-Alone Portable GPS-Based Anchor Alarm) during our third season cruising on Sea Gator, we had come to rely on the added insurance it provided on blustery nights. In the past, we used sighted landmarks to assess our position but landmarks offer little help at night. It has been very comforting to glance at the anchor alarm and immediately see that we are still within a fixed and expected distance of our anchor's position.

Anchor Alarm The anchor alarm has also helped while setting our anchor. We still use landmarks to see if we are moving as we back down on our anchor but we can also glance at the screen's calculated distance to verify. We even use the anchor alarm while tied to a mooring ball because, well, we have it and we'd like to know sooner rather than later if we somehow come loose in a crowded mooring field.

The prototype's success raised two important questions:

I don't yet know.

To answer these questions, I decided to build a small number of anchor alarms for others to use and test. I expected to learn a great deal in the process. But before I began, I had improvements in mind.

Room for Improvement

One of the goals of any prototype is to provide a proof of concept. Will this thing work? As advertised? I was pleased that our anchor alarm prototype worked well but I saw room for improvement.

Controls Simple ergonomics identified the first essential improvement. Even at its lowest setting the LCD backlight is bright enough to disrupt sleep so I would always turn off the backlight before retiring for the night. But the backlight button is adjacent to the alarm button so I risked inadvertently disabling the alarm each time I adjusted the backlight. My solution was to add a dim red LED that is lit when the alarm is activated. The LED's dimness does not interfere with sleep and a quick glance in the dark lets me know the alarm is active.

Another practical application revealed a second important improvement. In one anchorage with particularly poor holding, it took several attempts to successfully set our anchor. This episode lasted nearly an hour all while the anchor alarm was running on its internal battery. The internal battery's capacity was exhausted before we could mark the anchor's final position. Later, I realized the LED backlight was the culprit. With the backlight off, which is really not needed outdoors during daylight, I calculated the anchor alarm should run for over an hour on a fully charged internal battery. But the backlight - although we couldn't discern it - was on, so here it only lasted a little more than a half hour.

The above episode led to several battery management improvements:

First, I updated the firmware to always set the backlight off after the unit is first powered on. Next, I added the necessary hardware and firmware to detect whether the device was powered by either an external power source or the internal battery. When the external power source is removed, which is the case when the alarm is brought forward during anchoring, the device now turns the backlight off. This should help avoid unnecessarily draining the internal rechargeable battery. Finally, I updated the firmware to display "BATT" on the LCD and to issue an audible beep when the external power is lost. Using the LCD brightness button while running on the internal battery also issues an audible beep warning.

To China and Back

For time's sake I built the prototype's circuit board using stripboard. For the new batch of anchor alarms I wanted a professionally fabricated custom circuit board.

For my water level gauge project I had designed and built my first printed circuit board (PCB) using the EAGLE Layout Editor and the do-it-yourself toner transfer method. For this project I wanted a professionally manufactured PCB. I knew that there were many PCB manufacturers who will produce the boards given my design so I set out to design my final circuit using Eagle.

The first hurdle I encountered was that the board area for Eagle's freeware Light edition is limited to 100 x 80 mm. For the through-hole components I can easily solder, I need more board real estate for the required components. I could either spend heavily for an Eagle version without the small board size restriction or I could split my design into two boards.

Schematic Aha! The Radio Shack project box I used for the prototype already has PCB slots to hold multiple boards so I chose the latter solution (I'm not quite ready to add more expensive software to my toolbox). Using two boards would also give me a more modular design. Should I ever need to send a user a hardware update, it might be easier if I split the power supply circuit from the microcontroller circuit.

I divided my circuit into separate circuits for the power supply and the microcontroller (MCU). Then I used Eagle's board editor to transfer the circuit to boards sized to slide into the project box's PCB slots. This worked well with only four wires needed to connect the two boards. I used polarized molex connectors for all external board connections and for those to connect the two boards.

Next I began researching the available PCB manufacturers. I learned that most manufacturers require gerber files, which is a format used by the PCB manufacturing machines. A gerber file includes information used to layout all electrical traces and connections along with information to drill and mill the completed board.

Boards can be manufactured with affordable unit costs when ordered in large quantities. However, boards for prototyping or small batch quantities can be costly. I scoured the online discussion forums for recommendations and I settled on BatchPCB, which is a unit of Sparkfun Electronics a favorite electronics supplier.

BatchPCB charges a fixed price per square inch of board area. So I followed their online tutorial to create the required files from Eagle then I uploaded them for design rule validation. The process is completely automated and their software emailed me a reply with a good description of the errors found. I had to modify text on the silkscreen layer and resubmit. Another automated reply indicated my design passed their validation and included log and image files of the board. I could now use their online order processing to request a quantity and complete the order.

Printed circuit board BatchPCB uses a PCB manufacturer in China. In a few days my small board designs were added to larger panels with other boards and the panel design was electronically transmitted to the manufacturer in China. BatchPCB provides online order status so I could track their progress. When BatchPCB receives the finished PCB from the manufacturer in China they separate and ship the individual orders.

3-4 weeks after submitting my design and order I received the finished boards. When they arrived in Wyoming, I stared in amazement at the finished product. They were my first professionally manufactured boards and they had traveled to Wyoming all the way from China.

Some Assembly Required

While the anchor alarm's two PCBs were being manufactured in China, I created a complete bill of materials for all the board components and everything else necessary to build the anchor alarm. I ordered bare PCBs so it would be my job to install and solder each resistor, capacitor, diode, transistor, connector, etc. There are electronics manufacturers that will populate the boards with all the required components. But that is a job only practical in large quantity orders.

Selecting and ordering each required component was a time-consuming task. I began to appreciate what is involved in the manufacture of consumer electronics. Surely, there are full-time jobs devoted to the sourcing and procurement of the necessary components and assemblies. Digikey's on-line catalog was a great help in selecting the required components.

After the PCBs, project boxes, hardware, and components arrived I set to work building multiple anchor alarms. My electronics bench resembled a small assembly line. First, I soldered all of the components on the PCBs, added the MCU and tested each board.

I created jigs to mark all cutouts and holes in the project box and box lid. With the boxes and lids prepared, I installed the LCDs, GPS engine boards, antenna connectors, switches, power connectors, battery holders, and handles. Far more time than I anticipated was required to build each anchor alarm.

Creating a single prototype was easy. However, assembling multiple devices revealed much inefficiency. Proper electronic manufacturing design could greatly simplify the manufacture and assembly of the anchor alarm. If this project goes much further I'll probably seek help for that effort.

We Don't Need No Stinking Manual

Users Guide Many software developers loathe wasting valuable time writing a user's manual or contributing to the process. In that spirit, I rationalized that a two or three-page instructional insert would suffice. Alas, in the real world documentation is a necessity and, to be useful, documentation must be thorough.

With inquiries coming from Australia to the Netherlands I needed a proper anchor alarm user's guide. I shed the stereotype and wrote the necessary documentation. The Portable Anchor Alarm User's Guide (PDF 400 KB) has detailed operating instructions and specifications.

Testers to the Rescue

Pat and I have been using the new and improved anchor alarm for three months now and it continues to operate well. We can't imagine anchoring without its reassurance. But I built the latest batch of anchor alarms with others in mind.

I've given a couple of units to boating friends and sold a few units at cost, all with the expectation that comments and feedback from other boaters will result in further improvements.

Check back later for further updates and use the email link on the Contact Us page to send me your comments and questions.

Anchor Alarm - Available for Purchase

In response to popular demand, a limited number of anchor alarms are now available for purchase at Rick's cost for materials + users' feedback and comments. See Anchor Alarm - Purchase Information for details.

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Ship's Manifest

Anchor Alarm Now Available for Purchase

In response to popular demand a limited number of Rick's portable GPS-based anchor alarms are now available.
See Anchor Alarm - Purchase Information for details.