Illuminated Buoys for use with Paula III (or other boats)

new.gif (1957 bytes)  10 Mar 00 - Buoy animation added.

A kit is available for the Paula III, which includes 2 non-illuminated buoys. After building these, I decided that it would be nice if they were illuminated. It would have been easier to have converted them from the start and the following text reflects the modifications from the start of building. All of the diagrams are grouped together at the bottom of the page.

My requirements for the design were:
        1.    Install a flashing LED.
        2.    To minimise the weight gain (more than 12g and the buoy might sink).
        3.    To use a rechargeable battery and have an easy charging facility.
        4.    Automatic on/off operation using the simplest circuit possible.

Common Features to both Designs:
        1.    To save weight the magnetic strip was cut in 1/2 along it's length.
        2.    The battery packs were to be fitted at the base of the lower tube to replace the ballast weight.
        3.    Two small holes were drilled in the top deck of the buoy and 2 way 0.1" spacing PCB header inserted.
        4.    The mercury tilt switch is installed at about 30 from the vertical. This allows storage upside down or 1 side (LED off).
        5.    The brass tube was made as short as possible. Additional weight could be saved using a plastic tube.

First Design

I decided that, if possible, to use a single cell battery to power a flashing LED as batteries are heavy. Most of the flashing LEDs will operate below 3.5V and therefore would require 3 cells. The LM3909N LED flasher i.c. was chosen as that will flash a standard LED when using a 1.5V battery. Performance was poor on a single cell and so 2 cells were used. The smallest available single rechargeable cell I could get at the time was a 35mAh NiMH cell which weigh about 2g each. To enable automatic on/off operation a miniature mercury tilt switch was to be used (<2g). The remaining components comprise the LM3909N, a 100F capacitor and the LED. These components produce a flash rate of approximately 1Hz. This first attempt worked reasonably well, but it worked better when I increased the battery to 3 cells. The brightness of the LED was disappointing so I decided to use a different design for the 2nd buoy. This buoy started out weighing 26g and the final weight was 30g.

Second Design

Having been experimenting with NE555 timers, I decided that the 2nd design should use on of these i.c's. Unfortunately the NE555 requires a minimum of 4.5V, but all was not lost as the ICM7555IPA timer (a CMOS low power equivalent) has a minimum voltage of 2V. After increasing the 1st design battery to 3 cells, which is a common computer back-up pack, I'd looked around for better batteries. I'd discovered a really neat 3 cell 15mAh NiMH PCB battery pack which is much smaller than the cells I use in the 1st design. Additionally, this pack of 3 cells weighs less than 2 of the cells I'd used in the 1st design. This was selected as the power supply.

I wanted to obtain a flash rate of 1Hz and the frequency of these timers f = 1.46/((RA + 2RB)C) Hz. To keep the weight down I wanted to use a fairly small value capacitor, and a 0.22F fitted the bill. Therefore the value of (RA + 2RB) had to be 6.6M-ohm. The LED on time would be given by 0.693((RB)C) and the off time by 0.693((RA + RB)C). If the on pulse to the LED is relatively short the LED is capable of running directly from the 3.6V supply without a series resistor. This has 2 benefits, being less weight and a brighter flash. As I wanted a short pulse I decided that RB should be about 1/10 the value of RA. Looking in my resistor draws I picked 4.7M and 470k resistors. This would produce a flash of 70ms duration every 800ms, which was near enough.

This 2nd design is far superior to it's predecessor. The LED flash is much brighter and more consistent. Even better this buoy started out weighing 25g and finished up at 24g.

Flashing Buoy - Schematic & Board Mk 1  Flashing Buoy - Schematic & Board Mk 2

Making the Illuminated Buoy - (looks long, but it includes all building instructions and is easy)

1.    In the top cover (#1) drill the 1mm holes indicated in the instructions plus 2 additional holes 0.1" (2.5mm) apart as indicated on the sketch.
2.    Make and bond the hoops (#2) to the top cover.
3.    Cut 2 pieces from the brass tube (#3), 25mm and 37mm long.
4.    Bond the bottom cover (#4) to the centre body (#5).
5.    Bond the tubes (#3s) into the top & bottom covers (#1 and #4&5) using a piece of dowel (or similar) through the tubes to keep them aligned.
6.    Solder the battery pack to some thin wire (servo lead is ideal) and insulate the connections & exposed contacts.
7.    Feed the power wires up the lower tube (#3) and epoxy the battery to the tube ensuring that the bottom of the tube is completely sealed.
8.    It is recommended that the battery be waterproofed. There are several ways to do this, but the easiest is to paint a thick coated of gloss paint all over.
9.    Check that the 3mm LED fits inside the lens (#10). If not carefully scrape the excess material away with a sharp knife.
10.   Solder leads to the LED and feed down the upper tube (#3). See sketch for LED polarity.
11.   Solder the leads to the 2 way header plug and epoxy the plug to the underside of the upper cover (#1) with the pins through the 2 additional holes drilled.
12.  Make the PCB, then solder the LED, charging point and finally the power leads to the PCB.
13.  If there is any power in the battery the unit the LED should flash when the unit is upright.
14.  Bond the top cover (#1) to the lower assembly and paint.
15.  Fit the lens over the LED and bond to the top tube.
16.  Cut the magnetic strip in half along it's length and stick in place.
       Both buoys can now be made using only 1 of the strips - use the other for to make fridge magnets or anything other job you fancy.
17.  Charge & Use. When the buoy is in the deck chocks rotate it so the LED off - when picked up the LED will flash.

Flashing Buoy - modifications required  Flashing Buoy

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