This page describes how I construct my backup transmitters. It is meant to be understood by persons with at least a little technical electronics knowledge. This is how I do it.....I am sure there are better, faster methods, but I don't have access to any but the basic tools, etc. There is a surface mount version using the same methods and values that performs the same.
This transmitter was designed to be used as a back-up to a main transmitter, or to attach to a newly found turtle quickly in the field, to give time to aquire another transmitter without loosing the turtle in the meantime, and without taking the turtle from the location found.
It was designed to fit inside the same plastic flip-top container used for thread-trailing, and since the plastic container is not removed when thread-trailing a turtle and is put on hold for a while, a spare transmitter on-board is a good idea. It only weighs about 6 grams complete with battery. The battery is changed every 60 days or so, and would probably last another 30 or 60 days if need be.
This page is not meant to be a step-by-step kit construction, but they DO work first-time every-time for me, and it is a simple circuit using off-the-shelf components. The board and foil pattern can be changed to almost any shape and size.
(A surface mount version would take up a fraction of the board space, maybe making room for a second battery, a 3 volt version.
The board is cheap single sided copper foil board.....any thickness will do. I cut the disks out with a hole saw on a drill press after removing the pilot bit first.
I have a drilled master disk I use as a template for all hole drilling. This is it. The holes are drilled first then the foil is etched, when using this method. The holes guide the etching pattern drawn on the foil side.
Neither the hole placement or the foil pattern is critical, as long as they match.
Freehand the pattern with a sharpie pen, then use finger nail polish to cover the areas to keep. Use ferric chloride etchant or a n acid mix to etch the boards. Wait, Inspect. Clean everything with solvent and polish the copper....tin if you want.
Mount the (through-hole axial) components as per schematic and photos. The wire that retains the battery is a piece of a paper clip. The battery pad was a small piece of brass salvaged from something, cut, shaped and soldered on the foil side.
I have a lot of 3.3 meg resistors so use 2 in parallel as photos show. One 1.6 meg resistor also will do. Try slightly different values for different beep rates.
Probably any UHF or VHF NPN RF transistor will work. This is what I use.
Nearly any small crystal package can be used. Just make sure it is placed so it fits into the container with the lid closed and snapped. Most of the time I solder the crystal on temporarily and test everything, then mount it in it's final position.
Cylinder types work well. These photos are what they look like after most of the assembly is done.
The battery I use is from Ebay and cost $5 or less for 50pcs shipping included, on Ebay. (that's 10 cents each).
FYI: The steel battery cases are not rust-proof. These are cheaply made batteries, but 8 or 10 cents for 2 to 3 months of use is pretty good.
Antennas are made from stainless steel multi-strand, plastic coated, 80 or 100 pound fishing leader. The lighter versions seem to invite chewing by small mammals. Tiny brass or copper crimps, called sleeves, are included in a package. They can be soldered.
The crimps need to be cut into thirds, and crimped SOLIDLY onto an exposed end of the steel strand. Stainless steel does not solder, so don't try, and ruin a transistor by overheating the board.
6 to 8 Inch antenna lengths works O.K. Trying to tune the antenna of a micro-watt transmitter sitting 2 to 4 inches off the ground is nearly impossible. The signal changes from horizontally polarized to vertically polarized depending on range, so remember to try all antenna orientations on the receiver when tracking..
T1=MPSH10 transistor (others may work)
L1=100 Nh (0.1 uh)
C1=0.47 uf Tant.
C2=.1 to .22 Uf
R1=1.65 Meg 1/8w
Q=crystal 3rd overtone
Nearly everthing I used to build these was bought on Ebay or at Digikey. There is no advantage to using any but the smallest, lowest voltage/wattage components. I try to use tight tolerance components (i.e., 1% 1/8 watt resistors). They are usually no more expensive and give some piece of mind when field temperatures are in the extremes.
After final assembly and testing, thoroughly coat everything on the board except the battery contacts with fingernail polish.....both sides. Clear, five minute epoxy can then be used to embed the entire board if desired, except where the battery sits of course. I no longer embed the boards: the lacquer do a good job of water proofing.
The 5ml plastic sample containers I use for thread-trailing have a small hole drilled in them for the thread to run out of. This hole is used for the antenna also. It needs to be TINY to keep the rain out. It is a pain getting the thread through a tiny hole, but more of a pain to deal with a wet transmitter. (hint: carry a needle threader when thread trailing)
I inspect and dry the battery and remove moisture inside the plastic container on every turtle location. (This goes for thread trailing also)
Approximate size of the brass battery contact.
A strip of nickel battery tab salvaged from old nicad battery packs will work, too, and doesn't corrode as fast as the brass.
Anyone who has test equipment or breadboarding or prototyping skills and wishing to participate in making this transmitter project better should contact me. I will be willing to share a few different transistors and such.
WARNING: Note that a recent Ebay order of 5ml sample vials had a different inside diameter than previous purchase. With the newly purchased vials, neither standard size thead bobbins (for thread trailing), or the above size transmitters fit inside. The photo shows the new and old comparison. To order the correct size vial, look closely at the auction photo and only purchase those that have identical features to the ID photo. A U.S. quarter fits loosely inside the correct size sample container.
The tabs indicated are present on the correct vial.
Instead of making battery holders from a paper clip and brass scrap, I have tried a commercial battery holder made specifically for the AG13 battery. These will fit inside the plastic vials, but don't stop the rusting problem of the paper clips. These holders cost 26 cents each, making them the second most expensive component on the board, and don't work as well as hoped.
UPDATE: A number of used AG13 batteries from these transmitters were re-installed in transmitters during the winter, and run to their end. Most powered the transmitters to a total of at least 4 months, and a number were still transmitting past 5 total months. Unfortuately, one battery quit after only 72 days. For the cost of 10 cents per battery, changing them out at 60 or even 30 days is a good idea.