I'll do this in back-to-front order - start with question two -
As I am sure you are aware, there are a huge range of cells on the
market, however most of the small ones (watch and hearing-aid type)
are not rechargeable, and offer a very low current making them
somewhat ineffective in running something like a small camera.
Looking at the link you posted, it seems a 9V battery weighs it at
around 50g - which, based on some very unscientific testing, does not
seem too much for a cat to carry on it's collar, in so far as it does
not seem to affect it's ability to move (or scratch) however, in the
case of my test subject, some cats may not really enjoy the
experience. But based on your reply to Missy's comment below I will
assume that your cat is more open to such ideas than mine.
If you think your cat can deal with the size and approximate weight of
a standard 9V cell, then perhaps a rechargeable one would suit? I have
found the following at RadioShack (I could not find a UK retailer with
a good online catalogue of batteries):
NiCad 9V Cell - 8.4V, 120 mAh
NiMH 9V Cell - 8.4V, 150mAh
Both of these cells have much lower current ratings than the Varta
battery you mentioned, although as we don't know the draw of the
camera, it's hard to know whether this is going to last the four hours
you need or not.
Also generally available are NiCad portable phone batteries, which,
link the one in this link, seem to be just button cells, wired in
series and plastic wrapped. However, it seems very difficult to find
those basic button cells on their one. One option may be to buy some
of these batteries (typically 3.6V) and dismantle them and reassemble
them on a collar. The one I have linked two seems to get 3.6V from two
cells, so they should rate at 1.8V each - five of these button cells
should give you 9V. The dimensions are not listed, but my personal
experience with them leads me to believe fit could be fitted to a
collar without too much difficulty.
I believe that a 9V cell, attached length ways to a collar should not
get in the way too much although might be a little large when combined
with the camera. The only way to know for sure is going to be to test
it with the cat involved.
Otherwise, I think the base cells from NiCad portable phone batteries
are your best bet for creating a power source in the shape of a cat's
Another, less practical but more environmentally friendly, option is
solar power, two of these panels should power the camera, although a
rectifier maybe needed to drop the voltage to a usable level, and with
a larger harness (rather than a collar) they could be strapped to the
Now, as for the RF emissions, from your description of the device I
assume that it operates in the UHF range, as a 2.4GHz transmitter
generally requires a much more substantial antenna than a wire. I
would estimate that the power of such a transmitter would be less than
50mW, in fact based on looking at the products on this page
it is probably closer to 20mW.
This power level is so small that it is unlikely to be any more
dangerous to an animal or person than simply living in the
transmission area of a UHF-based TV station. And even then, I have
been able to find no real information about any dangers associated
with UHF transmissions. Most of the RF transmissions that people are
concerned about are in the microwave range, where direct exposure can
be a danger.
Some other thoughts on the issue:
You may find some problems with the range, 100m is likely to be in a
clear line-of-sight, while UHF signals will pass trough solid objects,
the do attenuate quickly, and something as simple as going under a
building or being on the other side of an earth mound may affect your
ability to received your cat-cam signal.
Also, you mentioned the magnet within the cat's current collar set up.
You may find that it interferes with the camera's image. While CCD and
CMOS cameras are quite resilient to magnetic fields, a strong source
nearby could still have some affect. This is probably worth testing
before hand, if you are able.
I hope this helps, your project sounds very noble, and I am sure
entertaining as well. Let me know if I can help more on this issue,
and also let us know how it turns out :) I am sure we'd all love such
a collar for our own cats.