I need to build two devices to transmit and recieve simple data
packets.  It would work something like this:

DEVICE A - TRANSMITTER

A battery would be connected to a switch, which would be connected to
a transmitting unit.  When the switch is pressed, the transmitting
unit will send a signal to DEVICE B.  When the switch is released, the
device stops transmitting.

DEVICE B

A battery would be connected to a recieving device which would be
connected to an additional battery/electronic pulse emmitting device.
When a signal from DEVICE A is recieved, the battery/electronic pulse
emmitting device discharges an electric current.  This device is
connected to the skin, which will allow the wearer to feel the
electric current (the current should be easily detected by the wearer,
but not painful).

My questions are as follows:

A) What parts would I need to build the following two devices?
    -I would prefer transmitter/reciever to be pre-built, but a simple
transmitter/reciever could be built if VERY CLEAR instructions are
provided
    -The cost for all parts can be no more than $40 (not including
batteries)
    -I have a soldering iron and solder, so such things do not need to
be purchased or factored into the $40

B) How do I build the two devices?
    -I have very basic electronics knowledge (i.e. I can solder things
together), but in general need very simple and clear instructions how
to build the device
    -The devices need not have a range of more than thirty feet, and I
would prefer a transmitting range as close to 30-40 feet as possible
(though I will accept other ranges if necessary)
    -The smaller the equipment, the better - all parts need to be
easily concealed behind clothing

C) What size battery should I use to power the shocking unit of the
reciever?
    -Each individual pulse needs to be disconcernable (the device will
be used for silently relaying coded information), but not painful. 
The wearer will have to distinguish between patterns of shocks to
decode the message.

If you have any questions, please ask before answering, there is no
rush to answer this question.  I am very open to prebuilt solutions,
but please run them by me before posting them as answers so I can make
sure they meet my needs.  Kits are also fine.

You'd probably get an answer if this was priced appropriately. Just my
$.02

"$2 - $5
-Can be answered with a single link or a single piece of information.
Sometimes, if a researcher is personally interested in the question's
subject, they may provide a longer answer.
-Not appropriate for multipart questions. 
-Only 60% of the questions asked in this price range are answered."


Google Answers Pricing Guidelines:
http://answers.google.com/answers/pricing.html

I presume you are in USA but I hope the following UK related
information gives you some ideas.

To comply with radio transmission laws you will probably need to use
type-approved low power pre-assembled transmitter modules such as
those listed at

http://www.maplin.co.uk/Search/Results.asp?Level=2&Menu=25322

There are a number of different powers, frequencies and prices. 

For the 'shocking' aspect I would caution you that even low power
devices can have the ability to cause injury or death. You might want
to look at adapting a TENS pain relief unit or muscle stimulation
unit, but I cannot advocate such a course. To get a 'tingle' you will
need quite a high voltage - say 70-100V - but at carefully restricted
frequency, current and duration.

If a vibrating and almost silent alert is acceptable you could look at
the small vibrator motors found in radiopagers - sometimes available
in electronics surplus shops now that pagers have been superceded by
mobile phones.

Have just had a completely separate idea - radio controlled model cars
etc have servos for forward/reverse/steer. Could you pull the bits out
of an r/c model and use that. I bet some lucky kids got new toys at
christmas and their old ones will be heading for charity shops etc, or
the outer casing of a car has been crashed but the electronics still
work. Put the servos on different limbs of the recipient and you can
tap left arm, tap left leg, etc, which may be more easier for the
recipient than counting shocks!

Owain

Your right, a vibrating device seems much easier.  I was basing the
shocking device on the idea of something like an invisible fence
system (for dogs), but at a lower power.  A vibrating device would
probably work, but would individual signals of about a .75 second
duration sent about one second apart be distinguishable?  Any other
comments with regard to how a silent signal can be detected or
communicated to the wearer would be greatly appreciated.

The recipient should be able to count vibrations faster than one per
second with reasonable accuracy. Old telephone dials worked at 10
pulses per second and many people could count the clicks and work out
the number dialled fairly accurately.

A continuous vibrate, slow pulsed vibrate, and fast pulsed vibrate,
should be easily distinguishable. It depends on how many different
conditions you need to indicate and how quickly you need to send them.

If this is for use in a pre-arranged and controlled environment you
could look at using an audio-frequency induction loop, of the type
used in theatres etc, together with an in-the-ear induction loop
receiver. This would allow you to talk to the recipient. These are
used to pass instructions covertly from police officers to someone
interviewing a child witness, for example. The induction loop can be
wound using an ordinary PA or hi-fi amplifier and a *carefully
calculated* loop of cable, and you might be able to use an old hearing
aid as a loop receiver. Professional versions of these use a radio
receiver beltpack powering an a/f loop worn around the neck of the
interviewer and will be outside your budget.

There are some comments on loops here:
http://www.ibs.org.uk/archive/iloops9910.html
http://www.assistiveaudio.com/facts.htm


As maxhodges suggests, if you increase the value of your question a
Researcher (I'm not one) might take on and develop these ideas.

Owain

If you're building this for the application I presume you're building
it for, humbly suggest you be very careful in how you use it.  These
types of devices have been *commonly* used for a particular
application and this fact is well known.  Just my two cents to save
you some heartache.

Actually I have no idea what this is going to be used for.  I was
approached by a friend who agreed to pay me if I could build something
of the sort.  He didn't state what he wanted it for.