I would like to buy a minature AM transmitter,the size of a five pence
piece if possible.Using perhaps a small mercury cell (rechargable)for
its power supply.The range of the transmitter need only be two hundred
meters.The project is part of a alarm system.Please indicate any
special sources who could supply such a device.

AM does not define the frequency, though I would guess you are
referring to ?medium wave? which is imprecisely termed the ?AM band?
-- in the region of a megahertz. Devices are very simple but you would
still need an aerial of several feet to get much out of it. For that
reason (and a bunch of others) most miniature transmitters are now VHF
(or higher frequency), also inaccurately termed the ?FM band?. So
before trying to hunt them down, are you committed to AM?

Best

Your comments are very interesting.VHF transmitters would be suitable.Please advise?

OK, VHF or UHF. I take it you are UK based (ref to 5 pence piece). VHF
can often be picked up on a standard VHF radio. Though sometimes
available are not legal for usage in the UK. Not that this is
necessarily a major consideration because there are holes in the usage
where you wouldn?t upset anyone. UHF systems are legal but require a
special receiver for the frequency. Both transmitter and receiver can
be built quite cheaply from modules but require expertise. With brief
search I didn?t find micro-miniature transmitters but no reason why
they shouldn?t be available. Suggest you search......
{?micro-miniature? VHF transmitter}
.........and variations on the theme.

You don?t say what kind of signal you want to transmit. Data is easier
than audio. Mercury cells BTW are not rechargeable and you will also
find that transmitters demand at least a couple of volts.

What thoughts?

Best

I found these,digital
transmitters/receiver,,http://www.finchdata.biz/qradio/modules.asp,under
816mhz .they should be straight forward to set up I think.For my alarm
I require the digital output to set off a alarm.At first they require
a digital input ,perhaps digital tone burst generator,this of course
would have to be miniture.To receive the output I am not sure,please
advise.The transmitter is the difficult part since it has to be
minature..The volts I can pick up somewhere as power supply.The
following are details
FM 868 MHz T6/R6 Transmitter/Receiver Modules Ref: 108-163
Miniature SIL package , Fully shielded, Data rates up to 64Kbits/s ,
Range up to 200 meters, Single supply voltage, Industry pin compatible
Transmitter :- 

Temp range -20C +55C 

European versions; 

868.4 MHz for 10% duty cycle band 

869.85 MHz for 100% duty cycle band 

American version; 

915.0 MHz 

Bandwidth efficient PLL technology 


Receiver

Single conversion FM superhetrodyne using RF SAW and ceramic IF
filtering at 10.7 MHz (image rejection 60dB)

Analogue, digital outputs 

Signal strength output (RSSI) 

Incorporates AGC resulting in improved dynamic range 

Dynamic range better than 120dB 

Single 5V supply 


The T6 AND R6 miniature UHF radio modules enable the implementation of
a reliable telemetry link at data rates of up to 64 kbits/s.

The T6 is based on a classical phase lock loop using a crystal
reference oscillator. This results in an accurately controlled RF
output in the frequency domain. A significant advantage of this is
that narrow filtering can then be used in the receiver, which results
in high interference immunity. In addition, the modules are fitted
with an on-board voltage regulator which enhances performance from
improved supply filtering and also ensures a constant RF output level.

The R6 is based on the single conversion superhet principle utilising
a crystal based phase lock loop for accurate generation of the local
oscillator. This allows use of high Q bandpass filters resulting in
good adjacent channel selectivity and high interference immunity.

The T6 and R6 modules will suit one-to-one and multi-node wireless
links in applications including building and car security, remote
industrial process monitoring and computer networking. Because of its
small size and low power requirements, these modules are ideal for use
in portable battery powered wireless applications. These modules may
be used within areas of high interference, or 'radio traffic'. Due to
the design, they provide a highly reliable radio link.

Dleivery B see below for prices

Eventually found the item on :-

http://www.finchdata.biz/cgi-bin/sh002323.pl?ACTINIC_REFERRER=http%3a%2f%2fwww%2efinchdata%2ebiz%2facatalog%2fsearch%2ehtml&REFPAGE=http%3a%2f%2fwww%2efinchdata%2ebiz%2facatalog%2fcatalogbody%2ehtml&WD=868%20t6%20modules%20transmitter%20receiver%20mhz%20fm%20r6&SHOP=%20&PREVQUERY=SS%3dFM%2b868%2bMHz%2bT6%252FR6%2bTransmitter%252FReceiver%2bModules%2b%26ACTION%3dGo%2521%26PR%3d%2d1%26TB%3dA%26SHOP%
3d%20&PN=868MHz%2ehtml%23a108_2d163#a108_2d163

You?ll also find quite a number of similar from :-

http://cpc.farnell.com/jsp/home/homepage.jsp

I presume you downloaded the pdf. Aerial notes are only advisory
because you really need some way of detecting how efficiently they are
matched. Loops are quite simple but you?d still have to experiment.
Whips are easiest and require least ?frigging?. Don?t try the helical
because these are very critical. Perhaps you should consider purchase
of an aerial book to get a handle on things.

Depending on what sort of data you want to convey, if very simple,
there should be much more miniature devices to be had. Just need a bit
of hunting. So how much data and what rate do you intend?

Best

As per 6.02 PST,my reply describes a simple tone generator
input,combined with a triggered output.Since the circuit I hope to
build is a alarm circuit only a simplified input output is required
from transmitter/receiver.Please advise.

Very interesting transmitter at following link,what do you think the
range could be,also what receiver should be
used,http://cpc.farnell.com/jsp/endecaSearch/searchPage2.jsp?x=38&OrgTT=transmitter&Nty=1&N=1000026+401&=gensearch&y=6&No=0&Ntt=transmitter&Ntk=gensearch&comSearch=true

I?ll try to get back to you tomorrow if that?s OK. Bit pushed for time
tonight and last couple of days.

Best

Sorry for the delay.

Ah, no these are ultrasonic transducers. You want radio don?t you?
Keep searching CPC and I can check out what you find. If you can?t
find specifically what you want I could suggest CPC parts. Don?t want
to make it too easy for you :-)

You are in UK aren?t you? CPC are pretty good but you need an account
before you can buy from them. Dead easy to set up and they?ll send you
a catalogue which is much more convenient for browsing on the toilet.
Actually I highly recommend that you do this because one often learns
much more from comprehensive catalogues than from technical books.

I shall keep a watch.

Best

Please comment on these transmitters,as reguard UK law ect,they seem
good value,as I say all I want to transmit is a alarm
tone.http://www.bullnet.co.uk/shops/test/bugkits.htm#QX900--------------------------------------------------------------------------------

QX900 NBFM signalling/tracklng transmitter
The QX900 is a crystal controlled NBFM signalling/tracking transmitter
which will find many uses in alarms, warning devices and location  of
missing items. It generates and transmits a stream of audio pulses
whose tone and repetition rate are both variable from low to high and
short to long with two miniature on board controls. ft can be fitted
to vehicles or other packages whose location can then be identified
with a receiver. If a directional receiver is used this can actually
provide a bearing on the signal. It can be used as a wireless  alarm
system if linked to some form of contact and with the ability of
varying the audio tones, several can be put on the same channel and
monitoring with a single receiver can identify which transmitter has
been triggered. Connected to a mercury tilt switch it can remotely
signal if something has been moved. In fact the list is endless. The
0X900 board measures 2Omm x 67mm and the unit operates from a 9V
supply. Current drain is around 10mA which will give 50 hours
continuous transmission from a Duracell MN1 604. The output power of
I0mW is sufficient for a range of 1000 metres. Operating frequency
180,000MHz.


Quantity Item Description Price: Cart 
 QX900 SIGNALLING/TRACKING TRANSMITTER 55.36  

--------------------------------------------------------------------------------

TKX9OO Wideband signalling/tracking transmitter 
The TKX9OO is the broadband version of the QX900 signalling/tracking
transmitter described elsewhere in this catalogue. It's output signal
is fully tuneable from 100-120MHz making it suitable for use on normal
VHF FM or airband receivers. The audio bleeping signal is fully
adjustable with miniature on-board controls in bleep length and pitch
making it ideal for all forms of signalling or alarm use. ft operates
from a 9V supply and has powerful 100mW output providing a range of
3000 metres. Current drain is around 50mA so a continuous transmission
time of about 10 hours can be expected from a Duracell MN1 604. Board
size 25mm x 63mm.


Quantity Item Description Price: Cart 
 TKX900 WIDE BAND SIGNALLING/TRACKING TR 35.34  

--------------------------------------------------------------------------------

I am still looking for minature transmiters,as far as I can see CPC
only do the printed circuit type similar to the type stated earlier in
comments,if you know different please let me know.Since I am planinig
on buying a few.FFor alarms,therefore only a basic input tone
ignal,its all getting very interesting now.

No, there *are* other transmitter / receivers available from CPC.
Keyfobs and very cheap 418/433 MHz AM transmitter module. Searching
catalogues on the web is very poor in comparison with having the
catalogue in hand. Do sign up for an account with CPC, they?ll send
you a catalogue and it?ll answers many of your queries.

Best

Thanks,I am in process of geting a cpc catalogue,good advice.