| Aim: The project was to build
a wireless link (HF) between 2 HP48/PC to exhange datas without being annoyed by the
walls.
The dialing distance could vary from 200m to 300m without blocking elements or relays. The
main difficulty has been to find a mean to
find how to accomplish a wireless link without special HF material such as an Spectrum
Analyser. I had in mind that it should be
as tiny as possible to plug into the extension port for example and standalone.
Functional Block:
Schematic:
Components location / copper side :
Documentation/ Pinouts
Technical observations:
I scanned the technical documentation for the mipot AM
434MHz modular system. It is very easy to use as only power and transfer signal (ie.
Send signal or Receive signal) are required. The price is about 200F (30
dollars) . There are alternative components such as Aurel modules (pin compatible).
As you can guess, this is only a interface problem, always the same between the TTL and
RS232 world so MAX232 chip is welcome.
If you don't take care of the inverters inside the max232 , your module will ever emit
something even if you don't need to send data..
Other thing, the 12V batteries go down rapidly because they are
not enought powerfull to drain current required by the AM carrier. As the modulation rate
is equal to m=90% , the two bilateral strip representing the usefull signal are
m²/2 ie 40.5% of the carrier power (that is not bad at all in AM).
Unfortunately, the remaining 59.5% are to be supplied continously by the batteries
les piles even in iddle state. Thus, the batterie should at least deliver the carrier
power defined by the relation Pp=Pmax(1+m²/2) with Pmax=1mw soit 0.71mW (ie 60mA
under 12V) wihtout forgetting to take into account the transfert power
equal to Pr= (Pmax-Pp)/2 of 0.144mW (ie in total 24mA under 12V) .
As sumary, the batteries should supply at least 60mA and in the
worse cases (during transfert) 84mA. This is imossible with such batteries (it last only
one minute!) so I use external power for tetst. I have written programs in the
System RPL language to run it faster although only a serial exchange at 2400 bauds
(max speed of mipots modules). I expect to add an antenna but I did not caculate the
length , L=(c/f)/2=34cm so It is not long enougth. I f you try to connect
my tiny antenna you will only get noise.
Test has been made with two HP48SX linked to this module by the serial port.
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I have successfully echanged data (and then programs
between the ceiling and the attic. That is funny. It was only raw transfert with no
security.
I have checked that the serial port is truly full duplex as it can get the message
it sends.
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