Paging system - which way to go?

[Sagacity61]

Hi all

I'm starting a project where I have 8 rooms which require service attention.
Using an RF link, a room push button signals to a central store which receives the room number requesting help on a single 7 segment display.

There are many ways of achieving this simple task, but currently I'm thinking the simplest way is for each room to send a pulse of differing duration.
The base station measures the length of the pulse, detects the room number and outputs the number to a 7 seg display via a BCD to 7seg decoder.
This can be done using 08Ms.

A colleague argues that it's simpler to generate ascii data from the room transmitter (RF link) and directly drive the 7seg display using 18Ms

This seems like overkill to me.
Any input from members or links to previous relevant threads/posts ?

Ta

 

[womai]

ASCII data (followed by a suitable preamble) will be much more reliable as well as in the end easier to code. The 08M can do it very easily (been there, done that). There are numerous examples for it on this forum (search for "preamble) for example). Measuring pulses would mean you need to measure pulse width accurately, which is tricky in the presence of noise, or to have greatly different pulse widths, which would mean very slow transmission and still unreliable. What's more, typical 433MHz transmitters aren't built to transmit such pulses, they need quickly varying logic states (ideally Manchester encoded, but the preamble approach works well, too).

Wolfgang

 

[hippy]

Welcome to the PICAXE forum.

I too would go with ASCII data, or at least something more than just pulse width.

Firstly there's the issue of how to measure an RF carried pulse-width accurately, then the problem of what happens when two people push their button at the same time ? You've got a potential data corruption problem anyway if people do that, but a two-length pulse overlaying a three-length pulse may appear as a four-length pulse which is worse than missing two calls; you would miss two calls and attend one which wasn't an actual call.

Totally asynchronous RF communications isn't as simple as putting RF transmitters around the place and using an RF receiver.

 

[manuka]

I've seen many such systems in hospital wards/old folks homes (my wife is a nurse), & point out that reassuring user feedback may also be needed- as may time stamping at the central base.What is your budget? How professional and reliable does the setup have to be?

You're not mentioned nature of the calls, ranges,local interference sources or room cladding etc- these may influence valid signal reception. For a simple proof of concept perhaps grab a couple of 433 MHz wireless doorbells and set them to different alert tones/tunes. Arlec offerings => http://www.arlec.com.au/display.php?pid=6 (try Bunnings) are cheap(start at ~US$10), with 30-50m range, come with multiple alert options & "channels". Perhaps best of all -most battery powered ones snooze at µA level, so AAs may last ~a year.

 

[Sagacity61]

1.
The pulse idea involves generating a 5mS reference pulse followed by a 20mS identification pulse at 20mS multiples.

ref #1
|____|--|_________________ room 1 TX

ref #2
|___________|--|__________ room 2 TX

ref #3
|__________________|--|____ room 3 TX, etc.

Timing's not critical, since I'm look for the centre of the ID pulse after the ref pulse.

2. Never woould have thought of the word "preamble" for a project like this - I have no lateral thinking skills !

3.
I can see the problem with simultaneous requests.
Wouldn't the some of the problem remain if two ascii codes were sent?
Would it be necessary (with either system) to block all other room transmitters to overcome this? (requiring TX/RX in each room)

4.
Budget is AUD$500.
It's located in a professional environment and so needs to be reliable.
Nature of the call is just "come here to room number # when you can"
Range is 30 metres.
Interference sources typical of an office environment (PCs , Printers, etc.)
Timber / cinderblock / plaster structure, so RF should be OK (?)