Hello!

For those that want the background waffle, it follows below.

Otherwise, here is my query:

Is there an easy way to write code in the PIC to respond to a pulsed 2KHz input
and not one at any other frequency? The pulses are about 8 microseconds long and
happen about every 500 microseconds. I am receiving bursts of 2KHz signal that I
want to time the length of. Phase locked loops take to long to respond.

My current plan is to manually wait for a pulse to start, time it, wait for
another one, if another one starts within the acceptable bracket, it's a 2KHz
signal (ie a binary pulse has started). The end of the binary pulse would be
when the little pulses stop. This seems a bit clunky, so I thought I would try
the PICLIST out!

I have been a subscriber to the list for a couple of weeks now - it's great! My
vote is for all of the indirect PIC stuff to continue. Perhaps if people with
other, non PIC information included 'notPIC' in the subject it would help people
to have them auto deleted.

Thanks!

Here's the background:

I am building a bizarre communications system that flashes binary messages
across a room as an exhibit in a hands-on science museum. It uses ordinary
torches (with LED's instead of bulbs) as the transmitters and they flash really
slowly so that you can see the binary coding easily. On the other side of the
room is a de-coder that translates the binary codes back into messages. The
demonstration is to help illustrate what a digitally encoded binary message is.
The interactive bit is that you type in the message on a keyboard and watch as
it is transmitted. Each end of the room has a transmitter and receiver so that
you can send messages backwards and forwards. You can wave your hands in front
of the torches and garble the messages. It's great fun!

The neatest thing about it is that you can control the flash rate from about 1
bit / second to about 20 bits/ second to choose from being able to see the bits
and to transmit the message at a reasonable speed. The receiver automatically
adjusts to the modified transmission speed.

The whole thing is done with a 16C57 for transmitting and a 16C55 for receiving.
I'm using standard PC keyboards for the input of the messages and they are
displayed on 2x40 char LCD's. The display shows the binary code of each letter
as it is transmitted. I am really pleased with it!

Anyway, as you can imagine, I am using visible light and have all sorts of
problems with ambient lighting. I am using photodiodes positioned in the middle
of a couple of parabolic reflectors taken from torches and they work really well
(the distance is 10m), but I am unhappy with the arrangement since it is really
only a threashold light detector and it needs careful adjustment to get it set
up. If the sun comes out or someone turns all of the lights on, it gets messed
up. I have recently built in a comparator and an ambient light sensor to
compensate for this, but to make it rock solid I have also decided to transmit
the binary pulses using a 2KHz 'carrier' signal. Each binary pulse is actually a
2Khz flashing pulse, but you can't see it as that.

I look forward to hearing your comments!

Many Thanks,

Roger Coleman.

--

Roger Coleman,
Curator,
Guildford Discovery
Ward Street,
Guildford,
GU1 4LH
England

Tel/Fax +44 1483 37080
101722.3022@compuserve.com