Interesting,
The thing that imediatly jumps out at me is the cyclic rate of the relays.
At a 8 second cycle rate, that works out to be ~10,000 cycles a day - more
at four seconds. I wouldn't expect a mechanical relay to have a very long
lifespan at that rate. If you actualy need this switching rate I'd
reccomend you look into some other options. Solid state relays are an
option, or you could control it with triacs.

A pic is a good match for this type of application. I would generaly
reccomend the 16F877 / or 16F876 for a beginner to learn with. But
something of this size could be done with a much smaller part (16F629
probably)

Most of the people on this list program with assembly. Since you mention
you have some BASIC experiance maybe you should look into ME labs PicBasic
Pro. http://www.melabs.com  They even have a demo version that's probably
enough to do what you're asking.

There's also some good PBP specific resources out there like this one:
http://www.picbasic.co.uk/forum/

The basic program would actualy be quite simple, it'll probably look
something like this:

lightsensor  var portb.0
relay        var portb.1

on           con 1 'these depend on how you wire your relays
off          con 0
day          con 1 'depends on sensor type and how you wire it

input lightsensor
output relay

Main:
if (lightsensor = day) then
   relay = on
   pause 8000   'number of milliseconds to pause
   relay = off
   pause 8000
else
   relay = on
   pause 4000
   relay = off
   pause 4000
endif
goto main


This is just a quick hack, and I imagine you will want to add features and
such, but it should give you some ideas.

-Denny




>
> Sorry for not being clear. I am working on a project using a PIC (16F877
or 16F628 ) or even AVR (90S2313, 90S8515) to control 4 relays (2 pairs);
on and off for say, 8 seconds interval for 24/7. Powerheads (fish tank
type) operating at 220v will be attached to the relays. During the day,
when the fish tank lights are on, I want the on/ off interval to be 8
seconds. During the night when the lights are off, I want the on/ off
interval to be 4 seconds. Thus, I think of using a light sensor as an input
logic so as to use different interval routines within the PIC.
>
> As I am totally new to MCU and electronics hence, if I can get hold of a
project that is as close to my requirement as possible, that will greatly
help. The only plus I have is, I have some BASIC programming background.
Hopefully, I can pick up quickly on modifying the source code to suit my
needs.
>
> Thanks in advance for the help.
>
> cheers,
>
>


-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist