Simple timer project - help needed

[Charles_Smith]

Hi,

I would to build a simple circuit, using a logic out from a control circuit, to turn on a low power relay for a given time period (about 30 seconds) then turn it off for about the same time interval, and after this cycle reset to sample the input again. I thought that I should a simple 555 timer circuit, but I would very much appreciate some help in getting the design right.

Many thanks

Charles

[Theatronics]

WHile you can get nearly a 50% duty cycle out of a 555 with a little work,
it is easier to use a flip-flop to do the work for you.

If you want a 30 second on and a 30 second off:
Set the 555 timer to pulse once every 30 seconds.
it doesn't matter what the duty cycle of the pulse is.

Run the output of the 555 to a 74LS74 or equivliant chip.
(30sec pulses into pin 3)
(Tie Pin 2 to pin 6)
(Perfect 30second ON, 30Second Off pulses out of Pin 5)
(14 is +5v , 7 is gnd)

is that helpful?

-Mike

[Charles_Smith]

Many thanks, I have some electronics knowledge (from many years ago) so I

[Theatronics]

You may have broken down your solution the wrong way.
Lets look at the rules:
1) Sample the Air only if the fan has been off for 30 seconds or more.
2) When signaled, switch on the fan for 30 seconds than shut it off.

It sounds like there is a definite ladder logic to this.  You might want to look
At a device called an IDEC Smart Relay. 
VERY easy to program (There are only about
8 commands and they are mostly delay timers and logic gates)
They run 110Vac or 12/24Vdc with 4 relay outputs and 6 Inputs they are very handy.

It will save you TONS of circuit development time.

If you want to DIY then I’d suggest something like this.

-Mike

[Charles_Smith]

Brilliant Mike, many thanks, I can see you got the idea.

This control circuit is part of a project which I am involved in with students from our local further education college to build a wood drying kiln; their budget for bits it very limited so I think I will try out you DIY circuit which looks very simple and I