Q:

Variable time alarm using 555 timer

I have to make a circuit in which i have to produce three outputs(on speaker buzzer sounds)

First is for 20 seconds on 5 sec off
10 sec on 5 sec off
5 sec  on and 5 off( this alarm sequence  for 3 minutes)
Second one is 5 second on 5 second off (this alarm for two minutes)
Third is 10 seconds on 5 sec off, 5 seconds on 5 seconds off (this alarm for 3 minutes)

I have to use 3 push buttons/switched for this.can anyone guide me how to use 555 ic for this.. I have resistor and capacitor values for times but i am not sure about circuit.

A:

The complexity of the sequence and the relatively long times makes a 555 timer completely unsuitable for this task.  Actually, you'd need quite a few of them for timing the various parts of these signals, and then some logic to gate and sequence things.  That would turn into quite a ratsnest of stuff.
Use a microcontroller.  What you ask is relatively simple to do in even a modest micro.  All you need from one is 3 inputs and 3 outputs, which just about every micro with 8 or more pins can do.
In the firmware, divide the clock down to get 1 second events.  Then write code that waits for N one second events between doing things.  I'd set up a 1 ms periodic interrupt to debounce the input buttons, and use that to also count to 1000 to create the 1 second events.
This is really a rather simple task for a microcontroller.

