developing control logic

Hello, guys, I have one problem and I hope you'll help me to solve it.
I need to develop a control circuit:
I have two switches like on the picture, one relay, voltage source and load.
Between source and load there is a control circuit. The task of control circuit
is the following:
In the begining thre must be a current flow from source to the load.
When I press switch A (change contact position), current countinues to flow (nothing happens)
after A is pressed after some time I press B and then current flow is disabled (there is no current flow
through load).
After some time (time interval is not important) now B is pressed and also there must not be
current flow. After some time A is pressed and now, there must be current flow. Circuit is breaked
after switch B is pressed and process repeats again and again.
In short:

initial condition: there is a current flow from source to load
after A is pressed (switched to second contact) =>current yes
after B is switched =>current no

B is switched again (return to previous)=> current no
A is switche =>current yes
B is switched => current no
and again and again.

My question is:
Is this problem is solvable with two switches A, B and a relay R?
If it is, can you give me a hint how to solve it?
My biggest confusion is that in the begining switches acts like they are in paralel, but in
second cycle theay acts alike in serie, and again in paralel...?
I was thinking with something like:
In initial cas switches are in paralel, but when both "throw", relay R change their connection
in serie. Problem is I don't know how to draw this...
If you can, please help...

Thank you very much

Sounds like basic three wire control to me.

      A            B            R

------] [-----+----]\[---------( )----

              |

      R       |

------] [-----+

Thanks for reply, but this isn't that easy. In the begining there must be a current flow to the load, and switches A and B must be swithced in strict sequence A-B, B-A, A-B, B-A...
here is how I tried to solve thi on picture. On initial , there is current flow, A is switched and still there is current until B is switched. Aftre B is switched current is off, but know I need to control R so that after B is switched R disconnect rung with B so when B is switched again current is still off. Only after A is switched R enable rung with B, current flows to the load and whole process is repeted...

Pandiani said:

In short:

initial condition: there is a current flow from source to load
after A is pressed (switched to second contact) =>current yes
after B is switched =>current no

B is switched again (return to previous)=> current no
A is switche =>current yes
B is switched => current no
and again and again.

Click to expand...

Following the above, would make this true:




But this does'nt make much sense to me, as it makes switch A pretty much useless.

I agree with Alaric - it's basic 3 wire control to do what you are asking.

Are you looking to do this in ladder? Is this homework? If it's a practical problem you might need to explain it a little more.

Thanks for the reply
No, I'm not making ladder diagram and it's practical homework.
I have practical problem. I have temperature sensor which is connected to one old instrument (220 V supply)
that has two points which correspond to tempertures 40 deg Cel. and 80 deg respectively.
At the back of the instrument there are "switch A" (No and NC contacts) and "switch B"
(also NC and NO). I also have one relay 220 V. I need to develop control logic that will
control temperature in the followin way.
When all is started, temperature is lower than 40 deg Cel, so heater (load) must be On.
As the is working , temperature increase and it comes to 40 deg so switch A reacts. At this point nothing
has to be change since heater is working. Temp. continuos to raise to 80 deg and switch B reacts,
and in that moment heater is off. Temperature is now a little more than 80 deg and with time it start
decreasing again to 80 deg and switch B racts. But this time heater must not turn on. Temp is
falling to 40 deg and then switch A reacts and only then heater is turned on. Temperature is now a little
lower than 40 abd start increasing causing the switch A to react, but heater is still on until 80 deg and only
then, heater is turned off.
So sequence is the following

Initial case (heater is on, temp low)
temp 40 deg -> switch A, heater is ON
temp 80 deg -> switch B, heater is off (temp continue to raise some time, then start falling)
temp is back to 80 deg, heater is off
temp is back to 40 deg, heater is back ON (temp continue to fall some time, then start to raise)
temp is back to 80 deg causing heater to swithc off

This sequence is repeted maintainig temp between (40 and 80 deg roughly)

In Alaric example, when switch A is not activated, load is without current and I don't want that.

Thanks for your time, I hope you'll be able to help me although it has nothing to do with PLC.

I hope this helps

Thanks bernie, it was exactly what I needed, I cannot believe how simple it looks now. Tell me, is this experience, pure intelligence or ... or it can be learned?
Do you know any good links to recommend where I can learn this and similar stuff?

Are there any tips and tricks that needs too be followed when working with this kinds of problems?

Anyway thank you all for your time and effort.

There's no special tricks involved. As others had already pointed out, what you described is a simple control circuit used in most any machine that exists. I just adapted the standard drawing to match the way you seemed to be viewing the controls. Most here would have viewed the sequence from an initially OFF state and that may have confused things a bit. The circuit is also usually constructed with momentary buttons. The button you have desginated as 'A' is wired 'normally open' and the button 'B' as 'normally closed' giving a circuit such as Alaric drew in ladder program format.

Thanks,
I guess this is what I was needed in ladder:

       A            B           R

 ------]\[-----+----]\[---------( )----

               |

        R      |

 ------] [-----+

Beacuse in intial case, there must be current flow, so A must be normaly closed... am I right?