"Fun with relays"

Discussion in 'The Projects Forum' started by richard3194, May 16, 2014.

1. richard3194 Thread Starter Active Member

Oct 18, 2011
68
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Hi. Can anyone suggest circuits where the main operating element is a relay - that would be fun and instructional?

What I have at the moment that show what I'm after:

#1 making a relay a buzzer from a general purpose relay (NC contacts in series with relay coil)

#2 making a latch from a general purpose relay (NO contacts in parallel with coil switch)

#3 use of a flasher relay

#4 use of a protection relay

#5 use of a timer relay

Maybe there are more circuits, or relays, where the relay is the main circuit element?

Thanks. Rich

Jul 18, 2013
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#6 Single P.B. toggle (flip-flop)?
Max.
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3. richard3194 Thread Starter Active Member

Oct 18, 2011
68
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Just come across: "ladder logic". Maybe a useful source of ideas.

Oct 18, 2011
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5. richard3194 Thread Starter Active Member

Oct 18, 2011
68
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Is #2 a form of logic circuit?

6. richard3194 Thread Starter Active Member

Oct 18, 2011
68
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strantor likes this.

Apr 17, 2014
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8. Sparky49 Well-Known Member

Jul 16, 2011
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strantor likes this.

Jul 18, 2013
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The only thing is Ladder is based on Boolean arithmetic and you can do many things in R.L. that is impractical with physical relays.
The basics are there though.
I should qualify that a little as early control systems were all relay logic before the advent of PLC's.
Max.

Jul 18, 2013
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Why not?
Max.

11. ErnieHorning Member

Apr 17, 2014
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If you think about it, they all are.

Jul 18, 2013
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Exactly my point.
Max.

13. richard3194 Thread Starter Active Member

Oct 18, 2011
68
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As I understand things, one simple on-off switch, in series with a load is not a kind of logic gate? Am I correct? I mean, no other switches in the circuit. Thanks.

Last edited: May 16, 2014

Jul 18, 2013
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Not really, in terms of Boolean.
A (or not A) output B, in the simplest sense.
You need to define what you consider 'Logic'.
Max.

15. richard3194 Thread Starter Active Member

Oct 18, 2011
68
2
You draw a outline shape of some design (say a box shape) with an input or inputs and an output. You call the thing (the box shape) a logic gate.

I think with logic gates there is to be no reliance on the state (on or off) of any switch inside the box, if the state is not entirely dependent on an electrical input (Lo or Hi).

In other words only voltage inputs must be relied upon, to produce an output.

If there is only one input, and the single switch inside the box is open then there is no output. Apparently, we don't have a logic gate.

If the switch inside the box is closed, then we do get an output. Again, apparently not a logic gate.

Why? I'm not sure. Possibly because there in no logical operation performed.

However, if the output of the box is an opposite of the input, that is a logic gate. A logic NOT gate.

16. richard3194 Thread Starter Active Member

Oct 18, 2011
68
2
In fun circuit #2, that is a latch circuit. If the start switch is pressed and represents an input going high, then there is a logical high on the output. That on it's own though does not represent a logic gate. However, when the input goes low, the output stays high. I suppose that is some kind of logical operation. Not sure.

We also have a stop switch, I suppose another input.

Jul 18, 2013
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If you are refering to some other reference to logic other than cause and effect of Boolean arithmetic which relay and ladder logic is based on then you would need to explain what form you are working in?
Max.

18. richard3194 Thread Starter Active Member

Oct 18, 2011
68
2
We're dealing with boolean arithmetic I believe, and logic gates, but we are looking specifically at relay circuits.

I see that you can make an OR gate out of relays. In a schematic of an OR gate you have two inputs A and B and an output. Of course, when talking about the pin states you say A, B or Ouput (Q I think that is) is either 1 or 0 - Hi or Low. There's an associated truth table.

Okay, now, putting one's focus on relay circuits, we see that we can make a circuit to operate like an OR gate.

The circuit is a lamp and in series with it are two contacts (switches) in parallel. What you do is call one contact A and the other B. The output can be the state of the lamp, whether lit or not.

What is happening is that we are associating the voltage state of A and B on the OR gate schematic, with the contact state of A and B in the relay circuit/schematic.

I see that if you take the output of an OR gate and feed it into pin B, you get a latch of the output.

Turning our attention now to relay circuits, you get the latch of the output if the load is a relay and it operates, that is closes contact B.

So, by building the relay circuit as described we have:

* created a circuit that will operate like an OR gate.

* created a circuit that operates like an OR gate whose output has been fed back to an input. A latch.

When you add another contact (a stop) in series with the two paralleled contacts, you can stop the latch and output goes low. We might be getting into combinatorial logic doing that, not sure. Then you are creating a S R latch.

So, fun circuit #2 is basically a SR latch and there is an element in the circuit of an OR gate.

Fun circuit #1, which is a relay coil in series with it's NC contacts, I see is akin to a NOT gate whose output is fed into the input. That brings about a multivibrator or astable condition.

I see that we are getting into bistable and astable circuits and multivibrator territory.

Last edited: May 17, 2014
19. eetech00 Active Member

Jun 8, 2013
819
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Hi..

Logic is a form of reasoning, basically a thought process. The use of relays, or PLC's is an implementation logic in hardware and/or software.

Boolean Algebra is a mathematical method for the expression and simplification of logic, and is used in both relay and PLC logic development.

Hope that helps...

eT