Design me a circuit please

[Marvo]

Okay here's one for the trainees. The background is that a friend of mine has an electric fence system that's got 2 zones. If anything touches the electrified fence wires it sets off an alarm and a little 12vdc flashing beacon on top of the control box. There are 2 of these flashing beacon lights, one for each zone.


He wants to add a floodlight that will activate only when one of the zone alarms activates. The idea is that he wants the floodlight to help him investigate the cause of the alarm especially at night.

To make it more difficult he can't use any outputs on the actual fence controller or even open up the control box because it will invalidate the warranty so he needs to pick up his floodlight control signal from the two 12vdc beacon lights on the top of the enclosure.

The floodlight will be a 230v 30watt LED.


When the fence alarm activates it also make a squealing noise so he wants to be able to clear the alarm condition on the fence controller and the floodlight remains on for a preset time (10mins).


Finally he wants a manual reset button that will switch the floodlight off if he doesn't need the full 10 minutes to investigate.




So here's your mission; as you can see in the drawing I made for you I've mounted the floodlight and an empty DIN rail enclosure already and I've run the supply cable to the floodlight. All you've got to do is come up with a circuit that works, give me a complete materials list and I'll need a materials only quotation as well

 

[Marvo]

Oh and the prize goes to whoever can come in the cheapest (least number of components) and obviously it must work as per customer requirements.

Any questions just shout, I'll be back here for a project progress meeting Tues evening.

 

[Floody]

Jesus that's a rough one. Ill leave this one alone and see what others come up with

 

[Marvo]

Awww cmon it's not that bad. Start by designing a basic circuit that brings on the LED floodlight when one of the fence alarms activate. I'd be looking at using 2 x 12vdc relays and a 230v relay.

Once you've done that we can look at adding a timer to make the floodlight stay on for the 10 minutes he asked for.

 

[Floody]

I've not had any experience with circuit building like this and relays yet so I am literally clueless. Ill sit back and watch what others come up with

 

[Marvo]

Okay, no experience with relays isn't going to get you off the hook I'm afraid

A relay is similar to a contactor. It has a coil which causes switch contacts to operate. Sometimes those contacts are normally open in which case applying power to the coil will close the contact. Other times the contacts are normally closed in which case applying coil power will cause them to open.


When a relay is shown on a circuit drawing it's always in it's 'natural' position (ie as if the power is off) so if the contact is shown open it's called a normally open relay.

Bear with me and I'll put you through the relay 101 course but in return I'll be needing my circuit.

Okay there's numerous different relays you'll come across but they're all just a variation on a theme. The most basic relay is the single pole single throw (SPST). The word 'pole' refers to a single set of switch contacts and the word 'throw' refers to the fact the switch contact only has one output contact. It looks like this;



There's two types of single pole single throw relays, the one shown above is normally open which means the switch will close when power is applied to the coil. The other type is normally closed which means the switch circuit will break when coil power is applied.



Finally there's a variety that covers both bases, it can be use to make or break a circuit and it's called a single pole double throw (SPDT)

---

Okay, as mentioned the word 'pole' refers to a set of contacts.


Relays can contain more than one pole, usually 4-poles is the max unless it's a special relay. Usually when the relay operates (coil power is applied) all the poles change state together. There are also special relays that have early and late operating contacts but they're not generally used in simple circuits. Shown below is a 2-pole (double pole) relay, it's full name is double pole double throw but it's know as DPDT to it's friends.




We've covered all the switch combinations commonly found in relays so that just leaves the coil. The coil is actually part of a solenoid, when it receives power it causes magnetism which in turn operates the switch. Coils have two ratings you need to worry about immediately. Firstly is it an AC or a DC coil, secondly what voltage does it operate with? This info is almost invariable written on the coil itself and on the body of the relay.


So when you think about a relay you can consider it as two separate circuits, one circuit is the coil which is going to cause the relay to operate, the second circuit is the switch(es) which are supplying or interrupting power to another device such as an LED floodlight in this case.

 

[Floody]

So ill assume the relay is wired into the fence so when the fence becomes live it closes the relay switch causing the circuit that the beacon/lights/buzzers are on to become active?

 

[Resu]

Not done anything with relays/contractors but recon I'd manage, its the override switch that's throwing me. Would go more in depth but on iPad just now.

 

[Happy Jack]

Jesus that's a rough one. Ill leave this one alone and see what others come up with

...but I'd like to see more fault finding questions from our mentors, pictures with faults that we can identify...

You asked for it, James. Aha!

No, in all seriousness, thanks Marvo for putting this up, really appreciate the time. I couldn't answer this off the cuff, but I'll try and return over the next day or two with my effort, as laughable as it may be.

 

[Sintra]

As with any test read the question carefully until you fully understand the requirements of the circuit.

 

[Marvo]

The way to approach this is to start with the most simple circuit and the modify it stage by stage to accomodate all of the requirements.


So we want the supply that goes to 12vdc beacon lights when the alarm activates to be able to switch the LED light on. So lets start with a circuit where one of the beacons will switch the floodlight.

We know the beacon is 12vdc so we can take a supply from one of the beacons to a 12vdc relay. We can then use this 12vdc relay to operate the floodlight.

The circuit would look like this;



Now we know there's 2 beacons and they operate with different alarm conditions. If we take the supply from both beacons to a single relay were going to cause both beacons to illuminate together so that's not an option. We now need to modify this circuit to allow either beacon to make the LED floodlight come on without making the other beacon light up when it shouldn't. I'll give you a clue, you're going to need two 12VDC relays.

 

[Floody]

After reading it a bit more I can see that you ONLY want the floodlight added. My mind has been going nuts because I first read it as you wanted us to design the whole thing... (You should see my notepad right now).

It doesn't seem so bad now, my apologies. So just the floodlight added onto the alarm?

 

[Marvo]

...I first read it as you wanted us to design the whole thing... (You should see my notepad right now).

Lol, maybe we should start over. Yes, the electric fence system and the alarm beacon lights are already there. He wants to add a floodlight to the existing system.

The first drawing in the opening post shows the existing layout and I added a din rail enclosure for the (yet to be decided) floodlight controls and I've shown the floodlight itself with its supply cable.

Don't underestimate the task though, the floodlight controls are already a bit complicated with the timer and reset button requirements. I've also got a sneaky feeling he's going to want to incorporate a daylight sensor later on as well but we'll cross that bridge when we get there

 

[Floody]

I'm trying to do it on my notepad still it's annoying the hell outta me now. At this point, I've added a second relay to the other beacon, I seem to think you have to link each relay out to not set the other beacon off. or am I wrong?

Heres a pic of where I am.... But whether I'm on the right track or not I have no idea.

 

[Marvo]

Not done anything with relays/contractors but recon I'd manage, its the override switch that's throwing me. Would go more in depth but on iPad just now.

Don't worry about the override switch for now if you have a circuit that allows either of the beacon lights to operate the LED flood then you can share that with us. I'd bail the ipad if I was you and go for the more traditional pen and paper

 

[Marvo]

I'm trying to do it on my notepad still it's annoying the hell outta me now. At this point, I've added a second relay to the other beacon, I seem to think you have to link each relay out to not set the other beacon off. or am I wrong?

Heres a pic of where I am.... But whether I'm on the right track or not I have no idea.

There's a couple of minor hicups with the circuit. On the 12vdc control circuit you've linked both beacon lights together so if one of the beacons operates when a fence zone alarms then both beacons will come on together. The beacons have separate 12vdc supplies from the fence control box and need to come on independently to indicate which zone is alarming, linking them together will not allow this to happen.


The second problem is on the 230v side. The neutral directly to the floodlight is fine but the live needs to be switched to the floodlight when either relay operates. The switch contacts of the relays need to be wired in parallel.


Think about a basic light circuit with two switches. If you wire the switches in series you need to operate both switches for the light to come on. If you wire the switches in parallel then either switch will operate the light. These are referred to 'AND' and 'OR' circuits and form the basics of boolean algebra which no doubt you'll be taught at some stage.

 

[Marvo]

We'll have another brainstorming tomorrow evening. I think that now the ball is rolling I should be able to give the client a decent progress report on thursday and maybe even get a stage payment for the early design phase.

 

[Happy Jack]

My attempt at a circuit will go up later tonight. I do have a question, though, regarding the preset time for the floodlight to remain on. What came to mind when I read the brief was that that end of the circuit would need to be fed via a 555 timer, a watchdog timer or something similar and a latching relay. That seemed a little excessive for the exercise. And I've since read that such circuits have the tendancy to fail over such relatively long periods of time.

However. This time-delay relay set to interval mode from RS appears perfectly serviceable. Can someone confirm that it is? Clearly a cheaper alternative will need to be found.

Buy Time Delay Relays Timer relay switchoff delay 2W CT ARS 22 ABB 1SVR630120R3300 online from RS for next day delivery.

 

[Marvo]

So you're already adding a timer...nice.

The timer you have linked will work but I think stick with the simplest timer possible, one with a single changeover contact like this one. Timers with multiple contacts, multiple functions and remote potentiometers can get confusing especially if you're unfamiliar with them.

As you can see in the operation drawing when power is applied the timer relay remains dormant for the preset length of time, then it operates and stays operated until power is removed again. It has a changeover contact so it can be used as a delay-on or delay-off depending which contacts you use.

 

[Marvo]

Just to explain the operation graph above;

 

[Happy Jack]

The timer you have linked will work but I think stick with the simplest timer possible, one with a single changeover contact like this one. Timers with multiple contacts, multiple functions and remote potentiometers can get confusing especially if you're unfamiliar with them.

Thanks for the link Marvo, you are quite right. I'v never had to spec a device like this, so cheers for the heads up.

 

[Marvo]

It's difficult to know what level to pitch these projects at. To be honest I thought you might have had a little experience with relays but I suppose those who haven't are just going to learn more.

Happy Jack, if you have a circuit that will operate the floodlight when either of the fence zones alarm then attach it for the others to see. You can always add a revised version in another post once you've added the timer.


I actually don't have a solution to this myself yet. There's so many ways it can be done with relays and a timer you'd run out of fingers trying to count them so there isn't just one correct answer. Different designs and circuits will all have their good points and bad points so it's more about the process of the design than actually coming up with the right answer as such.


The thought process for me would be that the LED floodlight is going to be operated by a relay. This relay would get it's initial operation from one of the 12vdc beacon lights.....but.....it would need a self hold circuit because that 12vdc supply is going to disappear when the alarm panel is reset.

So you need a relay that operates (indirectly) to a 12 vdc signal then holds itself in via one of it's own contacts (poles).

This self holding circuit can then be interrupted by a delay timer and by a manual reset button with a normally closed contact.


One other consideration is that once the fence alarm condition is reset you no longer have a 12vdc supply.....so....the relay that switches the floodlight would need to be a 230v relay and the self hold circuit would be 230v.

 

[Happy Jack]

Hey Marvo, I'm won't be at home until a little later; if my Teamviewer connection was active via my home desktop I'd retrieve my drawing and upload it now. It is reassuring to know that the arrangement you describe is essentially what I've drawn. Not that there won't be errors or suggestions, of course but...

There's so many ways it can be done with relays and a timer you'd run out of fingers trying to count them so there isn't just one correct answer. Different designs and circuits will all have their good points and bad points so it's more about the process of the design than actually coming up with the right answer as such.

This is why this particular exercise has been brilliant, so again I really appreciate your efforts. Once my drawing goes up I'll respond in kind, as it may not be terribly clear.

An edit just to add that I may have had an advantage. Our last unit we covered motors and motor controls in some depth and, as such, I have the DOL circuit etched into my subconscious. It was the arrangement there between contactor and N/O, N/C contacts that informed my response.

 

[Marvo]

Okay so once I've got my head around the requirements I usually draw a plan on paper. The plan isn't a circuit diagram it's just a very rough schematic of who's who in the zoo. It might suggest what components are going to be used and their relationship to each other ie what is controlling what.

Here's the 'Plan A' schematic that I've come up with;



It's called 'plan A' for a good reason...mainly that Plan B and usually Plan C end up superseding it at some stage.

What it shows is the two 12vdc alarm beacons on the left which will each operate a relay. These relays are configured so either of them will cause the 230v relay that supplies the floodlight to operate. The 230v floodlight relay will have one of its contacts supplying its own coil, this is a 'self hold' arrangement. The self hold circuit will include the timer contact and the reset-button contact both of which are normally closed.

And that's as far as the plan goes. Next step for me would be to split the circuit into two pieces, the 12vdc side and the 230vac side. I'd start drawing these circuits separately to start with, then when it looks like they both might work I'd look at connecting them together.

 

[Happy Jack]

Haha. Even down to the orientation of the relays...

I'll try and upload my drawing later. Thanks again Marvo, that really makes it crystal clear.

 

[Marvo]

.... Our last unit we covered motors and motor controls in some depth and, as such, I have the DOL circuit etched into my subconscious. It was the arrangement there between contactor and N/O, N/C contacts that informed my response.

The way a DOL starter self holds is very similar to the way it would work in this circuit. It's great to see you're using existing knowledge and adapting it to new challenges, this is what designing new systems is all about. You already know all the bits and pieces it's just a matter of rearranging them into a new bigger picture.

There's no mad rush to get your circuit finished, I'm hoping some of the other tear-aways like Floody and Resu are going to pitch in with an attempt as well

 

[Happy Jack]

Below is my first attempt along with a number of products that are referenced to it. I shall run up this materials list over the next day or so, as well as explain my circuit and reasoning in a bit more detail, even if it is proven to be incorrect.

What struck me is that, in fact, the N/C override switch could be located in a number of positions. But something irks about where it is currently, and I just can't place it. Anyway.



12V-R1 & 12V-R2 -
PT2S7LB2 - TE CONNECTIVITY / SCHRACK - RELAY MODULE, DPCO, 12VDC | Farnell United Kingdom

230V-R1 -
RT3S4T30 - TE CONNECTIVITY / SCHRACK - POWER RELAY, SPDT, 230VAC, 16A | Farnell United Kingdom

230V-TR1 -
M1SMT 24/230V - BROYCE CONTROL - TIMER, MULTIFUNCTION, 6 FUNCTION | Farnell United Kingdom

 

[Marvo]

Wow, looks impressive!! I'll have a good look at it later when I have some spare time.

What struck me is that, in fact, the N/C override switch could be located in a number of positions. But something irks about where it is currently, and I just can't place it. Anyway.

Often the final layout decision is influenced by the layout of the existing system being retrofitted or even keeping cable runs short as possible etc. These decisions can often be made later on in the life of a project, even as late as during the actual installation. Like I said there's not necessarily just one correct answer.

Good work Jack

 

[Marvo]

Okay, looking at your circuit when either of the zone alarm beacons get 12vdc power it operates one of the two 12v relays. Both of these relays contacts are wired in parallel (both the common contacts have a permanent 220v supply) so it works as an 'OR' arrangement where either will operate the 230v relay (R1).
So far so good

Operating relay R1 will perform 3 different functions. 1. It will supply the LED flood with a 230v supply. 2. It will supply the normally open timer contact with a supply. 3. It will supply the timer to start the timing process.

You've used a multifunction timer so I'm going to assume it's set so it will operate the contacts immediately on receiving supply and hold them for a timed period before dropping them out.


If this is the case then the timer will self hold over its own contacts and those same contacts will keep the LED flood illuminated.

Any time the reset button is pressed the timer and the relay will both drop out as long as the initial alarm has been reset and the 12vdc signal has stopped.

It's a very nice circuit. As far as I can see it will work as per requirements. Well done and congratulations on a successful circuit.

The only thing I would suggest is that you could get rid of the relay R1 entirely. If the timer contacts operate immediately on power-up then relay R1 could easily be made redundant with very minor changes, the only purpose it's serving at the moment is to supply the timer with power. I've added a clue in the drawing below.

 

[Happy Jack]

Yes I see it now, thanks for the pointer, Marvo. With your comments, R1 does appear redundant. I should have reproduced the DOL a little more accurately, in hindsight. And I think that was what was bugging me.

Thanks for all your efforts. Will come back early next week with a price. Haha.