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