Want to operate a relay from a change in input; hi to lo or lo to hi

Hi community, please help me with a simple circuit for an alarm. The input is from a limit switch activated by a rotary cam. The input can initially be either high or low but the output to the relay must be provided when the input changes, i.e. from high to low or low to high. The relay responds to the first change only and ignores subsequent inputs. Simplicity is key!

Things can be too simple. You don't indicate a way to turn the relay off once it's on. Maybe a simple push button would do.

I'm not sure what you mean by "ignores subsequent inputs". If it's already on, what could it do? If you turn it off manually don't you want it turn on if there's another change?

There are lots of ways to do this.

I'd use a dumb-as-dirt PLC. Automation Direct has one that will do nicely for under $100 with programming software; but that requires you to learn logic ladder programming (not THAT hard but the really cheap PLCs have a smaller instruction set so they can be confusing); or find someone to program it for you. I could write a program to do this but it's too early in the morning to think too hard about it now.

If you want to use only relays then this is not that simple. It has bunches of parts and wiring and will cost lots more than a PLC. PLCs were designed for this sort of thing.

An impulse relay outputs a pulse when the input changes from off to on. You either need an impulse relay to trigger on the change from off to on and either a second impulse relay to trigger on on to off or a circuit to swap the input to the second relay, or an impulse relay that can be set to trigger on either off to on or on to off.

Impulse relays are expensive. A swapping circuit can use a relay that is powered when it is off, and the input is of and a pulse is output, and the power is interrupted when the input is on and a pulse is output. This could probably be done using a common 4 pole double throw relay and both the n.o. and n.c contacts from the impulse relay.

I think I can make this circuit but it's fairly complex and it'd be a bother. A PLC would be much better.PLCs were invented because complex relay logic makes your brain hurt. If you're sure you want to use only relays, I can figure this out. Let me know.

The output from the impulse relay latching relay goes to a latching relay that latches on an impulse and unlatches from another input, such as a push button. This circuit can also be done with two simple relays wired as a flip/flop, which would be cheaper. Again I can draw this circuit if you want.

This is a lot of relays and wiring. It would be MUCH easier to use a PLC. A small microprocessor could also do this but then you would have to have an output to drive the relay. The output could even be sized to output a high current (AC or DC) and skip the output relay. This requires finding or making a suitable controller and programming it, much harder than using a simple PLC.

- Stephen

Yes, it is unclear how the relay "resets" between events. Or does it toggle? What you are describing is an "edge triggered" event. So, which is it:

Case 1a:
* Rising edge > relay closes, then opens by some other undisclosed event
* Falling edge > relay closes, then opens by some other undisclosed event

Case 2a:
* Rising edge > relay closes
* Falling edge > relay opens

Case 3a:
* Rising edge > relay opens
* Falling edge > relay closes

Or when you say, "the relay responds to the 'first change' only" do you mean it responds only to a falling edge event ("high to low"). If so, then are one of the follow your scenario:

Case 1b:
* Falling edge > relay closes, then opens by some other undisclosed event

Case 2b:
* Falling edge > relay opens, then closes by some other undisclosed event

Case 3b:
* Falling edge > relay opens
* Falling edge + t > relay closes

Case 4b:
* Falling edge > relay closes
* Falling edge + t > relay opens

Thank you for replying Stephen. Turning it off is not really a concern; I was thinking of using a pushbutton. PLC was the last thing I had in mind, although I've done a course in that. Let me say, I'm wary of overkill; Arduino also came to mind, at < $100. Your advice is sound but the price tag is intimidating.

About using only relays, I'd like to stick to just one. There must be a simple way to do this. Supposing the input was simply a low to high voltage, eg. involving the N/O contact of a magnetic door switch. This could then drive a relay directly. The actual scenario is just one step more advanced.

What I had in mind was logic gates, logic latches, perhaps an adder. Otherwise an LM741 possibly as a comparator. This hombre's gut feeling is that since all that's involved is a change in voltage, whether up or down, there must be a simple circuit buildable to sense this and consistently give the required output.

Thank you Steve for your detailed reply; perhaps my post to Stephen will make it a bit clearer. Case 1a describes what I'm aiming at.

the problem with logic gates, logic latches, perhaps an adder is that there's lots of other stuff. These parts take at least a nice regulated power supply and drivers to drive a relay. They also have to be wired and mounted. If you want this to work for awhile a breadboard won't do. A simple in-circuit programmable processor would be easier, possibly cheaper and probably easier to prototype. It would also be much easier to change if your logic was faulty.

A PLC would take care of most of your issues. It has a power supply, it wouldn't have to be breadboarded, you wouldn't need a case, or a regulated power supply and it would be easy to change the program. All said and done less than $100 (I thought it was $70 but I might be wrong) might be cheap.

Yes there is...

U4 & 5 is a CD4013B with R tied to ground and S tied to the switch. [~CLR is same as R]
and U8,9 & 10 is a CD4093B

The easiest way to achieve this is using embedded micro

Then it doesn’t matter what state your port pin is You simply programme the input port to detect what state the pin is when you first initialises Or you can use the for example " interrupt on change port method" , which is a standard feature of the pic12fxx , pic 16f88 series

All explained here http://ww1.microchip.com/downloads/en/appnotes/00566b.pdf
Write your method for activating relay when port changes state in the interrupt method This will be outlined depending on what compiler you use

Thanks for the additional input Steve and Mark too for your contribution to the thread. The CD4093B costs about $20 which is about the price of a PIC 12F629 programmer. That Pic, itself, costs less than a third of that amount.

Thus, using logic gates as posted by Steve is both expensive and a lot of work but ready to roll. Using a Pic, as Mark described, requires a little hardware, software and learning but seems the better option for me.

Where do you get $20?!? It's 48¢ at Digi-Key [unit quantity - p/n: 296-2068-5-ND] and 39¢ at Jameco!

http://www.ebay.com/sch/?_nkw=cd4093b
but taking a closer look, it seems to be a package. So your circuit's viable as a once-off production and simple enough too. There's an element of risk, however. Supposing the cat nuzzles the circuit while nobody's looking and fries a CMOS NAND gate. The Pic route is more attractive to me.

Well, then don't buy them at eBay.

And, once the part is in circuit, it's no longer vulnerable to ESD. Also, if the parts are properly stored in anti-ESD packaging, they will be fine, regardless of clandestine fur.

Hi applogies for not being able to get back to you this week Lack of credit on phone Moving on I've drawn a flow chart for you using edraw max available from www.edrawsoft.com so that you can transfer the algorythem to langauge of your own choice for pic micro or any other processor of your own choice This explains, I hope says I, may not be 100 percent, but should give you at least rough idea on how to achieve this

I have based this on the pic12f675 or similar so that you can move to a solution

See image below

Also I should add the gosub state changed is where your relay is switched on

This should be handled by a separate function Also not forgetting that you would want to reset relay in which case you would add another function which detects switch push to reset relay

This could be handled by a timer interrupt or hardware interrupt so as to reset relay to open state just as you would possibly achieve with an alarm system for zone reset

Just about to write code myself with a simple test circuit to check flow chart algorythem but will post shortly with schematic and code for you Thats if you want an answer,besides which I dont have anything to do on a friday Boring, boring

Yes terrible state of affiars so alas my amusement is to see if I can achieve what you want and meet this challenge

Dont say it Ill say if it for you uhm. " get a life perhaps", hmmm possibly correct, whish it were possible right now but circumstances dont allow this So here i am attempting to improve overall status, disposition in life

23:50hrs Just finished schematic and have included dsn file in proteus proffesional format for you plus schematic in pdf format

Have to wait till Sunday for code etc Check the diagram etc also included is parts listing

See attached 7zip file

Diagram below

Im going to handle relay reset or zone reset using hardware interrupt on GPIO.2 which is the gpio interrupt pin but may change schematic slightly

This should make this clearer now If you have any corrections that you can see let me know

Mark www.harrington.force9.co.uk
Till Sunday, Monday have a good weekend

Time 00:34 15-12-2012
Here is revised flowchart with interrupt procedure handled by gpio.2

Thank you Mark for all your trouble. Sorry about the silence; was away for Christmas and beyond.

The flowchart is very instructive. Your willingness to adopt my challenge as your own is commendable. Perhaps you'll face a future task which will be all the simpler because you got your hands dirty scaling other people's obstacles. You couldn't have made better use of the time.

As suggested earlier, I'm inspired to learn to use Pics. Although other methods may be simpler in some ways, the experience and knowlege gained can be applied elsewhere. Regarding this project, the customer's said wait, which buys me some time. The first step will be to build a Pic programmer (possibly JDM) and buy some Pics.

Thanks too for the contents of the .7z file although I haven't opened it yet. Am using Linux and must repartition before downloading further software. Linux readers of this thread can do the following to unzip .7z
[code]sudo apt-get install p7zip-ful[/code]
l In case you're wondering, I'm using a desktop --with a serial port.

Welcome

Hope you had a good xmas and im pleased its encouraged you to want to learn more Thats a good sign which which for me is equally as pleasing


We could do with a few more people around like you it would be even more encouraging from all angles

With reference to pic programmers have a good look at pickit3 which is compatible with mplabsX Id have a look at microchips website possibly download this although im not sure if its linux compatible You will have read up on this

Im about to set up a linux box in the near future but having no internet connection plus a whole other manor of hurdles is proving difficult at the moment as you need to download all other updates packages etc to get started including gcc gtk, gimp etc etc which looking at the market plus microsofts charges This may be the way to move for many people with these difficulties


Not that id do away with windows but. ...... Well say no more here windows 7, 8 then office and Im starting to think why when you can have free office free ide' ,free compilers, free java, free webservers, turn your box into a router, print server, no issues with liscences Could be the big answer for bussiness Low cost you see which is what this is comming down too How do we save money but produce good quality engineers and provide training without the costs of courses some £800 plus per course just to be micosoft approved Getting silly some of this especially when you are supposidley trying to create employment

On the not so serious side would it be advantage to become dear old uncle scrooge, well Better than being mickey mouse or donald duck It could be another quacker lol

Patiance i guess it will move soon enough

In the meantime good luck and hope to hear from you soon

Try this not finished this yet but you can have a look see
http://www.eeweb.com/project/mark_harrington

This should follow two posts down but no reply button was available.

Sorry to keep everybody waiting, but here's the conclusion of the matter. Finally, I got to see the files compressed with 7z; it's nicely done. However, the client's selling his house rather than going on with the alarm. Greener grass yonder? Would seem to be. Nonetheless, it's been a good learning exercise which is far from over.

Enquiries about Pickits turned up nothing within spitting distance, i.e. 400km radius. Pics can be had, although one salesman offered guitar picks! Seems better though-- and Scrooge-like-- to use the Arduino Uno I already have with a breadboard and components to do the programming. There's a Pic12 howto which could help me find my fee /(here.)":http://www.reenigne.org/blog/a-pic12-programmer-from-an-arduino/ Enjoyed your site too, Mike but don't yet see myself building my own programmer from scratch. The bistable Pic12 circuit would be very useful and is my first Pic pick.

Hi Stephen

Have a look at the projects page I’ve put a project on this site already tested that will do this for you all code pcb layouts included

You might need to change the input to your relay for your switching that’s about it hex code is there as I said the whole project plus flow chart diagrams showing how it operates

http://www.eeweb.com/project/mark_harrington/single-pic-micro-single-shot-bi-stable-relay

For a programmer you want a pick kit 2 or 3 available from ebay or go to digi key, Rs components Farnel or similar

That should solve your problems simply obtain mplabsX IDE www.microchip.com Free

Enable your programmer and select com port via the ide

Set the ide for your chip via the settings tab top of ide

In other words select your chip example 12f xxx

Read the chip

*NB !!* Dont forget to do this first


Make a note of the value at address 0x3FF This is important as it contains a calibration value for the local internal RC oscillator

Dont be tempted to wipe the chip unless you have done the above other wise its recalibration time NOT DIFFICULT but also NOT EASY !!

Import your hex file, example from the main menu its File, Import and browse to where hex file is located , Click open or Ok

Blow the chip

Should work a ok
If not, ask, I blow a chip for you and send it to you via post inclusive PCB

Any doubts what so ever contact me and Ill see what i can do to help

Thank you for all the input and thank you Mark Harrington for the offer to blow a PIC. At present, I'm trying to program a PIC16F628 using an Arduino Uno, simple shield and Ubuntu Linux over USB.

"Your text to link here...(The post is here
"http://www.eeweb.com/electronics-forum/program-pic-with-arduino-shield-linux/)"