24 VAC circuit alarm.  Ideas to convert to digital? - Page 1
Forum section: electronics / beginners

author: mpl00
author_rank: Newbie
timestamp: April 08, 2015, 07:37:00 pm
content: Hello,I am rather unsure where to begin with this project as I am not well-versed in electrical wiring; however, I am very well-versed in network engineering and the likes so hopefully I am able to follow along.Consider the following PDF:http://www.alpinehomeair.com/related/Crown%20BWC070_090_120_151_sola_installation_manual%2012.2012.pdfEDIT: JPEG snips of relevant portions of PDF attachedBeginning on page 59 you will see Alarm Contacts in figure 10.3 along with an explanation of its functionality following on page 60. Figures 10.5 & 10.6 further detail the wiring.I have been having issues with my boiler entering a "hard" lockout phase - most often, naturally, while I am out of state on vacation - which requires manual intervention to reset. This results in the heating system shutting off and the house dropping to unsafe temperatures.What I am hoping to accomplish is to develop some sort of alarm which, when triggered, will send an alert SMS/Email to myself. First things first, what would I (or an electrically inclined friend) need to create a crude trigger that would, say, illuminate an LED to verify the circuit works? Second, what methods exist to convert this signal into a digital signal which I could then interpret using a network-attached device?Please ask for more details or explanation. As you can tell, I'm not even sure where to begin!Thanks in advance

author: SL4P
author_rank: Super Contributor
timestamp: April 08, 2015, 09:12:22 pm
content: Can't see the PDFs...Have to register on the vendors site...You may get more help if you snip the relevant part of the circuit and repost here...

author: mpl00
author_rank: Newbie
timestamp: April 08, 2015, 09:20:08 pm
content: Quote from: SL4P on April 08, 2015, 09:12:22 pmAhh, thank you for the heads up. I've edited the original post to include JPEGs

author: dr_dan
author_rank: Contributor
timestamp: April 08, 2015, 09:32:42 pm
content: The alarm contacts are just a switch - there is no voltage on them (the 24vac in the subject line is therefore misleading!).The contacts are rated at 24vac maximum. You should be able to hook up a couple of AA batteries and a torch bulb for example, and see the bulb light when the boiler enters its lockout state.If you've got a computer nearby to send the message to you, you just need to hook the contacts into that computer somehow. GPIO on a Raspberry PI would be a contemporary way to do it, and you should find no shortage of online tutorials showing how to connect a switch to the Pi.Another possibility would be to connect those contacts across one of the buttons in an old mouse, and set up your computer to respond to that mouse button click by sending a message to you.Good luck!

author: Ian.M
author_rank: Super Contributor
timestamp: April 08, 2015, 09:44:41 pm
content: The third jpeg (the short paragraph of text) says the boiler already has an alarm output, as a pair of low voltage contacts that close when the boiler enters a hard lockout state. They are intended for connection to an alarm system.All you need to do is to hook them up to any sort of I.O.T network device that can monitor a simple switch closure, just like a N/O pushbutton.However for your stated aims it would probably make more sense to get either a standalone landline speech dialler or a GSM SMS dialler from a security alarm parts company, power it from a regulated 12V PSU and connect the boiler alarm contacts between +12V and one of the dialler's trigger inputs. Then you program your mobile number and a suitable message into the dialler and test it by shorting out the two alarm terminals for a moment which should ether call you or text you depending on the chosen dialler.

author: mpl00
author_rank: Newbie
timestamp: April 08, 2015, 09:54:28 pm
content: Oh. Well I feel this makes things a lot simpler. I clearly misunderstood the short paragraph of text which has sent me on a wild goose chase for the last couple of hours (admittedly, reading/learning a fair bit along the way). So, rather than 24VAC being sent through the circuit upon hard lock, it merely closes the circuit through which up to 24VAC could be pushed through by any source I choose, correct?I do have a computer nearby and plenty of CAT cable. By connecting one end of a pair to the contacts and the other end to an appropriate input device I should be able to get the Alarm signal from the boiler out of the garage and to an appropriate destination. Am I understanding all of this properly?

author: dr_dan
author_rank: Contributor
timestamp: April 08, 2015, 10:06:22 pm
content: Quote from: mpl00 on April 08, 2015, 09:54:28 pmCorrect! If you're happy to rely on a computer running 24/7, you can rig it up to detect the contact closure and send a message to you in some convenient form.Or if you just want an off the shelf solution, as Ian.M. suggests, get a stand-alone auto-dialler for an alarm system and set it up to call you when the contacts close.

author: SL4P
author_rank: Super Contributor
timestamp: April 08, 2015, 10:42:26 pm
content: ok, let's revisit the requirement...You now know you have a clean pair of contacts to indicate the alarm condition.(check they actually do what you need before digging yourself into a solution...!)-- a battery and LED will show you if/when the contacts are closing and re-open.Using a PC is a an added point of failure, and power hog - to simply send an alarm message twice as year... especially when the house is empty (on vacation).Not wanting to start any flame war, but one ofmanysimple tech solutions could address the requirement for well under $100Hacked cellphone for SMS, Arduino to LAN/email, PIC etc, or simply jumpered across to your monitored alarm system - or even 'whatever is needed' to restart the boiler (if that's legal).Cell/SMS require an active SIM, LAN/email require net connectivity while the house is empty (same for PC).The contacts are not 'limited' to 24v -or- AC... simply that's what they're rated for as max values. Switching 12v or 5v DC is completely legitimate (make sure the LV wiring is properly secured and isolated from any other associated supplies!). The only case where this may be an exception is where the switching element is an open-collector driver (not in your case).

author: orion242
author_rank: Unknown
timestamp: April 09, 2015, 04:15:10 am
content: Might want to look at Mamac they have some pretty cheap devices that can monitor this and shoot an email.http://www.mamacsys.com/IP-Maverick.htmlFixing the root problem with your boiler might be a better idea. Its going into a hard lockout for a reason and if ignored may result in cost repairs latter.

author: mpl00
author_rank: Newbie
timestamp: April 10, 2015, 12:03:21 am
content: Thank you for all of the input. This has sparked a lot of interest and reading re Arduino & Raspberry Pi if nothing else!Any idea if an Arduino's 5v output be able to span a 100'+ run of CAT cable in order to detect switch closure?ARD5v---------C1_| \ARDGnd-------C2¯C1 / C2 = Alarm Contacts

author: SL4P
author_rank: Super Contributor
timestamp: April 10, 2015, 01:12:11 am
content: Probably will work, but you need to factor in the voltage drop (use fatter copper) - as well as introducing interference back into the Arduino.Don't put any long distance voltage on the switch at all.Simply add 2K2 pull-up to +5V at the arduino, and detect the contact closure to 0V. That will also tell you if the wire goes open-circuit (as mentioned below). Simple opto isolation is an easy way to protect from unexpected voltages.Also worth considering if this is all going hi-tech, is to provide a sense (or N/C contact pair)atthe boiler - so that if the boiler loses it's power supply (while alarm is still powered), or if the alarm wires are broken for any reason - that will count as an alarm condition.The best alarm contacts are designed as N/C contact pairs to support fail-to-fault notification, and open-circuit alarm wiring.

author: Ian.M
author_rank: Super Contributor
timestamp: April 10, 2015, 01:41:47 am
content: I certainly wouldn't run 100' of cable with simple contacts on the end and expect to be able to detect the contact closure as a low current 5V logic signal. That's just asking for trouble.The first thing to check is if the boiler's alarm contacts are closed with power off. If so, it makes total failure detection a lot easier as you wont need to add heat sensors on the output pipes or optical sensors over the boiler's status LEDs.Its probably best to put the Arduino near the boiler then use a digital signalling system designed for long cable runs to connect it to whatever you have 100' away. If you've got a CAT5 or better cable run, why not go straight to Ethernet right next to the boiler? The 1.5KV isolation that typical 10/100baseT Ethernet magnetics give you will prevent a lot of problems.

author: SL4P
author_rank: Super Contributor
timestamp: April 10, 2015, 02:03:03 am
content: Quote from: Ian.M on April 10, 2015, 01:41:47 amPretty much agree... the 100' run isn't all that bad in a domestic environmen with decent cablet, and detecting 0v will be a lot more reliable than detecting a positive voltage contact in most environment.

author: Ian.M
author_rank: Super Contributor
timestamp: April 10, 2015, 02:23:32 am
content: if you use very low voltage/current you get to find out about dry contact reliability problems. A good rule of thumb is that if you don't know if the contacts are precious metal or mercury wetted (or even electronically switched), to design for 12V open, 10mA closed. It avoids a lot of trouble . . . .