Alarm wiring

[tonkski]

There?s probably a more specialist group for this question but I?ve never failed to get a helpful answer here... I?m connecting my old house alarm to a new interface unit which talks to the internet. The interface unit isn?t specifically designed for my ancient house alarm. The house alarm outputs +12v on its interface when the system is armed and I?d like the interface unit to act on this. However, the interface unit is expecting an input pin to be connected to ground when there?s an event. I?d thought to achieve this using a 12v relay which will connect the interface unit?s input to ground when the house alarm is armed. However, is it a bad idea to have a relay ?on? 24/7? There?s all sorts of relays, modules, optocouplers etc available which might do this but what?s the correct device for this job please?

[T i m]

Depending on the design of the output, it mast be able to sink and source some current therefore you might be able to connect your (12V) relay between 12V and the output pin, meaning it will only be 'active' when it's unarmed?

You could use an opto-isolator equally easily (and retain some isolation between units), but you might need a few more peripheral components. You might find a little PCB / module designed for interfacing an Arduino easier to rig up.

Cheers, T i m

[tonkski]

Thanks Tim. The house alarm is configurable so that I can instead have the 12v removed when it?s armed. Trouble is, especially during lockdown, the system can be unarmed for days on end. I?ve seen the arduino modules - can they be left on full time?

[Martin Brown]

Do you mean armed or tripped?

Most alarms have several states, eg mine has

Daytime/Off - optional door chimes Nighttime (external doors only) Armed Triggered - Silent alarm Triggered - Alarms ringing

If you just need to invert the sense of a 12v output that needs to be converted into a pull down to 0V then a simple transistor should do it. Pull up resistor say 10k to +V and 100k base resistor to the 12v trigger signal. Your insurer may not consider a jerry rigged alarm acceptable.

Just check that the units do agree about the value of 0V first!

[tonkski]

Thanks Martin. I?d meant armed. There are two configurable outputs but I?m using ?armed? at the moment rather than tripped, just because it?s easier to trip. I?ve checked that the units agree about 0v by measuring the voltage between them. It was about 0.03 volts drifting about. There?s about 100ft of wire between the unit, if this matters. I?m afraid my knowledge of electronics isn?t quite up to understanding your transistor suggestion! If one of the cheap Arduino circuits is just as good I?d rather go with that.

[tonkski]

Thanks Martin. I?d meant armed. There are two configurable outputs but I?m using ?armed? at the moment rather than tripped, just because it?s easier to test. I?ve checked that the units agree about 0v by measuring the voltage between them. It was about 0.03 volts drifting about. There?s about 100ft of wire between the unit, if this matters. I?m afraid my knowledge of electronics isn?t quite up to understanding your transistor suggestion! If one of the cheap Arduino circuits is just as good I?d rather go with that.

[T i m]

You are welcome.

Could be handy.

Ok.

If you go for an opto-isolated module, as long as your interfacing is appropriate (correct driver current into the opto LED etc) then it should be not more or less happy being 'on' 24/7 than any of the other alarm components. ;-)

Cheers, T i m

[T i m]

<snip>

It would basically be the same, it just might make it slightly easier to handle, it being on a small PCB rather than just loose components.

Something like eBay item 274499061740

Cheers, T i m

[tonkski]

Ah, thanks. Can that give a ground output to the interface unit? I'd thought something like 263560749851 to connect the interface unit's input to its ground when the 12v is applied.

[T i m]

Potentially yes ... as the chances are it doesn't actually need a ground, just a voltage close to ground (that you will get with yer opto-coupler).

Well, with it being a relay then you will get your solid ground but are back to having a relay coil energised all the time the alarm is activated (or d-activated etc).

Cheers, T i m

[tonkski]

Thanks, that makes sense. Thanks for your help.