convert Battery powered alarm to mains with battery backup?

[TonyF]

Hi,

I have an simple PIR/Siren alarm in my hobbyshop, (glorified shed in the back garden) which is battery powered (4xAA). I now have mains power to the shed (UK, 240V) and as the batteries last about two weeks I want to convert the alarm to mains power. I can get a simple kit to do this from Amazon, but if some evil tool thief unplugs the PSU the alarm will stop, so I want there to be battery backup.

Now, I'm not an electronics guy but I am reasonably practical and I understand the principles of household ring mains etc. although probably not all of the supporting maths. Anyhow, in my simple mind I feel I should be able to put four rechargeable batteries into the alarm and then take a battery charger and wire it into the circuit somehow. I'm guessing I will need something to manage the flow of current as the device is DC, a diode or something maybe? (Notice I'm edging out of my limited puddle of knowledge now).

So, is there a commercially available and cheap solution to this problem? and if not why not! mumble mumble mutter mutter. And if not, is it a simple thing to construct or should I give up and try to get my wife/accountant to approve expenditure on a powered alarm with battery at around £140? on the plus side these units have GSM so it can call me to tell me I'm being robbed, that way I can go get beat up as well presumably. Then again I'm far too tight to buy my shed a mobile phone, ask my kids.

Thoughts and advice greatly appreciated.

Tony.

 

[Tha fios agaibh]

A simple solution would be to use a relay.

Hook up your mains 240v to the relay coil and the supply side of your dc adapter.
Then wire its 6v dc output through the relays open contact and the wire from the battery through its normally closed contact.

When mains power is lost the relay will default to battery power.

The downside is you will have to periodically check that the batteries are still fresh.
If you wanted to get more advanced you could design one that could trickle charge the batteries.

 

[duke37]

I would suggest using a 6V lead/acid battery and buy or make a power supply, set to 6.9V to continuously charge the battery..
Burgler alarms which have passed through my hands use this method at 14V. A pair have been used to supply 24V to a station clock system. You may be able to get a used 12V box and put in a 317T to get the right voltage.

 

[TonyF]

thanks for the replies folks. something to think about. any thoughts on how big these solutions will be in terms of physical size?

 

[Tha fios agaibh]

how big?

Approximately 2 inches/5cm or smaller if need be.

Maybe something like this (on ebay) relay and socket, although you only need one set of contacts.

https://www.ebay.com/itm/Relay-MY4N...817518?hash=item3f80addcae:g:Nx4AAOSwNRdX-H~C

 

[ChosunOne]

A simple solution would be to use a relay.

Hook up your mains 240v to the relay coil and the supply side of your dc adapter.
Then wire its 6v dc output through the relays open contact and the wire from the battery through its normally closed contact.

When mains power is lost the relay will default to battery power.

The downside is you will have to periodically check that the batteries are still fresh.
If you wanted to get more advanced you could design one that could trickle charge the batteries.

Approximately 2 inches/5cm or smaller if need be.

Maybe something like this (on ebay) relay and socket, although you only need one set of contacts.

https://www.ebay.com/itm/Relay-MY4N...817518?hash=item3f80addcae:g:Nx4AAOSwNRdX-H~C

NIX on an AC-to-battery switchover relay to power an alarm system! Don't try to do it that way. That doesn't work for modern (since the early 80's) equipment.

Specifically, when a PIR motion sensor loses power for a split second, it needs more than a few seconds (depending on the model, up to a minute or two) to "stabilize", during which time its status will be in alarm state. It's an inherent characteristic of PIR sensors: They "watch" a mosaic pattern background and look for changes in that background; but when they first power up (after a 1 millisecond power-out) they have to assess what the background normally looks like. That can take a minute or two, depending on the PIR model.

That's why most professional-grade alarm control panels that power up in the Armed state, ignore sensor inputs for the first couple of minutes: To give PIRs time to stabilize. Otherwise, we'd have massive false alarms in every neighborhood that has an extended power outage long enough to cause the alarm systems to power themselves down.

Duke37 has the right idea: A charger feeding the battery, so that the charger & battery together keep the system supplied with power. When AC fails, there's no transient switching over.

 

[bertus]

Hello,

When using the charger & battery combination, use a diode between the charger and battery to avoid draining when the charger is down.

Bertus

 

[mww]

Start with the basics, are you using rechargeable NiMH batteries, or alkaline LR6 ?

If NiMH, then yes you keep them on charge by feeding about the voltage you get at 80% charge. That means the cells stay at 80% but you are not actually charging them unless mains has been off and you need to charge them.
Typically the alarm is going to work from ^V down to 4V, so even a 5V/100mA supply might be good to go. Diode not required.

If LR6 then you must not charge them and will supply the PIR from mains near.above the upper voltage, and the batteries have a diode in series. For this you need to know the top voltage the PIR will accept as you will be up near that level to be above the 4 new alkaline cells (ie 6V or so)

 

[Tha fios agaibh]

NIX on an AC-to-battery switchover relay to power an alarm system! Don't try to do it that way. That doesn't work for modern (since the early 80's) equipment.

Good point about the pir sensor needing to be stabilized if power is interrupted.

I figure during the ~15ms opening of the relay, the filter caps in the power supply would easily sustain the voltage during the transition.

 

[ChosunOne]

Good point about the pir sensor needing to be stabilized if power is interrupted.

I figure during the ~15ms opening of the relay, the filter caps in the power supply would easily sustain the voltage during the transition.

It might or might not, I couldn't say--but why bother? We stopped using transitioning relays about 40 years ago, and all power supplies in the field have standby battery connections integral to power supplies so that there is no transition time in the event of AC failure. It stopped a lot of problems we used to have with power-outs. I can't imagine why I would design a system with a transition relay now.
https://www.ebay.com/itm/Altronix-S...244876?hash=item216436f84c:g:HFIAAOSwYv9dqfQK
This is the first example I happened to find on ebay, but you can buy a 6V power supply with battery terminals for less than that in North America. I'm guessing he could find something similar in the UK.

I'm sorry I'm on the wrong side of the pond. I have a surfeit of old but perfectly usable 6VDC power supplies (500mA) that I'll probably wind up taking to e-scrap one day unless I can unload it on eBay. I'm guessing the cost of shipping would be prohibitive.