1970s alarm system - still has power but from where?

[SomedayStache]

We have remnants of a late 1970s alarm system throughout our house.  There is an old bullhorn in our attic (I guess to scare the burglar away with really loud noise???), some wires running to various windows, and a centralized control station with toggle switches and pushbuttons that would blend into the original, Spock era, Enterprise decor.

The centralized control station was plugged into a wall socket and also has wires going up into the attic.  We unplugged the wall-socket power and I assumed that everything would be dead.  But after removing the faceplate of the controls I found a low voltage at various pins behind the faceplate.  My husband poked around a bit in the attic trying to figure out where the power is coming from - but he couldn't even find the wires.

What should we be looking for?  Is there an inverter somewhere in the attic tied to the house AC power?  Does anyone have any idea what this type of thing might look like?

I really want to get rid of this old control station and now that I know some of the wires have power I'm a bit concerned because our windows were replaced a few years ago and many of the window wires were just snipped off - I'm not sure if they had any voltage on them that could cause us problems.  I'd really like to find the source of the power and disconnect it.

[guitar_stitch]

The low voltage from the AC system is not a concern.  Most likely, it's 18v or less and extremely current limited.

As far as power, open the main control box.  There may be a 12v battery that provides standby power in the event of a mains failure.

[SomedayStache]

I never thought about a battery providing backup power.  I don't remember the exact voltage we measured but I think it was in the range of 1 to 3 V.

That'd be crazy if a battery still had a charge because I can't imagine this legacy equipment has been used in decades.  I'll take a look around for one.

[Cyanne]

We had the same situation. There was a battery backup that was providing power.

[ShoulderThingThatGoesUp]

Yep, I had a fist-sized battery sitting inside a control box in a closet.

You may also have a weird-ass floor safe somewhere if it's like the 1971 house I had the security system in. It had a bag marked "$5000" but it was empty.

[SomedayStache]

I can imagine the excitement I would feel upon finding such a bag.


And then the disappointment when it was empty.


But still a hidden floor safe is a pretty fun find regardless.

[paddedhat]

Yea, a system that old might have a cabinet, roughly 1/2 the size of a modern electrical panel, with a fairly large, and fairly dead old school battery, still hooked up to a trickle charger. The batteries were very similar to the ones you find on lawn tractors and motorcycles. The way to confirm this would be to dump the main disconnect breaker in the service panel. No low voltage available means that you have a 120Volt to 12 Volt converter still connected somewhere (unlikely).  Continued low voltage means that you are still draining a very tired battery.

[Spork]

In the case of my parents' house, there was a wall mounted control box about the size of a good size wall mounted first aid kit.  Theirs was in the back of a closet.  It had a 12v gelcel in it that would run it in power outages.

Theirs is "smart" (?) enough that if the battery gets low, the damn alarm goes off.  We had a week long outage a few years back and the damn thing kept going off until I tracked it down and pulled the leads off the battery.

[SomedayStache]

The way to confirm this would be to dump the main disconnect breaker in the service panel. No low voltage available means that you have a 120Volt to 12 Volt converter still connected somewhere (unlikely).  Continued low voltage means that you are still draining a very tired battery.

These are the steps I'll be taking soon.

[GuitarStv]

I found about four or five transformers attached to light boxes in my basement (with really long cable runs up through the rest of the home) that provide low voltage power to the old (unused) security system.  They were invisible unless you pulled the light out of the box and then unscrewed the electrical box from the ceiling joist supporting it.

[Kaspian]

Is it possible an old alarm system could be connected to (and drawing some small power from) the phone line?  (Especially if it was once set to dial an emergency number?)  We don't think of phone lines as having power but they were really headed that way for awhile--all the luminescent button handsets with chirping ringtones drew their power directly from the line and not an outlet.