Sorry if my title is misleading its the best I could come up with.

I want a relay that is normally on, and when it receives any electric impulse it turns off and stays off until manually reset.

Most relays are on when receiving current and off when not receiving current (or visa versa). When mine receives current I want it to turn off and stay off even if it loses current.

The project, if you must know, is a security system. I got a $70 IR beam detector from radioshack that you mount at the end of your driveway. It rings a little chime if a car or person enters the driveway. I'm trying to modify it so instead of ringing a chime it flips a relay to shut off power to my speaker system. I'm throwing a party on new years and I want the music to turn off automatically if the police show up. It would also be nice if I could find a relay that works the exact opposite to the one I described, always off, and when it gets a signal it turns on a stays on until manually reset. I could use this to turn on a red warning light inside to the house so I know someone is there.

If no such relays exist, what kind of circuit do I need to make a relay function like that?

To make the relay work with the speaker system I was going to butcher an extension cord and place the relay in series with one of the wires then electrical tape it back together to cover any bare wires. That way I really have a 120v AC wall socket that is controlled with a relay. I could plug anything I wanted into it. But would that really work with AC power? I think it would just turn the speaker system on and off very quickly. Too quickly for the amp to warm up, but it might damage the system. Perhaps I should have it flip two relays. Cut and extension cord in half, attach a relay to each wire in series, then tape it back up. So when the relays are flipped its equivalent of cutting the extension cord in half and would kill all power.

seems like you need a 'latching relay'

It seems like you sir are correct

I'm looking on the radioshack website and I can't see any. I need it by Sunday. No shipping on the weekends and today is friday so I really need to buy it in a store.

Is there anyway to achieve the same effect with regular relays using only ratshack parts?

I'm drawing something up for you, should be pretty simple ..

http://i40.tinypic.com/2e48zye.jpg
Ok, this *should* work, you'll need a single pole double throw (spdt) relay, preferably with a 120v(or 230 depending on where you are) coil, otherwise you'll need to use a transformer to drive the coil, and instead of the 'load' you'd have another relay which would connect to your load ... you connect your load (whatever you're running from it) to the leg that will be 'on' when there's no power to it, and the 'alarm' to where it will be otherwise. The 'alarm' will need to be relatively high power (a few big lightbulbs should do fine) to keep the relay 'held in'. Where the 'alarm' is connected you loop that back to just below your sensor relay to give power to the coil, which keeps it latched off, you have a 'reset' switch in series with that to turn it back on again ..
Hope that makes even a little bit of sense,a bit tricky to explain ..

Alright, I'm a little confused.

At the top left there is a little circle thing, what is that?
The hot 120v AC?

The 120v spdt relay is the the item in the dashed box, correct?
So what is the sensor relay? A google search tells me its an inline relay that activates when the voltage reaches a certain level. So that will react when the circuit receives the impulse? So is the circle thing when I connect the positive lead of the impulse?

The load is the item that will lose power? (aka the speakers?)
The alarm is the item that will gain power? (a couple lightbulbs)


Thanks for all your help by the way

Alright, I'm a little confused.

At the top left there is a little circle thing, what is that?
The hot 120v AC?

The 120v spdt relay is the the item in the dashed box, correct?
So what is the sensor relay? A google search tells me its an inline relay that activates when the voltage reaches a certain level. So that will react when the circuit receives the impulse? So is the circle thing when I connect the positive lead of the impulse?

The load is the item that will lose power? (aka the speakers?)
The alarm is the item that will gain power? (a couple lightbulbs)


Thanks for all your help by the way

Sorry for not being a bit clearer ..
I was assuming that your IR thing you were going to put at the end of your driveway switched a load on and off, I guess you could use a standard PIR sensor to do the same, pretty much at that point you need something that'll switch on and off .. I just meant 'the relay in the sensor' there .. now with your 'chime' thing it really depends on what the output is , if it is sufficient voltage then you might just be able to put it straight to the relay coil .. and yes the relay is the thing in the dotted box.
The circle at the top is your incoming power the hot AC, at the bottom is the neutral lead .. The load is whatever you want to power, the alarm is what would activate when your load goes off ..
It is quite hard to explain over the internet, it's the sort of thing I could probably whip up in a few minutes if I had the parts but it's much harder to explain to someone else :)

Very generally, you need it so the sensor trips the relay, which then powers ITSELF, keeping it at a steady state, until an actual physical switch is hit.