4 small signal diodes for the bridge or 1 miniature bridge rectifier.
1 10k resistor
1 1µf capacitor
1 IRL530
Max.

 

Would these be ok?

http://www.maplin.co.uk/p/kbpc104-2a-bridge-rectifier-aq99h

http://www.maplin.co.uk/p/irf540a-mos-fet-transistor-to220-case-n10ah

http://www.maplin.co.uk/p/metal-film-2w-10k-ohm-resistor-d10k
I cant find a 1µf capacitor, could you recommend another?

 

Would these be ok?

I cant find a 1µf capacitor, could you recommend another?

The bridge can be the smallest one shown, the cap is not that critical.
A little smaller if necessary.
You may get away with the much smaller 2n7000 in that application instead of the IRF540
Max.

 

My shopping list so far.....
http://www.maplin.co.uk/p/kbpc1005-2a-bridge-rectifier-aq98g
http://www.maplin.co.uk/p/metal-film-2w-10k-ohm-resistor-d10k
http://www.maplin.co.uk/p/2n7000-mos-fet-transistor-to92-case-uf89w
I have a few 470uf caps kicking about if these would be suitable?

Do you have a higher res image of the diagram you sent me please? I cant make out which leg from the transistor goes where..

 

The Source goes to supply common, the Gate to the res/cap and the Drain to the output (Alarm).
https://www.fairchildsemi.com/datasheets/2N/2N7000.pdf
Fig 11.
Max.

 

Spot on thanks, ill pick up the parts in the morning and get it all set up on a spare bit of vero board. Does it matter which way the bridge rectifier goes? iv never applied one before

 

 

The input is not important, either ~ in , the output is, +Ve to the Gate, and -Ve to alarm supply Common (0v).
The 450 µf is overkill, anything smaller than 1µf = .5µf is ok.
Max.

 

Sorry but the above +ve to the gate is making no sense to me. Could you label the diagram the correct way please.

 

What do you see wrong with it??
Max.

 

The +ve is connected to the pin 1 on the siren which then pin 2 is connected to the drain, the gate then connects to the cap and the resistor which is connected the the ac? thats how its showing on the diagram.

 

I had assumed you wanted to pick a relay up again for the 12v alarm, I showed a FET instead of a relay, what signal does the alarm need? I had assumed it just needed the auxiliary 12VDC?
Max.

 

so where would I place a relay on that diagram?

I have a 14v output ac and a 12v siren and PSU for it. I would like the 14v ac to somehow close the 12v circuit and activate the siren.

I need a circuit to close the section bellow

 

If I could convert the 14vac to dc then just use a suitable relay

 

If the siren just needs a contact closure or equivalent, then that is what the circuit achieves in the one I showed you.!!!
Max.

 

but in the diagram it doesnt show the +ve to the gate, it goes to the siren pin 1 first? the siren then connect to the drain via pin 2

 

Max already has the solution but if you would rather use a relay..... Both do the same thing but what Max did uses fewer parts.

 

but in the diagram it doesnt show the +ve to the gate, it goes to the siren pin 1 first? the siren then connect to the drain via pin 2

It doesn't need to go to the gate, the gate is turned on by the AC coming out of the first siren which is changed to DC by the bridge.

<ADDED> Think I see his confusion Max said "the output is, +Ve to the Gate" This is the rectified DC output of the 1st siren not the 12 volts that powers the 2nd siren.

 

Thank you everyone for the help, I built the circuit bellow, unfortunately it isn't working, any quick diagnosis I can do? Im not at the property with the sirens I just have the circuit with me.

 

Personally I would have used a FET instead of a bi-polar which draws base current from the usually higher impedance speaker source.
You also should have a BEMF diode across the coil.
Max.