batt ----------------30 87--------horn-----ground
| +++++++++++++++ |
(+ are just filling space to format)
i realise the door circut here is not as you currently have it set up, however all you need is a momentary pulse of power to get it "latched" open, and i cant imagine why you couldnt just hook em up to a power source (a 12v battery might even work actually, pending on the SCR lol).
if theres some sort of timer you could put in, put it, as well as any on/off switch, between the SCR and pin 85. so that way it will function again if someone opens another door, and if you turn it off, you wont have to worry about sounding the alarm lol. im sure theres something wrong with this diagram, or i have my anode and cathode backwards (it was over a year ago and we spent maybe a day on em ok? and i hate transistors :P)
also, want to make sure the power supplied isnt going to fry the SCR, or go over the breakover voltage.
for more information, this is what i used as a quick reference: http://www.allaboutcircuits.com/vol_3/chpt_7/5.html
and remember, the simpliest circut is the best
the floor is once again open *phew*