alarm to a car horn

Yep, that's the right way- what do you use to draw the schematic? Looks good. The transistor turns on whenever you get a positive voltage on the gate, so it basically switches the relay coil to ground, turning on the relay. When the gate to source voltage is greater than about 2V or so, the transistor turns on, when it's less, it turns off. When your alarm goes high, the diode conducts, charges up the C1 capacitor, which turns on the transistor. When your alarm pulse goes back low, the D1 diode prevents the cap from discharging back through the alarm trigger and turning off the transistor again; instead, the cap has to discharge back to ground through R1. If you have a larger value for R1, it does it slowler, so the transistor stays on for a longer time; once it does eventually discharge, the transistor turns off again, and the relay opens up. Also, if you have a small cap, you can see that the time required to discharge would be shorter, so the pulse is correspondingly shorter.

Oh, I notice you have the relay hooked up to the normally closed side of the contact; it should actually be connected on the normally open side. That way, the siren only gets powered up when the transistor is turned on, energizing the coil.

Handy program, I'll have to see about loading my pc up with it too. I started to try using autosketch to draw it up, but it was too ridiculous trying to draw the components out when it should have needeed just a single mouseclick- If you don't want to buy a hundred of these things to play with like I did, you could SASE me and I'll drop a couple in for you to try out; I have plenty-

It's also equivalent to a BS170, but that probably isn't any easier to find. Hell, these things may be made there, in Mexico for all we know-    You can use pretty much any MOSFET transistor, since all it's doing is switching on and off; using it to drive a relay takes very little power, so anything you can find would probably work fine. The 2N7000 and BS170's just happen to be small and cheap, since there's no point in using the expensive ones that can handle hundreds of amps and volts, when 8 cents will do the job-