relay behavior

And if anyone wants to know anything about radiated and inductive interference, let me take you back one day to suppressing (or trying t) deal with) a 70s Lotus Europa or nearly any Renault or Fiat in those days, even down to bonding the exhaust! There's a classic post by Mr. Idiot about finding out about the spike in a painful way.

Hello Folks, the alarm does what it is supposed to do when i push the button, im just worried about the + signal sent back to the cpu of the alarm. that is why i wanted to get rid of that +. i just wanted to be precautious about not frying the cpu. and for the signal sent, one is latch, one is 90 sec timed. Eric so, no way to prevent that + effect on the relay? and no danger to cpu?Eric

Not if you diode it as shown. Having said that there are two schools of thought, one says modern units are protected internally but I won't take the chance. Done it myself and seen it happen on this site.

gargouille - if the alarm output is intended for a relay, it should have its own internal protection - namely a reverse biased diode from that output to its +5V or +12V supply. As to the +12V on the relay coil, it is +12V because its other end is connected to +12V and no current is flowing. When you ground that "switched" end, it becomes 0V (GND) and will have a current i flowing thru it's coil resistance R where iR = V = 12V (Ohm's Law), or R=V/i = 12/.2A for 200mA => R = 60A or higher for a current of 200mA or less to flow if 12V. Hence my comment earlier about a relay coil of at east ~70 Ohms or rated for under 200mA at 12V or 13V etc. Most circuits function that way. A voltmeter will read whatever voltage an open connection is connected to. An ammeter will act as a short (closed switch) and measure the current (eg, thru the coil).