View Full Version : Question about automotive relays?

04-20-2006, 05:40 PM
hey guys

i am working on a project where the ultimate goal is to have a few horns go off if the car door opens...the goal isnt all that important...what is important is knowing what to look for in an automotive relay...so help me out :)

i plan on splicing into the wire that runs to the little light that turns on when the vehicle door is adjar....this wire will be the signal turn on wire for a relay (i am thinking maybe 30 amps?) which will have power incoming from the battery (either directly or thru the fusebox) and sending it out to a couple of vehicle horns...

what power rating shoud i look for in the relay? is there any problems with my general explanation? what about fuses? cuz i am aware when it comes to an automotive audio amplifier, the main battery positive cable needs a fuse; so will i need to fuse the incoming battery power wire running to the relay? what size fuse to use? the link i have posted is where i was browsing different relays and i cant figure out how some of them differ, or why some 4 pin relays are better than other 4 pin relays just cuz the internal construction is a little different...someone please shed some light on this for me ...thanks!


project f-150
04-20-2006, 05:42 PM
why do you want to do this?

04-20-2006, 06:41 PM
simply put, somebody is messing w/ my friends truck...they might even have his keys, cuz they keep getting in there and messing with stuff...so we just need some way of knowing when they are breaking into it at night so we can scare em off/catch em

now what i want to do is a very cheap type of alarm that will just sound out when the door opens...i dont wanna buy some cheap 80 alarm system, i dont need all that....i wanna spend a few bucks on a relay and some loud horns and build it myself...this way i can do it for under 20 bucks...so now any comments/suggestions?

04-20-2006, 06:48 PM
A SPST relay doesn't need the trigger voltage to carry anywhere near 30 amperes. It doesn't need to have any stronger feed than an amp's remote turn-on input needs. But other than that the relay will need its own source of 12v+ and ground.

You then have the input which acts as a trigger for the relay to complete the circuit on whatever it is powering.

Then last is the output to give current to whatever is powered by the relay.

You can fuse the power input to the relay if you wish but it's a bit of overkill. The relay itself will have an amperage rating which if exceeded, will fry the relay. So in essence it acts as its own fuse.

04-21-2006, 08:06 PM
good point prowler...thanks for the post...

on some of the diagrams i have been looking at, i usually see 5 posts...one pair of posts are for the input trigger positive wire and ground....then there should be one for the 12 V positive input from the battery...then there is one that will recieve the 12V signal when the trigger is off, and the other will recieve the signal when the trigger is on? does this sound right?

05-02-2006, 08:15 PM
there are "sorta" diagrams in back of the relay package? i got mines at kragens but havent gotten around to install it yet

05-02-2006, 08:16 PM
looks like theres to outputs from the diagram that im looking at, that's where im gonna put the 2 piezo sirens, one to each post. now can anyone tell me if you can put more than one to a post?

05-02-2006, 08:22 PM
looks like theres to outputs from the diagram that im looking at, that's where im gonna put the 2 piezo sirens, one to each post. now can anyone tell me if you can put more than one to a post?
Yes you can so long as the sum total of current draw for each accessory attached does not exceed the amperage rating of the relay.

I have a 30a SPST relay that controls the remote turn-on for two amplifiers then also powers two 20" blue neon lights (one inside the car and one in the trunk over the top of my 4ch) so that's four things being run off of mine on a single output.

05-02-2006, 08:30 PM
Grrrr @ double posts when the server freaks out :pissed: