Well i finally got a BOSCH relay to do my fuel pump kill and i know what all the other posts do, but i'm a little unsure about what 87a does. Could someone explain it?

 

87a completes the circuit with 30 when the coil is not energized. This is how most alarm starter kill relays work.

 

Hmmm, i'm not exactly understanding that, but is there anything that i could use 87a for in making a fuel pump kill switch?

 

Sure you can, but its very involved and depends on who does the wiring. Put it this way, you could make a whole alarm system with relays.

But for a simple fuel cut, use 87 and 30 to cut power. (85 and 86 powered with the car on.)

Not recomended at all-use 87a and 30 to cut power. (85 and 86 powered with the car off, Which will drain the battery)

 

A while ago I thought i heard that you could use 87a to plug into another switchable source, and that switch or whatever would have to be activated before the relay and switches would even work. For example, if you pluged it into the rear defroster switch, you would have to push that it, then flip the switch, then start the car. By what you said, i'm guessing that what i was misinformed?

 

yes to the misinformed. pretty much the connection between 30 and 87A is "always on", and the connection to 30 and 87 is off when the relay pins 85 and 86 aren't getting power. When those two pins DO get power, the switch flips and now 30 and 87A is "off" and 30 and 87 is on

 

I may be wrong but I think one is normally open and the other is normally closed, so 87a is on and 87 is off but when the signal is received at 86, then 87 is on and 87a off.

 

check out http://www.the12volt.com/relays/relays.asp for the theory of operation of spdt relays

 

Consider this one: http://www.autospeed.com/cms/article.html?&A=0059
I thought it was pretty clever

 

[QUOTE]Put it this way, you could make a whole alarm system with relays. [QUOTE]
Has anyone tried to actually do this? I have a ton of relays and I am poor so I was thinking about doing it. I have it all figured out with the exception of some sort of timer to cut the alarm off 30 sec or a min after a trigger so the siren wownt run until my battery dies.


[Modified by Bigjon, 10:47 AM 4/6/2003]


[Modified by Bigjon, 10:47 AM 4/6/2003]

 

lol...I remeber this one. There really is no way to turn it off...its sorta set up
to be turned on off with a key or maybe you coud do a wireless setup for
on and off.

Man those were the days......

 

That's all an alarm is. Transistors, few resistors, maybe a cap, and a 555 timer chip (it's a 555 right? Man i forget now).

 

555 is for flip flopping. <-- not the tech term.

A friend of mine at work made an alarm with:

a 508 (programable chip)
resistors
cap
diodes
voltage regulator


He said you could do it all with relays, but it may draw too much.

He said he would email me the circuit.
I'll post it in a bit.

 

instead of using relays couldnt you use a bunch of or gates?