I have a negative output on my alarm which is used to switch on things like the tilt switch, microwave sensor etc. When the alarm is turned off, it switches to 11 odd volts and I wanted to use this to switch a mini relay but when connected, the voltage goes down to 7v and you can hear a tiny click from the relay. I assume there isn't enough current to actually switch the relay?

 

So is there any way of using this low current 11v to switch a relay?

 

Cheers

 

Vijay

you could try a solid state relay they take very little current and are super reliable. But they cant really switch big currents but i guess you could use the solid state relay to switch a bigger relay if you know what i mean

Cheers bud. I'm not looking to switch a large current, just a negative to a start switch ;)

Connect the 0volt side of the relay to the switch, apply 12v to the other side. Wire the switch normally closed?

 

If the alarm output is designed to switch 0 volt it won't be designed to pass current when not 0v

But won't that power the relay when the alarm is on (negative down the wire) and possibly drain my battery?

 

"If the alarm output is designed to switch 0 volt it won't be designed to pass current when not 0v" that sounds about right but I was hoping to use what ever is there to switch something, is there no way of doing that?

 

Vijay

the common misuse of terminology can lead to confusion

 

usually :

 

state 1: "output 0v" the unit doesnt as such output 0v, but instead internally conects its output pin to 0v to allow other units to connect to 0v via it. so a device with 12v connected will switch on when the alarm lets it connect to 0v

 

state 2: "output 12v" the unit is no longer connecting its output pin to 0v, and probably by means of some resistors or an IC is artificially holding the pin at a +ve voltage. any attempt top draw current from this "12v" will cause the voltage to drop imediatly.

 

without knowing exactly what you are doing its dificult to suggest. ignition feed 12v to the relay and 0v to the alarm?

a transistor would be a common way to switch a relay from a supply that cant provide enough current, but may be overcomplicating things

Right, what I'm trying to do is disable/able a start button from the alarm - so when the alarm is armed, it cuts the power to start button unit. This has to be done by cutting the negative supply to the start button control box.

 

The only usable wire, is this negative output wire from the alarm which is 0v when the alarm is on and 11 odd when the alarm is disarmed. There is no other wire that I can find that changes it's state when the alarm is abled/disabled.

if i were you id just do the solid state relay if the alarm is putting out 11 volts the relay only draws about 150ma-8ma (depending on what relay you buy) which even at 7 volts would be next to nothing, im not 100% sure on how the alarms internals work but im pretty sure a 150ma draw wont kill it if you already had a mechanical relay wired up and it didnt go pop as they draw a shed load more current than that!!

Right, I've got one of these 3 amp SSR's http://www.ebay.co.uk/itm/290833291797?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649 am I right in saying that when you put 12v to pins 3&4, then pins 1&2 make a connection?

 

Info on the relay is here - http://www.gavazzionline.com/pdf/rp1a48d5.pdf with a functional diagram on page 3.

 

Vijay