Zero voltage switch??

[Rescue1]

Hello everybody,hope everyones summer is going great!! I want to build a simple switch for my trucks alarm system so when the battery cable is disconnected,a seperate siren from the main siren is turned on and powered by a seperate 12v house alarm sealed type of battery.Now I figured out how to do it with a spdt relay,but having a relay latched all the time would not be practical.I was figuring something with a Transistor and a pull down resistor on the base,but I figured I would ask first.Also,it would be nice if the seperate batterys voltage would be maintained by the trucks electrical system.Thanks for the help....

P.S.-These kits can be purchased,but I already have the battery,and the circuit looks simple. Plus building stuff is way more fun... :wink:

 

[goodpickles]

Why not use an SCR?

 

[Exo]

You should make sure the resistors are high enough, so you do not waste more current then you have to, exact numbers depend on the transistors you want to use / are able to get...

Q2 should be powerfull enough to drive your siren...

if your siren is inductive you should put a reversly biased diode across it to protect the transistor when it gets turned off...

the siren will be on until you reconnect the main battery (or until the secondairy is drained :lol: )

 

[Styx]

Why not use an SCR?

he is using DC once the thyristor is latched, it is latched (for as long as the minimum forward conduction current exists)

 

[panic mode]

And that's good if someone tries to steal your battery ;-)
You can always have hidden reset sw. accross SCR to turn it off.

 

[Rescue1]

Thanks for the help Guys.. ...I am familiar with the basics of the SCR,but how would removing the main battery voltage trigger the SCR to "turn on".I really appreciate the help,this site is the best...

 

[Rescue1]

Thanks for the help Guys.. ...I am familiar with the basics of the SCR,but how would removing the main battery voltage trigger the SCR to "turn on".I really appreciate the help,this site is the best...