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low/no draw deadmans trigger


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jwhyte1445 
Member - Posts: 8
Member spacespace
Joined: December 08, 2014
Location: Nevada, United States
Posted: December 09, 2014 at 2:53 PM / IP Logged  
Thank you for the suggestion. I can understand the route you're explaining but this setup will be for an LED bar unfortunately.
jwhyte1445 
Member - Posts: 8
Member spacespace
Joined: December 08, 2014
Location: Nevada, United States
Posted: December 09, 2014 at 3:02 PM / IP Logged  
oldspark wrote:
A MOSFET...
An N-channel MOSFET with S to GND and D to ground the +12V connected "alarm" (siren, horn etc - or maybe a detonator?).
A resistor from G to +12V to turn it on - probably anything from 10k to 1M Ohms.   (10k means 12V/10k = 1.2mA draw; 100k = 120uA, etc.)
Also G to GND via your grounding link that is broken upon theft.
G needs to be typically 5V above S for the MOSFET (D to S) to turn on.
The grounding link keeps G at GND (0V) so it does not turn on.
You could add an optional resistor in the G to GND circuit - ie, from 100 Ohms up to one-tenth of the G to +12V resistor. That's to limit current flow in case of a D to G fault.
Maybe also a small capacitor from G to GND - ie, a 0.1uF or 0.01uF greencap etc to help guard against noise.
Having G tied to +12v and ground seams to be conflicting. Is the resistor some how isolating voltage +12v from the ground?
With this design is the horn (I like the detonator idea) able to be grounded for use still as the S and G terminals are only linked when the gate is active right?
I'm sorry if I'm way off..
davep. 
Gold - Posts: 639
Gold spacespace
Joined: May 27, 2011
Location: California, United States
Posted: December 10, 2014 at 10:26 AM / IP Logged  
jwhyte1445 wrote:
Thank you for the suggestion. I can understand the route you're explaining but this setup will be for an LED bar unfortunately.
Use speaker wire as the trigger.
One conductor is the trigger, the other to ground at the relay end. At the light bar end, insulate each conductor from the other, and from anything else. You could even use more than one run of speaker wire to increase the chances of the cutters grounding out the pair as it's cut because both conductors are in proximity to each other.
The larger the gauge, the greater the chance of the cutters making contact with both conductors simultaneously.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: December 10, 2014 at 6:03 PM / IP Logged  
G does to +12V via the resistor (10k-1M) and to GND via the shorting link. No conflict.
itsyuk 
Silver - Posts: 505
Silver spacespace
Joined: February 23, 2010
Location: United States
Posted: December 16, 2014 at 9:07 PM / IP Logged  
a little outside the box....
what if you wired the lights with power to them all the time and then had a relay ground them to turrn them on?
then you could connect an alarm to sense the (powered) ground wire at the relay. that wire goes dead....an alarm senses it and yelps... maybe needs a 528T relay to sense the "change in state" of that wire.
its not as difficult as it sounds.
yuk
quiet rural missouri, near KC.
If your system moves you physically and not emotionally, you have wasted your money.
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