Dilemma involving relays (engine immobiliser)

synoptica · 24th October 2006, 10:36 AM

Hey guys,

I've got a reasonably well-known, but now quite old, alarm in my car. While it does the job (single-point relay-operated immobiliser and central locking trigger), unfortunately the alarm ceases to perform it's job when it's unplugged (or the wires are cut - as the connector is right at the base of the siren).

The immobiliser (which the manual says must be installed on the wire to the starter relay... not so great when you drive a manual - so it's on ignition now) - is triggered by a relay. The relay is triggered by a wire that provides an earth when the system is armed. Unplug the alarm, no more earth. Open circuit - and the relay is NC - so the immobiliser is useless.

I've found a wire that provides a short earth pulse upon arm/disarm which might be able to be put to use... but other than that I'm out of ideas.

So basically, I've got the following:

SYSTEM DISARMED:
No earth, relay triggered (closed circuit)

SYSTEM ARMED:
Earth, relay not triggered (open circuit)

SYSTEM UNPLUGGED/CUT:
No earth, relay triggered (closed circuit) <-- but I want this to be the same as ARMED.

Any ideas guys? I've just managed to replace the whole audio system after it was stolen earlier in the year, I don't wanna lose it again.

Help!

Cheers,
- Chris.

krell · 26th October 2006, 11:21 AM

I would suggest that you update the alarm
(I have a mgoose trade account if you want a better price).

If you want to stop the car from being actually stolen, why not put in a seperate kill switch ?
It's cheap and can be as simple or complex as you want.
cut a fuel pump, ign, starter, etc.
If the alarm is bypassed the switch is still there.....somewhere....

synoptica · 26th October 2006, 6:49 PM

That's absoloutley the plan - though it would be nice for the alarm to just work

Already made up a reed-switch based killswitch, and have plans to implement another switch too.

krell · 27th October 2006, 10:55 AM

Reed switchs can be very delicate (mechanically), I dont trust them, seen too many break the glass, so please be carefull.

If the 'plan' is to replace the current alarm, why muck around with it ?
Or am I missing something ??

synoptica · 27th October 2006, 7:57 PM

Sorry separate killswitch is the plan - I'd like to keep the alarm for the moment.

You wouldn't believe how many reedswitches I've broken already - so I've just soldered them all to a little prototype board now

Goth · 27th October 2006, 10:52 PM

I'm sure you can get mechanically ruggedised reed switches. If those aren't good enough, a Hall Effect sensor could do the same job in solid state.

TERRA Operative · 28th October 2006, 9:01 AM

Get a reed switch used in house alarms. They operate exactly like the bare glass ones, the only difference is they are in a casing to prevent damage.
Something like this should be pefect to stick behind something.

24th October 2006, 10:36 AM	#1
synoptica Member Join Date: May 2002 Location: London, N19 Posts: 2,271	Dilemma involving relays (engine immobiliser) Hey guys, I've got a reasonably well-known, but now quite old, alarm in my car. While it does the job (single-point relay-operated immobiliser and central locking trigger), unfortunately the alarm ceases to perform it's job when it's unplugged (or the wires are cut - as the connector is right at the base of the siren). The immobiliser (which the manual says must be installed on the wire to the starter relay... not so great when you drive a manual - so it's on ignition now) - is triggered by a relay. The relay is triggered by a wire that provides an earth when the system is armed. Unplug the alarm, no more earth. Open circuit - and the relay is NC - so the immobiliser is useless. I've found a wire that provides a short earth pulse upon arm/disarm which might be able to be put to use... but other than that I'm out of ideas. So basically, I've got the following: SYSTEM DISARMED: No earth, relay triggered (closed circuit) SYSTEM ARMED: Earth, relay not triggered (open circuit) SYSTEM UNPLUGGED/CUT: No earth, relay triggered (closed circuit) <-- but I want this to be the same as ARMED. Any ideas guys? I've just managed to replace the whole audio system after it was stolen earlier in the year, I don't wanna lose it again. Help! Cheers, - Chris.

26th October 2006, 11:21 AM	#2
krell Member Join Date: Jul 2001 Location: Brissie Posts: 1,169	I would suggest that you update the alarm (I have a mgoose trade account if you want a better price). If you want to stop the car from being actually stolen, why not put in a seperate kill switch ? It's cheap and can be as simple or complex as you want. cut a fuel pump, ign, starter, etc. If the alarm is bypassed the switch is still there.....somewhere.... __________________ Swarovski jewellery & Suncatchers - New car audio and home audio OCAU Extreme Cooling Club H2O Member I was there - "No. of Users Online" - New Record 1059 set 06/03/06

27th October 2006, 10:55 AM	#4
krell Member Join Date: Jul 2001 Location: Brissie Posts: 1,169	Reed switchs can be very delicate (mechanically), I dont trust them, seen too many break the glass, so please be carefull. If the 'plan' is to replace the current alarm, why muck around with it ? Or am I missing something ?? __________________ Swarovski jewellery & Suncatchers - New car audio and home audio OCAU Extreme Cooling Club H2O Member I was there - "No. of Users Online" - New Record 1059 set 06/03/06

27th October 2006, 10:52 PM	#6
Goth Grumpy Member Join Date: Jun 2002 Location: /dev/null Posts: 9,303	I'm sure you can get mechanically ruggedised reed switches. If those aren't good enough, a Hall Effect sensor could do the same job in solid state. __________________ "How is anyone supposed to know that this isn't just a bunch of crap?" - Richard Feynman.

28th October 2006, 9:01 AM	#7
TERRA Operative Member Join Date: Jul 2005 Location: Niraikanai Posts: 6,531	Get a reed switch used in house alarms. They operate exactly like the bare glass ones, the only difference is they are in a casing to prevent damage. Something like this should be pefect to stick behind something. __________________ I DO NOT support internet filtering!!! Tereno Empireo Rapidmova Reakcii Armeo Check out my Youtube channel!

26th October 2006, 6:49 PM	#3
synoptica Member Join Date: May 2002 Location: London, N19 Posts: 2,271	That's absoloutley the plan - though it would be nice for the alarm to just work Already made up a reed-switch based killswitch, and have plans to implement another switch too.

27th October 2006, 7:57 PM	#5
synoptica Member Join Date: May 2002 Location: London, N19 Posts: 2,271	Sorry separate killswitch is the plan - I'd like to keep the alarm for the moment. You wouldn't believe how many reedswitches I've broken already - so I've just soldered them all to a little prototype board now