Single relay car alarm

[Larry 1300]

Hi I am trying to fit a car alarm to my Dolomite 1300 based on this video:
https://www.youtube.com/watch?v=_KlpGfhSSPI
the difference being that I have fitted a toggle switch instead of the reed switch.
I have taken the constant power off the purple wire in the boot. I have tapped into the purple and white wire for the interior light behind the door switch in the near side B post.
The alarm is working but not as it should.
When I flick the switch on the toggle the siren sounds, even when the doors are closed. But the siren stops when I open any door.
Can anyone guide me please on the correct place to tap into the interior light circuit as I think that this may be my problem.
This is my relay connections:
30=constant power, with in line 20amp fuse
86=connected to purple and white wire in the interior light circuit, tapped into the near side B post switch.
85=one side of toggle switch, the other side of the toggle is to ground
87=to positive side of siren, the other cable from the siren is to ground.

Any help is appreciated.

[Mad Mart]

The wire to the interior light is always going to have 12V therefore the relay will always be energised because you have the siren grounded. You need to have the +ve side of the siren connected to 86 and the -ve to the interior light wire after the lamp.

[Larry 1300]

Hi thanks for your reply. Could you please tell me where to find the post lamp courtesy lamp wire and let me know what colour it is please.

[Mad Mart]

OK, forget about the second part of my post. The problem is in the first part ie. the wire all the way to the door switch will be live at all times (even with the door closed). So, as said, you will permanently energise the coil in the relay. You need a feed that will only be live when the door opens.

FYI Purple is live to the lamp, purple/black is the ground side of the lamp (if the wiring is the same as a Sprint).

Meantime, I'll try to figure something out...I like a challenge.

[Larry 1300]

Hi I was looking at the alarm video again and noted that it says that a diode should be fitted between 87 and 86 terminals flowing in that direction. I have not fitted a diode.
Can anyone advise me on the correct diode to buy and any tips on fitting it would be appreciated.
I see ones on eBay but cant understand the wattage I should get.

[Mad Mart]

The diode is needed as if it is not fitted the alarm will simply turn off again when the door closes. When the door is open the coil in the relay is energised and the current flows to the siren but also flows through the diode and thus keeping the coil energised. That is why the switch is needed, to reset the relay.

To decide which diode is needed you will need to know the maximum voltage and current that will pass through it. The voltage is easy as it will be a max. of 12V. You will have to have a look at the siren to see what the current draw is (that's if it tells you). Otherwise you will have to try it and see what the max. current draw is using a multimeter or amp clamp. I have plenty of diodes here so I can send you one.

I'm still learning about electronics so hopefully I'm doing ok, as the experts haven't chipped in yet.

[Larry 1300]

Hi thanks you have been very helpful.
This is the siren that I have used:
http://www.maplin.co.uk/p/staccato-elec ... nder-yz03d
It says 20 watt on the casing.
I would appreciate it very much if you would let me know what diode to get please.
The web page says: Maximum current 350mA

[Mad Mart]

I don't know where the 20 Watts comes from as 12 Volts x 0.350 Amps = 4.2 Watts.

Anyway, a 1N4001 diode should be fine for this application. There are 1N4002 to 1N4007 diodes as well. Any of these should be fine also.

[Larry 1300]

Hi thanks. I fitted a diode as suggested but it made no difference.
The alarm sounds when the toggle switch is on but goes off when I close the door.
The video says that this is a positive trigger alarm.
I have changed from a reed switch to a toggle switch.
The video says that the reed switch is continually on.
One side of the toggle switch is connected to the relay, I have the other side connected to ground.
Should the side of the toggle be connected to a positive source and should I run a tap from the interior light for this?
I had a look at this web site: http://www.stangnet.com/mustang-forums/ ... ay.425432/
Let me know if you know please.

[Mad Mart]

Have you fitted the diode the correct way around? Diodes are like one way valves. The silver band around one end of the diode should be on the 86 terminal side.

[Mad Mart]

I've been looking into this circuit but I just can't see how it can work in it's current guise. The problem is using the interior light circuit for the alarm trigger. As I've said, there is always 12V on this wire so with your toggle switch closed (as per the circuit diagram) your siren will be sounding all the time. You need a circuit which only has 12V when the door is open. I've tried all sorts using my Circuit Wizard software but I can't find a solution. If you had another micro switch on the door, one that didn't actually earth the circuit to the car body, just made a connection, then it would work as a pull-up circuit.

[Mad Mart]

Tony (MIG Wielder) and I have both had a good look at this circuit. Tony actually came up with the solution (with a small tweak by myself)....

[Mad Mart]

OK, there is a slight flaw in this circuit as the siren will sound every time you open the door, whether your switch is open or closed. Back to the drawing board.

[Mad Mart]

Bill (GTS290N) has put me right by moving the siren feed to the other side of the switch. This works fine but he also pointed out that reed switches usually have quite low max. current ratings and that the current the siren will be pulling could burn it out.

[Mahesh]

Sorry to wade in, but I would recommend a car alarm from eBay around £20 or so.

The reason for this is, the type you are building is a simple make/brake contact to the horn, you could just put a key switch and wire in-between the horn and courtesy light.

Also you have no timer on this model to maintain the relay once a door has been opened, in other words someone would merely open the door and close it to switch the alarm off.

Unfortunately I binned my old moss 715 alarm which was complete with remote fob, may have something else, but can only check at my other premises next weekend, if I find anything your most welcome to it.