Connecting an alarm LED

In message , Rob writes

If it is a normal LED then putting a 1.8K resistor in series with it will be OK for a 12V supply. Don't worry too much about which way you connect it, reverse supplying it with the resistor in circuit will not blow it. So if it doesn't work one way try it the other! Maplins and others sell flashing LEDs with the "flasher" built in, these can look quite good.

PS transformers only work on ac not dc so would not be appropriate here.

10K Ohm resistor in series. Wire the positive side to permanently live and the )v/negative side to a switched positive. That way, when you turn on the ignition the LED goes out but when you turn it off, there's sufficient voltage drop over the LED to light it.

I fitted one of these to my SD1. Cheap, no extra parts needed, and very simple to fit. When the ignition is switched off, they automatically flash like a real alarm LED. Mike.

So you want it so you can sell a car under a false description?

running at 12

there's

500 ohm surely. (12-2)/0.02 assuming 20mA thru the LED

A bit less than that I would have thought. 500 ohms for a 20mA LED

LEDs will quite happily shine with a couple of mA.

I'd not use a 10k myself, closer to 2K. I'd not use a 500R because 20mA will be exceptionally bright with modern LEDs.

Pete.

Rob to make life easy for yourself rather than guessing at resistor values and wiring just buy a 12v flashing LED and wire it permanently into a 12V supply anywhere in the car, the 20mA it uses each pulse is a lot less than any alarm system will likely use so just leave it permanently connected.

If you do want it switch then connect the + side of the LED to ground, earth or -12v (whatever you want to call it) and then connect the positive terminal to the switched 12v supply that feeds your stereo, that's easily accessible in most cars and just requires you to route the cable for the LED behind the dashboard somwhere.

Chris

Bugger, should learn to proof read my posts, here's the correct version:

Rob to make life easy for yourself rather than guessing at resistor values and wiring just buy a 12v flashing LED and wire it permanently into a 12V supply anywhere in the car, the 20mA it uses each pulse is a lot less than any alarm system will likely use so just leave it permanently connected.

If you do want it switch it with the ignition then connect the + side of the LED to ground, earth or -12v (whatever you want to call it) then connect the NEGATIVE terminal of the LED to the switched +12v supply that feeds your stereo, that's easily accessible in most cars and just requires you to route the cable for the LED behind the dashboard somwhere use a scotchblock to splice into this wire if you can't or don't know how to solder.

Chris

You young whippersnappers. We've still got lots of the old sort at work.

It's going to be awfully dim with 10k in series.

1k is the 'norm' for an average indicator LED at 12 nominal volts.

If you wish to drive it harder, best to give the exact spec of the LED. Because they are not all the same. Different colours require different currents. Etc.

Hello,

If it was a boy racer (a bit thick), the type to wear a tracksuit with a cap and have a cloned friend, they probably wouldn't check it had any alarm. They go for things likea K&N induction kit to suck hot air in and decrease efficiency, big wide exhaust to hide the noise of a stuffed engine and bit of plastic glued down the windscreen. All it would need then is one of those massive bolt on spoilers. Guaranteed a sale if the fog lights are permanently wired on so they work in daylight.

It's a blue, 2 volt LED. Unfortunately, I don't know any other spec apart from that.

I can't recall ever seeing a blue, continuously illuminated LED used for an alarm. Red flashing is the normal type used. If you use the blue LED someone is morelikely to think that you left a mobile phone or laptop PC charger in the car and break in to get it.

Its from the latest Clifford G5 alarm system. It's the genuine LED and is described by Clifford as:

"New Bright Blue Status LED lets thieves know that Clifford G5 is on board!"

I had one in my GTR and my wifes got one in her Musso. They are great because you see them flashing from a mile away. And they scream "Expensive Alarm".

Fraser

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Fraser Johnston" saying something like:

Great. So somebody breaks in and steals the alarm.

Grimly Curmudgeon ( snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

You jest. I've heard of it happening. Several times.

England must have a better class of thief. Over here the height of sophisticated crime is sticking a screwdriver in the ignition and turning it. On older Aussie cars that actually worked.

Fraser