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LED with piezo element window burglar alarm? Answered

I would like to make an automatic lighting system for when i open my cupboard. I was wondering if its possible to hook up an LED to a window protecting (piezo element) burglar alarm? I have already removed the piezo element, and i'd extend the length of the LEDs placement across the cupboard, of course, with wires. 

My question is, If this works, how many LED's can i solder to one of these units without burning it out?

Thanks for any help.


I don't know how far you have gotten with this project, but at the risk of spoiling the surprise, I claim there is a reed switch inside your Intrudalert(tm), and this reed switch is the component that senses the presence, or absence, of a nearby magnet, thus telling the integrated circuit (IC) if it should sound the annoying alarm, or stay quiet.

You can read more about reed switches here,
but basically the way it works is there are two thin pieces of steel (called reeds) that are close to each other but not touching. That is they are not touching until you bring a magnet near these reeds, then they become magnetized and stick to each other, and the switch closes.  If you take the magnet away, then the springiness in the steel pulls the reeds apart again, and the switch opens.

Now, I don't know if you noticed or not, but this switch-closed-when-magnet-is-near and switch-open-when-magnet-is-far-away, that is really the opposite behavior of what you want for LED that turns on when the cabinet door opens, and turns off when it is closed.  So for that reason, there is a desire to "invert" the output from this reed switch somehow, so that you can get a switch that closes when you take the magnet away.

But how to do this? 

One way is to build a circuit (of some kind) that does the inverting. Such a circuit senses when the reed switch is closed, and causes another switch to open, and vice-versa.

Another way to do this is to carefully position your reed switch in between two magnets whose fields cancel.  That way when you take one of the magnets away, the field of the remaining magnet causes the reeds to stick together, and the circuit to close.

I think that this second way will be the most convenient for you and your plans for turning on LEDs.  You'll  need two magnets for this trick, but if you are buying these Intrudalert gizmos from a dollar store (that's how I got mine), then it is not too expensive to just buy two of them, because they are cheap. Or maybe you've some other source that happen to be a close match?

Although at this point you might be wondering: How expensive are just plain reed switches?  That is, if you could find a place that sells those, and I'll leave that as an exercise for the interested reader.

Erm.. speaking of cheap, you might have noticed from reading that Wikipedia article that quality reed switches come sealed in a glass envelope, but the reed switch in your Intrudalert(tm) is just a couple of funny pieces of metal soldered to a circuit board.  These cheap reeds can easily become misaligned just from handling them.  However, the good news is they're pretty easy to re-align too, and this can be done using a pair of needle-nose pliers.

Regarding the question of:  How much current can your reed switch handle?  I would guess maybe 0.1A = 100 mA, which is a lot for LEDs, although honestly I don't  really know. You might try looking up some similar sized reed switches to see what they are rated at.  Or you might just naively assume that it handle any amount of current, and then discover how much current it takes to melt it.

Also, in the process of taking apart my Intrudalert(tm) I drew you some pictures, in the hope that these might help explain what I'm writing.
One of these pictures involves me trying to figure out the approximate shape and direction of the field surrounding that magnet, using some iron filings.  I have no idea why the designers chose to put the field in that place, that direction, or if it even matters, but um... maybe that picture will meaningful to someone reading this.


I've been working on trying to make Piezo's LOUD on low voltage and this circuit seems to work nicely. I measured it at ~5volts after kicking around in my garage for a year. Jack's picture below identifies one if the parts as a transformer. Does anyone know how the 'black blob' drives the transformer/Piezo? cheers, tom

Ok, I figured it out! ... you can just pulse the transformer like this: http://www.ozitronics.com/docs/k15.pdf

there are already battery powered LED lamps with magnetic switch, to assemble it you only need to glue it using the included two sided tape or screws. I have seen them for sale for 1 or 2 bucks

All you need to do is retrieve the switch, and make sure its OFF when the door is CLOSED.

Now, most magnetic switches for alarms are the other kind, ON when the door is closed.


Right but how many LEDs can i attach to a single unit? It takes 3 batteries 1.55V each... And im confused as how to do the wiring to place the LEDs a foot apart from one another

White LEDs ? Can you point me at exactly which ones you have ?

Can you show some pictures of the bits ?


I think the gizmo OP is writing about is the same as this one:
It is an inexpensive battery-powered alarm controlled by a reed switch and a magnet.  These are sold in the FUS by the DollarTree(r) chain, for 1 FUSD per.

Interestingly enough, there is also an LED version of the same gizmo.  Similar  shape.  Same magnet and reed switch. Same batteries: 3xLR44. But it lights up a single white LED instead of an alarm.  The one I've got, also from the local DollarTree, calls itself a "LED automatic closet light"

dont know how many will be possible per unit but i'd like to light 3 LEDs

i didn't know they existed! thanks for the info... but still looking to make one :)

here are the pics... as you can see i burned the plastic trying to solder a torn wire back to one of the points but no matter i have a few other of these units. since i'm all new to electronics, i'm aiming to use LEDs with this unit without constructing anything further.... hope that's possible.

selina 029.jpgselina 024.jpgselina 025.jpgselina 028.jpg