Project Help - Alarm Circuit (Years ago in High School)

FreezeSukka

I remember years ago, back in my junior high school electronics class, a project from a kit... (I wish I could remember the name of the company, but for the life of me, can't.)· Anyway, the project consisted of a black box (approx. 3" x 3" x 5") that sits anywhere you like·and it·has about a 3/8" hole drilled into the top of it.· You would hot glue a coin or dollar bill to a magnet which would fit into the hole of the box, making the coin, bill, etc. look flush to the box.· Directly beneath the hole was some sort of reed switch, I believe, that triggered the·screeching siren·once the object was removed.· The catch though, was that once the object had been removed it couldn't be turned off by simply placing the object back into the hole.... You had to actually replace the object AND short out 2 of the screws on the bottom of the box to turn it off.· (which of course, the unsuspecting person did not know)· It was really neat and helped you find out who is honest and who isn't, when they are in your house.

I am not very good at all at designing circuits.· So if anyone has ever built this circuit or knows where I can find it I would really appreciate it.· I have searched and searched a ton of sites featuring alarms and kits and what-not and haven't found·this certain one as of·yet.· If I remember correctly it wasn't a very difficult circuit at all to build.

Maybe, I'll get lucky and someone here has already done this project!· Thanks!

~Jeff

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P.S. This is what this part of the alphabet would look like if "Q" and "R" were eliminated.

knightofoldcode

Jeff,

This type of thing could be done really easily, without the use of any Basic Stamp. Using my limited knowledge, the most convinient way I can think of would be to use a 555 timer. Once the reed switch is tripped it sets off the alarm for some 5 mintues, etc. Then the screws mentioned previously would be connected to the "reset" of the timer.

I think you could get away with using a reed switch, 555 timer, 1 cap, 1 resistor, the enclosure, possibly a 7805 and a 9v battery. You'd want to do some power filtering on the 7805. But I think that's about all that would be needed. Look into the 555 timer.

555 Timer IC

Knight.

Chris Savage

I would also make one more suggestion on this project.· Perhaps instead of gluing a magnet to the object (Especially a Dollar Bill), try using light.· For example, you could use a CdS Photocell, or even an IR pair to detect the object being removed.· That way the object would just cover the hole.· Removal would be detected either by light entering the hole (In the case of the CdS Photocell) or by the absence of a return signal (In the case of the IR Pair).· Of course the magnet would be harder to circumvent, but then nobody should know about the "trap" anyway.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

FreezeSukka

Thanks guys!· I think I even have all of the components that I would need, except for the 7805.· I really like the idea of using the Photocell instead of the magnet and also the 555 timer.· I am still in the "learning mode" when it comes to electronics and programming and such, so please bear with me.· I have never actually used the 555 timer (go ahead and chuckle!), but from what I've read on here from time to time, it shouldn't be that hard, I hope.· From searching sites about the 555 timer, I did find a circuit close to what I think I need...
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However,·from looking at it, it looks like it could just be turned off by covering the Photocell again.· How can this circuit be "rearranged"·to keep the alarm going and needing that "short" to turn it off?· Any help would be greatly appreciated.· Sorry, for being slow at this stuff.

~Jeff

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P.S. This is what this part of the alphabet would look like if "Q" and "R" were eliminated.

Post Edited (Jsjga) : 9/20/2005 8:41:06 PM GMT

FlyingFishFinger

I myself have not used 555's too much either. A solution to the keeping-on is to have the photo-cell drive a 2-coil-latching relay through a transistor. The output of the relay drives the 555 (replaces photocell). Connect the other relay coil to the 2 screws in line with the power supply for a reset switch.
Rafael

Orion

You can do this very easy with a relay configured in a holding circuit.· See the schematic I have attached.· Once the micro switch closes it will energize the relay.· The relays contacts will now power itself regardless of what happens to the micro switch.· The buzzer is simply paralleled to the relay coil.· To reset disconnect power or add a normally closed switch to the power in a hidden spot.

Chris Savage

You can also have a buzzer or relay activated by an SCR.· Then you can use a momentary pushbutton switch (N.C.) to kill the alarm.· Some early Alarm Kits did that very thing.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

FreezeSukka

I have built that simple circuit that I found earlier (substituting a piezo speaker for the regulat speaker) and instead of activating when a light is shining on the photocell, it does just the opposite. How do I get it to turn on when a light shines on it instead of a shadow? I would also like to implement the SCR in it, no particular reason, except for learning. I hate asking, but does someone have a schematic using: (parts that I already have): 555 timer - SCR - piezo speaker - photocell

I also like the idea of using relays as well, but I just don't want to involve that much money and parts in this "toy" circuit. I am reading up on the 555 timer as of right now, trying to gather as much as I can. But some of it seems a little over my head as far as electronics goes, only because I am a newbie. Maybe I should've picked a different interest I guess, but programming and electronics have seemed so fascinating to me. So I guess I'll stick with it. I am more of a hands-on designer/fabricator i.e. lasers (steel cutting), break press, welding, designing metal parts, etc. (I am almost finished with my Mechanical Design Engineering degree) When it comes to that kind of stuff I feel exceptionally confident. This stuff..... not so much! I actually got into the Basic Stamps while taking a class (Basic Robotics) and I have been hooked ever since! I have complete respect for you guys who know what you are doing in this field. And especially to those of you willing to help out others like myself, and not to mention for free. So thank you very much.

Thanks once again,

~Jeff

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P.S. This is what this part of the alphabet would look like if "Q" and "R" were eliminated.

kogerat

Hi Jeff,

Did Google search for SCR alarm circuit and found this site. http://www.technologystudent.com/elec1/thyris1.htm Check it out.

Jim K

Beau Schwabe

As Chris Savage originally mentioned using an SCR....

Although the basic concept of this circuit can be made with "non thinking" discrete components,
here is a·solution that uses a BS2 to "gate an SCR",· while also·keeping the theme of using Parallax
products.· When the circuit is "OFF" only 3 nano Amps are consumed. When it is "ON" you have all
the processing power of the BS2 to do with whatever else you wish in your design.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 9/21/2005 2:57:07 PM GMT

FreezeSukka

Awesome guys!! Thanks a ton! I haven't built them yet (1:45 am)... (the little one decided it's playtime [noparse]:)[/noparse] so I decided to check my email and, voila! an email from Forum Administrator!) But I will definitely do that tomorrow. I want to try all of the one's that you all have mentioned. Again, to everyone on here... THANKS A BUNCH!! I will definitely let you know how they work out for me.
Thanks,
~Jeff

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P.S. This is what this part of the alphabet would look like if "Q" and "R" were eliminated.

LoopyByteloose

By the way, with the 555 you doesn't really need a voltage regulator unless you are using the CMOS version [noparse][[/noparse]in some cases]. They usually have a wide battery voltage range. Check your spec sheet for the particular ones you buy.

I think you would have to use a 556 [noparse][[/noparse]two 555s in one package] - one would be a monostable trigger and the second would be your audio oscillator.

The 555 can be reset by shorting the battery terminals, but I· they also have a reset pin.

I think the SCR would require shorting or removing power.

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G. Herzog in Taiwan

Post Edited (Kramer) : 9/21/2005 2:36:26 PM GMT

Chris Savage

I would recommend against shorting power to initiate reset on any given system.· A novice may not realize how much current potential there is in a given system.

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Chris Savage
Parallax Tech Support
csavage@parallax.com