Example Circuit

This is a simple door alarm circuit. If you're in the US, Radio Shack will have a variety of alarms, books with circuits, and even all the parts that you'd need to build one from scratch, which is a measure of just how simple the circuit can be.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

20-30 SECONDS is a bit of a stretch for a 555 timer. I'd run the 555 as a higher-frequency keyed oscillator and use a divider chain to get a 20-30 second interval.

...Jim Thompson

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A CMOS 555 (eg. TLC555/ 7555) will be okay.

But a classic time delay circuit using a 2N6028 PUT to trigger a

2N5064 SCR to drive a DC-powered beeper would work nicely and would latch on until the power is interrupted (by the door closing again). Best regards, Spehro Pefhany

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Hell, if/since the amount of time isn't critical, you could use an RC circuit and a 74C14.

100K res, 220/330uF cap, into one gate the output of that gate can drive all the left over gates in parallel and their outputs go to a logic level FET or a transistor. The door switch shorts out the cap when the door is closed and your done. A 220uF cap will give you ~25sec a 330uF ~40. Use the 330uF and a 100K pot and make it adjustable.

Just use an ordinary switch, so that whenever the door opens, it buzzes a really obnoxious buzzer right away - this will startle the animals, and teach them that "open door=>really bad noise".

For that matter, you could equip the door with an automatic closer - you can get them at the "screen door" section of the home handyman store. :-)

Good Luck! Rich

I can do you one better. Use a resistor and a rather large capacitor across the buzzer.

Let's see, say the supply is 12V and the buzzer starts at 6V, rated for 100mA at 12V (=3D 120 ohms). A series resistor of 60 ohms will give

9V equilibrium, and a Thevenin equivalent resistance of 40 ohms. A time constant of 30s =3D R*C gives a capacitance of some 0.75 F, not actually too unwieldy. Three 2.25F, 2.7V supercapacitors in series would do.

A DPDT switch on the door turns this on and off. When the door opens, one set of contacts closes, energizing the circuit. When the door closes, the other set closes, discharging the capacitor through a noticably smaller resistor (1 ohm or less?).

Tim

Jim Thomps>20-30 SECONDS is a bit of a stretch for a 555 timer.

If he doesn't need precise repeatability, the 555 would work fine. With that wide a spec (charging from zero, the first interval of a

*series* will be 1/3 longer than subsequent intervals), you could even get it to work for minutes.

The automatic door closer sounds like the ticket.

The time delay will make the circuit more complicated than what it seems that you want. A simple wallwort 12v transformer,

12volt relay, buzzer and a magnetic switch will give you the alarm. The relay will need to be atleast single pole double throw. Connect the wallwort to the relay thru the Normally Open magnetic switch. Connect the buzzer thru the Normally Closed side of the relay to the wallwort. When the door is closed, the magnet will close the magnetic switch causing the relay to energize, opening the normally closed side of it's contacts and the buzzer doesn't sound. If the door opens, the magnetic switch opens and turns off the relay thereby closing the normally closed side of the relay allowing power to the buzzer... Buzzer sounds.

Con side to this arrangement is that you can never be in the office with the door open unless you wanna here the buzzer constantly.... otherwise you'd need to add a defeat switch... which would mean that you'd have to remember to turn the circuit back on... which it would be just as easy to just close the door... Dunno.. Seems to me that a door closer or a spring would be easier to install and wouldn't use power.... Other than the stored mechanical energy of springs. And atleast if you are in the office and want the door open... you can prop it..

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Use a CD4060. You get an oscillator and a divider all in one. If you want the alarm to remain on constantly, run a diode 1N914 from the selected output over to the oscillator's input to force the input high once the desired output goes high.