how to have a momentrary switch keep power flowing after it is released?

[thefif]

I have a normally closed switch that the door to my apt. holds shut.
(so its off when the door is closed and on when its open)
I am trying to have it so that if someone opens the door the buzzer goes off but i need it to keep going off even if they close the door.
its all going to be run at 9vdc

now, i understand that i could probably do this with a PIC or something like it but i don't understand the programing that well.
I am not opposed to using IC's i just don't want to have to program one.

sorry if your having a hard time following this i just cant think of a better way to describe what i mean.

 

[(*steve*)]

The easy way is for the switch to charge up a capacitor which provides base current to a transistor which switches a relay. The size of the capacitor and the amount of base current together will determine the length of time the relay stays pulled in after the button is released.

The relay operates the buzzer, the N.O. contacts being placed in the buzzer circuit where the switch is now.

Here is an example using a 555. This is a better circuit for a whole host of reasons. The 555 *may* even be able to drive the buzzer directly.

 

[Mitchekj]

There are a million ways of doing the same thing, great thing about electronics. Here's an all discrete solution (probably easier to use Steve's idea) I just submit it for educational purposes. Hehe.

This is an adaptation of a door alarm. J1 would connect to your door switch. While the door is closed, Q1 is biased, keeping Q2 off so the alarm doesn't sound. Q3 and Q4 are a self-latching circuit, the buzzer will continue to sound so long as S1 is closed.

D1, D2, D3 aren't really needed (short them if not installed)
R1 should be more like 100k-150k.
The buzzer BZ1 should have an internal oscillator, and rated for 9V+.
Resistors are all 1/4W, caps are 12V+ rated. (25V should be rather common for the electrolytic ones.)

 

[thefif]

thanks to both of you but I am probably gonna go with Mitchekj's first and see where it takes me.

thanks again.

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i don't have money to buy the parts need (i have to order them off line and it gets pricey)
so i threw this together with some of the things i had laying around

 

[klj12574]

I would use a one shot and a flip flop

 

[(*steve*)]

Looking at the youtube video, I'd probably have the motor open the door of a birdcage, allowing the bird to fly away. The release of weight would cause the birdcage to go up, a weight to go down, and hit a board that releases a ball. The ball would run down a track (releasing flags as it goes that spell out D-I-N-G D-O-N-G, before dropping into a glass of acid. The acid would react with the metal releasing hydrogen which would be captured in a balloon. The balloon would rise, lifting a lever on a large switch which turns on a lamp.

 

[Mitchekj]

But then the lamp ignites the hydrogen balloon, setting the draperies aflame... so you need some type of fire supression system built in. I'm thinking halon. Sorry birdie.

But that did look like a cool little contraption.