Looks like the magnetic switch was set to NC. But if I jump the input terminals it just keeps the bell on constantly. With the magnetic switch hooked up directly to the main unit, the voltage is 12V with the door closed and practically nothing with the door open.

These questions are probably going to show how much of a newbie I am, but why doesn't the bell just go off constantly with nothing hooked up to the input (wouldn't that be the same as an open door?) and why does it constantly go off with the inputs jumped? Wouldn't that be like having the door closed?

 

Apparently, there is a 12 volts coming from the main unit and going through the door switch. The switch is open when the door is closed, but when the door opens, the switch closes, which routes the 12 V back to the main unit. Likewise, when you short the main unit terminals, the 12 V causes the bell to ring continuously.

You will need to add a relay to the output of the one-shot, and route the 12V though the contacts (not the coil) of the relay. That will allow the door switch to work as normal, but the relay will only allow the 12V to pass into the main unit for the duration of the high from pin 3 of the NE555. You will also need to add a diode (1N4001, e.g.,) across the coil of the relay to protect the NE555.

 

So, to clarify, I made a system that is normally closed, and I need one that is normally open. And what you just suggested will invert that?

btw, I hooked it up to the door switch and the bell and it works exactly opposite

 

So, to clarify, I made a system that is normally closed, and I need one that is normally open. And what you just suggested will invert that?

btw, I hooked it up to the door switch and the bell and it works exactly opposite

Here's the sequence that I think currently happens.

The door is closed; the door switch is open and there is no path for the 12V from/to the main unit.

The door opens; the door switch closes and there is a path for the 12V from/to the main unit for as long as the door is open. The main unit rings the bell for as long as the door is open (and maybe a little longer depending upon the pot setting in the main unit.)

The door closes; the door switch opens and there is no longer a path for the 12V through the switch. The main unit stops ringing the bell.

Here's what I think needs to happen.

The door switch becomes the trigger switch for the new one-shot. When the door opens and the switch closes, it triggers the one-shot, and pin 3 goes high for one second. Contrary to what I said in the last post, I think you should be able to use that signal (without a relay) to trigger the main unit, thus making the bell ring for as long as the one-shot is high (one second.)

Have you tried it that way?

 

Yes, I just had it hooked up that way, but what's crazy is that the bell rings constantly until the door opens, then it stops for 1 second and continues to ring. The relay would just flip the output, right?

So I need this...
http://www.radioshack.com/product/index.jsp?productId=2062481
and this...
http://www.radioshack.com/product/index.jsp?productId=2036268

I'm a little confused as to where to hook up the diode. You said across the coil?

 

I have breadboarded the circuit as drawn and corrected below. It works, but for reliability, pin 4 should be connected to Vcc, there should be a .1 μF cap from pin 1 to pin 8, and pin 5 should be tied to ground through a .01 μF cap.

You should have the two door switch wires connected to the red circled points in the schematic. The output from pin 3 should be used as the input for the main unit. Polarity may be important in this connection and could be the reason the operation seems reversed. No relay should be required. Good luck.

 

IT WORKS!!!

I made your mods, but it still needed a relay, so I added that and now it works like a charm! Thanks so much for your help!

 

IT WORKS!!!

I made your mods, but it still needed a relay, so I added that and now it works like a charm! Thanks so much for your help!

You're welcome and congrats!