Time Delay Circuit - Door Buzzer

I'm trying to figure out how to get a door buzzer (12v) to stay electrified for 3 seconds after the buzzer is pressed. I figure I need some type of transistor RC circuit. Circuit needs to detect 12v for split second and send 12v to buzzer for 3 seconds. Any ideas?

Look up 555 monostable. That will certanly fit the bill.

Then look up driving inductive loads with a 555.

will do.
any other ways besides using the 555?

A time delay relay will do the trick but 555's and associated components are probably cheaper.

The 555 is called a timer. This is because it times.

There are lots of ways to do it without a 555, I doubt they will be simpler though. You could do the same thing with precision using transistors for about 4X the components.

You could also do it something like this. You will have to arrive at the values by trial and error, on of the reasons the 555 is so popular is it is predictable within the tolerance of the components.



I may have messed up, since you put this into the homework section. We don't give direct answers, we prefer to guide you into figuring it out for yourself. This one is a gimme (though you will have to design the rest for yourself), the rest will be harder.

The second Q1 should have been Q2. You don't want to use a prepackaged darlington (1 transistor) because they have resistors going from the first base to the second emitter, and I was going for the increased gain and high input impedance.

I cant imagine why you would use the 555 as a monostable to do this. As said using discrete components can achive this but in reality it would be very inpractical.

How about using a simple R/C circuit, similar to this circuit which holds a relay active for an extra few seconds after switch is released... http://www.circuitdb.com/circuits/id/92

B. Morse