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[on:]

Hi
I would like to build a simple buzzer circuit which beeps for some 5sec then die off when AC power comes on. I know circuit is kinda simple but i cant seem to figure out   here is wat  I came up with...
AC to DC rectification (say 6v)
then a capacitor in series with resistor in series with a peizo buzzer...
like capacitor take time to charge till that time peizo buzzer is on after that once capacitor charges fully to max voltage peizo dies off...
I am i doing this the right way? or is there a better way to do it...
Plz do help out. I want to minimise the component count for this.
I keep feeling its such a simple circuit but I just cant figure out how to....  :'(

[Reply #1 on:]

With an Arduino one could make one outputpin HIGH for five seconds in setup() when you want a nice "die off"consider digitalWrite(). see http://arduino.cc/en/Tutorial/AnalogInOutSerial, to get started.

[Reply #2 on:]

It not power failure circuit. just the opposite. the buzzer should sound say for 5sec then die off. just to let me know that AC power is back.
With Arduino I can simply do this. but come on for such an easy task I don't think I need to use Arduino.
Any idea if the circuit i explained will work? My house is currently under renovation so I cant actually practically work on this for a week. Thats why I cant implement this currently and see if it works.  

[Reply #3 on:]

So really want the AC going thru a relay that is Normally closed, and have some of that same AC be rectified into a low level DC (say by having the same AC drive a cheap wallwart as a 5V/12V source) , and have wallwart DC output go thru a current limit resistor to charge up a cap and eventually drive the relay into the open state to turn off the AC to your alarm bell.
System resets on power loss.

Shouldn't be that hard.

alarm bell:
http://www.signalguys.com/p/1005.html
(or just a cheap radio turned up really loud!)
wallwart power supply to drive relay (or as needed for the relay you pick)
http://www.mpja.com/prodinfo.asp?number=15178+PD
resistor & capacitor from your favorite supplier such that 1/2PiRC = 5 seconds (or something like that, maybe need a square root of 2 or something in there too, I'm a digital guy, go google it - big cap, small R)
relay:
http://www.mpja.com/prodinfo.asp?number=18255+RL
(is it just me, or is turn-on current/voltage not shown on the data sheet?)

Okay, this one is simple to follow
http://www.mpja.com/prodinfo.asp?number=18474+RL
5V across coils, 120V across the relay itself, good 1A (for that 125W radio, good & loud!)

Don't forget another R in parallel with cap to ground to make sure it bleeds off on a timely manner.

[Reply #4 on:]

This circuit will work? Its Buzzer which works at around 4V.
http://www.mediafire.com/i/?3gb6so3p8ck90pr
Shouldn't I add something to discharge the cap???

[Reply #5 on:]

I don't think so.  Even if the polarized cap you show wasn't damaged by being charged backwards, as I understood your original intent once the power came up & stayed up the cap would charge & stay charged so the buzzer would not stop until power went away.

You really want something that becomes turned off once the capacitor charges up, correct?
That's why I indicated the relay: power source thru Normally closed contacts to your +buzzer, -buzzer to ground.  
To control the relay: +6V to series resistor to +cap and to relay control. -cap to ground. bleed down resistor in parallel with the cap, value 10X of the series cap.
If it wasn't so late I'd draw a schematic & post it.

[Reply #6 on:]

You really want something that becomes turned off once the capacitor charges up, correct?

Yes, I would.
I really wouldn't like to have DC Source like a battery powering the buzzer. since its merely for the few seconds. wouldnt charging up a capacitor do?
I would really appreciate if you could draw up the schematic for the circuit you mentioned when you get time to do it.
Thanks in advance.
Seems like my simple circuit keeps growing bigger :-/

[Reply #7 on:]

Here's what I was thinking

[Reply #8 on:]

@above
This should do the trick... Thanx a lot for the schematic

[Reply #9 on:]

Let us know  how it works out when you're done. Rbleed should be 10x Rdelay, I think I said 10X of the series cap earlier, typo on my part.

[Reply #10 on:]

Sure will work on it in a couple of days time

[Reply #11 on:]

Yup it works... thanx for the circuit