Delayed Trigger - siren to activate few seconds after trigge

Hi all,

Wondering if anyone could help me with the following problem?

Basically, I have a speaker in a bellbox which "turns on" when the "trigger line" is reduced to 0v. While the trigger is above 4v, the speaker stays quiet.

The line is usually at 12v, until I want to make a noise, when the line is dropped to 0v.

I would like to set a "delay", so that the speaker is only "turned on" when the trigger line has dropped below 4v for a five seconds.

I was thinking an inline capacitor would be the way to do this (so that the line remains above 4v for five seconds) - however i'm really not sure if this is the best way, or what capacitor to get.

Please see images to aid my description:

Uploaded with ImageShack.us

Would anyone be able to help me out with this? Is it the best way to achieve such a "delay"? What capacitor do I need?

Thanks very much for your help

Shockuk.

If you actually want to build it yourself, instead of buying an IC timer from someone like Rapid or Maplins, or making a counter:

a way that electric overrun timers (e.g. fan timers) used to be made with simple materials:

incoming nnV DC through a variable resistor charges a capacitor. The capacitor takes nn seconds to charge up to nn volts

switch detects, is voltage of capacitor equal to incoming voltage yet? When "yes" switch operates (can be solid state or a relay)

In your case I think you would want to discharge the capacitor through a resistor, and when voltage drops to nnv (or zero) operate the switch.

Easiest is to vary the pot to change the charge/discharge times.

The advantage of this simple design is that it is easy to understand and knock up on a breadboard. You can add lamps to show the voltage rising or dropping over time and making ther lamp glow bright or dim.

If you wanted to you could make a water operated timer working the same way, water drips into or out of a can, the can is on one end of a seesaw, when the can reaches target wight the seesaw tilts and operates something.

Is this a school project or something?