Looking for alarm circuit ?

[Nicksan]

I am looking for a simple alarm circuit, trip a wire and alarm sounds, but with a timer that stops the alarm after 5 minutes or whatever can be set.
I have seen dozens of them online but none with a timer that allows the alarm to stop automatically. Anyone have such a circuit laying around?

Thanks.

 

[Gayan Soyza]

A fully controlled alarm with an adjustable preset time given by the user can do with a microcontroller. It can turn ON the alarm and OFF the alarm automatically by the given time.
But this is getting complicated for you.

What about a timer done in a simple timer IC like 555. There are a large number of circuits on the NET regarding 555 timers. Did you search?

 

[Nicksan]

Yes, I searched. I found many that use the 555 but none that turned off automatically.

 

[Gayan Soyza]

Jesus Christ! You did not search even one that turns OFF automatically.

**broken link removed**

Caclulate the R and C according to your required time.

Here is one link from 10000000000000 search results.

https://www.eleinmec.com/article.asp?4

 

[Hero999]

If the wires too long you'll notice two interesting effects:

One: the capacitance in the wire will cause it to trigger on power on.

Two: it will trigger randomly as the noise picked up by the cable will cause false triggering.

To stop this:

One: connect pin 4 to +V via a 100k resistor and to 0V via a 100nF capacitor.

Two: add a 100nF capacitor across the trigger.

The 100nF capacitor on the trigger will filter out any noise and the 100k and 100nF on pin 4 (reset) will stop it from being triggerd for a short perioud after turn-on.