Timed Power Cut-Off Circuit

[Hodgey]

Hi everyone,

I've got a burglar alarm that works off a 12V transformer. The only
trouble is when the alarm is activated via the system it doesn't stop
until the system code is entered into the keypad.

I'm wanting a circuit I can cut into the 12V siren feed (from the alarm
unit) that will shut the alarm off after 20 minutes but will re-set
when the voltage feed from the system is finally lost. It has been
along time since I looked at electronic circuits so any advice would be
greatly appreciated.

I have worked with 555 timers before. Could a monostable 555 timer be
used for this application?

I'm looking for a not too overly complex solution that I can easily put
together myself using breadboard.

Thank you for your time.

Regards,
Mark Hodgkinson

 

[John Fields]

Hi everyone,

I've got a burglar alarm that works off a 12V transformer. The only
trouble is when the alarm is activated via the system it doesn't stop
until the system code is entered into the keypad.

I'm wanting a circuit I can cut into the 12V siren feed (from the alarm
unit) that will shut the alarm off after 20 minutes but will re-set
when the voltage feed from the system is finally lost.

 

[Hodgey]

John, Thanks for your prompt reply which is appreciated.

To fill you in on the problem I've got here's the story:

To provide tamper protection the actual external siren is controlled by
a relay in the external alarm box (i.e. if the wire to the box is cut
the relay drops and a battery in the box powers the siren). Trouble is,
this circuit no longer works because I'm not getting any power from the
main alarm control box associated terminals to keep the relay up so the
siren sounds continuously.

Because my system is over 10 years old, I don't want to get a new
system on the grounds of the above system fault and the fact that it is
virtually impossible to get into the alarm box without great
difficulty, I've decided the tamper feature isn't critical and I want
to power the siren direct from a separate set of terminals I've found
that give around 13.66V across them when the alarm is activated.

I hope you can come up with something for me.

 

[John Fields]

John, Thanks for your prompt reply which is appreciated.

To fill you in on the problem I've got here's the story:

To provide tamper protection the actual external siren is controlled by
a relay in the external alarm box (i.e. if the wire to the box is cut
the relay drops and a battery in the box powers the siren). Trouble is,
this circuit no longer works because I'm not getting any power from the
main alarm control box associated terminals to keep the relay up so the
siren sounds continuously.

Because my system is over 10 years old, I don't want to get a new
system on the grounds of the above system fault and the fact that it is
virtually impossible to get into the alarm box without great
difficulty, I've decided the tamper feature isn't critical and I want
to power the siren direct from a separate set of terminals I've found
that give around 13.66V across them when the alarm is activated.

I hope you can come up with something for me.

---

I'm not sure I can, at this point. That is, I'm confused.

In your first post you stated that you can turn the siren (I'm
assuming that's what the "alarm" is) off by using the system keypad,
but in your second post you stated that the siren runs continuously
because of some problem in the control box. Which is it???

BTW, please bottom post. It makes everything so much easier to
follow for whoever's reading the thread.

Thanks,

 

[Hodgey]

---

I'm not sure I can, at this point. That is, I'm confused.

In your first post you stated that you can turn the siren (I'm
assuming that's what the "alarm" is) off by using the system keypad,
but in your second post you stated that the siren runs continuously
because of some problem in the control box. Which is it???

BTW, please bottom post. It makes everything so much easier to
follow for whoever's reading the thread.

Thanks,

Sorry to have confused you. I think I'm telling you more than is
necessary so I will try to be percise.

The siren did run continuously because of some problem in the control
box. To get over this problem I have done some re-wiring and have
directly connected the siren to a set of terminals which give a +12V on
alarm activation. Previously the siren was connected via a tamper relay
which activated the alarm when the relay was down (i.e. power was
lost). This relay was powered via a different feed from the control
box. It is this feed that is faulty.

Anyhow, the circuit I'm wanting is simply a timer circuit which can be
cut into the 12V siren feed. The circuit needs to provide a 12V output
to sound the siren for approx 20 minutes after a 12V input supply is
provided and then cut-off (i.e. provide a 0V output). The timer should
be re-set when the 12V input feed is lost (i.e. after the code is
entered into the keypad).

The basic problem I've got is that my alarm doesn't stop until the code
is entered but if no one is home it may run for days if not weeks!
There is a possible fine of up to £2000 ($3700) if an alarm runs more
than 20 minutes.

Regards,
Mark

 

[John Fields]

Anyhow, the circuit I'm wanting is simply a timer circuit which can be
cut into the 12V siren feed. The circuit needs to provide a 12V output
to sound the siren for approx 20 minutes after a 12V input supply is
provided and then cut-off (i.e. provide a 0V output). The timer should
be re-set when the 12V input feed is lost (i.e. after the code is
entered into the keypad).