[SOLVED] need 12v alarm system design help --I want the alarm light to stay on for 30 seconds

need 12v alarm system design help --I want the alarm light to stay on for 20 seconds

We have an alarm system on our boat that is very basic, it is 12v and about as simple as it gets.
(float-switch makes a connection when bilge water gets high enough and light and siren go off... Different lights for different compartments, one siren run off a bus)
Operationally this is functional except when alarms are being triggered for Just a second, and you don't get a light on the panel long enough to see where the problem is.

What I would like to do is build a circuit that will keep the light on (or flashing) at the panel for 10 or 20 seconds after the alarm has ceased being triggered. I am looking for the easiest, simplest circuit that I can build with radio shack parts I can get from whatever port we are in. All I need is the light to stay lit, or flash, for 10 to 20 seconds so I know where to look for trouble.

I am not at all electronics literate, and don't know where to start. Thank you for your patience.

(the intermittent alarm happens when bilge water is higher than normal, but not quite high enough to set off an alarm, until the vessel starts rolling and bilge water sloshing side to side in 30+ degree rolls trips the switch... needless to say when we are rolling 30+ degrees, knowing where the water is getting high in our wooden ship is important.)

This isn't a terribly direct method, but they are the simplest devices I can think of quickly for your situation. What you want in a monostable multivibrator, also known as a one-shot. The circuit is powered at all times (low draw, a few milliamps). When a single pulse comes in, a longer pulse comes out.

See this website for some quick reading/diagrams and see if it might be up your alley. 555 timers might be available at Radio Shack... I'll have to do some thinking about less complex methods of doing the same function.

Oh, you'll need a voltage divider at the input, to take 12V pulse down to 5V, but that's just two resistors that are easy to calculate. Additionally, you'll need a small linear regulator to take the 12V boat bus voltage down to 5V to run the IC... but that's one IC with two small capacitors beside it.

**broken link removed**

Re: need 12v alarm system design help --I want the alarm light to stay on for 20 seco

CaptainKnots said:

... float-switch makes a connection when bilge water gets high enough and light and siren go off... Different lights for different compartments, one siren run off a bus ...

Click to expand...

Does that mean that there is a separate switch for each light? Is/are the switch/es just a pair of contacts which close when the water is too high?

What is the "bus" which drives the siren?

Re: need 12v alarm system design help --I want the alarm light to stay on for 20 seco

Syncopator said:

Does that mean that there is a separate switch for each light? Is/are the switch/es just a pair of contacts which close when the water is too high?

What is the "bus" which drives the siren?

Click to expand...

If his boat power system is anything like I've seen before, it's typically a 12 VDC system with several lead-acid batteries (deep cycle/marine), that power the boat's electronics while the engine is off. Similar systems are used in RV's/campers.

Yes, 12v DC deep cycle batteries
The switches are separate, and are at their most basic exactly a set of contacts, although the high heat alarm for the engine room is a little more complicated...

The alarm system is accepting current or no current, no current is no alarm, and when current is sent to any light it will also sound the siren as the lights all share a leg with the siren at the bus.

Thanks for the one-shot idea we will look into it. That is the type of thing that we need. Are the bullet proof/last long, and can they be wired in a way that a failure will still sound the alarm as normal?

Well if you know how much current it gives out for that split second that it lights up you could get a 12v latching relay and wire it so that when the alarm goes off it flips the relay and turns on the light.

Fworg64 said:

Well if you know how much current it gives out for that split second that it lights up you could get a 12v latching relay and wire it so that when the alarm goes off it flips the relay and turns on the light.

Click to expand...

woah. That is stupid (edit: meaning super simple) and bullet proof.
If I understand how that works it looks like that could be wired so that a failure in the relay would not interrupt the intermittent alarms (a design requirement) and that a simple power kill at the breaker for the alarm system would reset everything.

I will seriously consider this option, the problem I see with this, and possibly why this never occurred to me is that the poor watch officer shouldn't have to be checking if alarms are still active or not. Generally speaking high water alarms aren't going off for no reason, they are generally accompanied by heavy seas, or sideways rain, or both. The human element comes into play here where a task saturated watch officer is sending all his resources to pumps when he should be checking that the hatches are all closed and or find the source of the extra water, and exacerbates the problem because the alarm panel needs to be turned off and back on to reset the relay.

Also with my design, if you know a place where you can always find 12v you could add a momentary switch to reset the relay, i would whip up another design in mspaint but im on a mobile device right now, im sure you can imagine it though

after talking with my engineer about the relay idea, and looking at how the siren interacts with the LEDs I think that the relay idea is the way forward. We are going to rewire the system so the relays do not also sound the siren. So when the alarm is going off the siren will sound, but once the siren has sounded, even if only momentarily the light will stay on so we know where there was trouble. I also like the idea of a switch. Thanks for pointing out a solution so simple.