Sharing a contact closure...

[beelzerob]

I've got a pair of wires that comes from a float and goes to a tank alarm. It's a simple mercury switch...so, when the float rises, it closes the circuit, which makes the alarm go off. On the alarm panel, it's just 2 simple screw terminal connections for the float wires.

I don't want to interfere with the alarm panel operation...in other words, I want that to STILL work. But at the same time, I'd like to be able to sense when that mercury switch has closed so that I can do HA-related events as a result. So, how do I wire is so that both the alarm panel and my own I/O sensor will detect that the switch has closed?

Is it as simple as putting my I/O sensor leads on the screw terminals of the alarm panel?

 

[BraveSirRobbin]

This is a tough one as you really don't know the specs of the float alarm panel. In other words a contact closure has some kind of voltage/current combination running through it. Just blindly paralleling this over to your security system could possibly damage it.

What would be better to do is to have the float trip a double pole relay, with one pole going to the float alarm and the other going to the security system.

This is my opinion, let's see what others may come up with.

 

[electron]

What I do is just run the sensor to one of the I/O devices, then use an output on that I/O device to drive the other I/O device (I do this with my Ocelot/M1). BSR's suggestion is probably the better one tho as you don't have to waste an output.

 

[znelbok]

The double pole relay is the way to go.

Failure of the device (ie alarm) will not stop the other device (HA system) from operating correctly.

relays are cheap as, they are very small as well so you should be able to squeeze one in somewhere with no trouble at all. You will need a power supply though to energise the relay coil, and this needs to not exceed the rating of the float switch (voltage and current).

Mick

 

[beelzerob]

The double pole relay is the way to go.

Failure of the device (ie alarm) will not stop the other device (HA system) from operating correctly.

relays are cheap as, they are very small as well so you should be able to squeeze one in somewhere with no trouble at all. You will need a power supply though to energise the relay coil, and this needs to not exceed the rating of the float switch (voltage and current).

Mick

Ok, I understand now....have the float go to a relay, which will then trip both the alarm panel and my HA I/O input.

It's not my first choice, just because I've still added a layer of failure between the float and the alarm panel...the relay. That's also why I don't want to do the double I/O route, though I considered that.

If I did go the relay route...where do I found voltage/current specs that I'll need....for the float, or for the alarm panel?

 

[BraveSirRobbin]

The double pole relay is the way to go.

Failure of the device (ie alarm) will not stop the other device (HA system) from operating correctly.

relays are cheap as, they are very small as well so you should be able to squeeze one in somewhere with no trouble at all. You will need a power supply though to energise the relay coil, and this needs to not exceed the rating of the float switch (voltage and current).

Mick

Ok, I understand now....have the float go to a relay, which will then trip both the alarm panel and my HA I/O input.

It's not my first choice, just because I've still added a layer of failure between the float and the alarm panel...the relay. That's also why I don't want to do the double I/O route, though I considered that.

If I did go the relay route...where do I found voltage/current specs that I'll need....for the float, or for the alarm panel?

I would use the schematic below. The Elk relay specified only requires 1.2 milliamps (.0012 amps) for a trigger, so that will not cause any problem for your float switch. Just make sure the float switch can handle 12 volts DC as an operating voltage (which I'm sure it can).

The relay itself only uses 60 milliamps of current draw on the supply side 12 volt so it will not cause any issues with your security system power if you chose that route.

As far as worrying about failure, I would worry about the float switch causing you problems more than the relay (thinking about it, you could just parallel two float switches together as it would only take one to trigger the relay)!

 

[beelzerob]

Well, that seems an economical way to do it. The only other resource I can think of is that if the alarm goes off, it's quite loud (like a smoke alarm), and maybe I could wire some kind of sound sensor there to detect that...but I'm thinking that's more clunky and complex than needed.