How to integrate nearly anything into an HA system using a transistor

signal15

Senior Member
Here's an example. I am looking at installing CO Experts carbon monoxide detectors in my house because they give *instant* alerts instead of the potentially several hour delay that UL listed devices give. They do not currently make a model with alarm contacts on it (but will next year). The red status LED only turns on when the device is in the alarm state. So, on the positive leg of the LED, we can splice in a transistor like this and have our alarm contact:

http://www.rason.org/Projects/transwit/transwit.htm

Of note, I have not tested this yet. But the engineer from CO Experts says that it is certainly possible. I might have to put a very low current relay in the mix also. But, if this works, I could use hook my ELK up to a bunch of stuff, including the low salt LED on my water softener.
 
Here's an example. I am looking at installing CO Experts carbon monoxide detectors in my house because they give *instant* alerts instead of the potentially several hour delay that UL listed devices give. They do not currently make a model with alarm contacts on it (but will next year). The red status LED only turns on when the device is in the alarm state. So, on the positive leg of the LED, we can splice in a transistor like this and have our alarm contact:

http://www.rason.org/Projects/transwit/transwit.htm

Of note, I have not tested this yet. But the engineer from CO Experts says that it is certainly possible. I might have to put a very low current relay in the mix also. But, if this works, I could use hook my ELK up to a bunch of stuff, including the low salt LED on my water softener.

Transistors are great, but you might want to consider an optocoupler. Your going to start to see lots of hairy things happening when you start connecting all these things together without isolation.
 
Here's an example. I am looking at installing CO Experts carbon monoxide detectors in my house because they give *instant* alerts instead of the potentially several hour delay that UL listed devices give. They do not currently make a model with alarm contacts on it (but will next year). The red status LED only turns on when the device is in the alarm state. So, on the positive leg of the LED, we can splice in a transistor like this and have our alarm contact:

http://www.rason.org/Projects/transwit/transwit.htm

Of note, I have not tested this yet. But the engineer from CO Experts says that it is certainly possible. I might have to put a very low current relay in the mix also. But, if this works, I could use hook my ELK up to a bunch of stuff, including the low salt LED on my water softener.

Transistors are great, but you might want to consider an optocoupler. Your going to start to see lots of hairy things happening when you start connecting all these things together without isolation.

Wouldn't a relay isolate the circuit? Aren't the switched side and powered side of the relays separated?
 
If I were to use a 4N27 optoisolator, it should just work to wire the input side either in series or in parallel to the existing LED, right? It would certainly be quick and dirty, but it should work.
 
This is a good method and a long time ago, I actually wrote up a "How-To" on interfacing a common smoke detector into an HA system by using a transistor. I had it working, photos and all.

On the advice of a few Cocooners who I had preview it, I didn't publish it here as they felt (and rightfully so) that adding external circuitry isn't a good idea on life saving devices. Plus, what if someone did it incorrectly and rendered the system nonoperational?

Not to put this method down (as it's a great idea), but give careful consideration on what your interfacing it with. ;)
 
I might have to put a very low current relay in the mix also. But, if this works, I could use hook my ELK up to a bunch of stuff, including the low salt LED on my water softener.


If that's the case why use the transistor just use the relay output
 
An output such as an LED might not have enough drive level to trigger a relay, but could easily change the state of a transistor.
 
If the LED stays lit during an alarm, you can use a phototransistor stuck to the led and covered to let no light in. You'd have to tinker with some resistors to get a good signal, perhaps pair it with a very sensitive solid-state relay. This way you don't tamper with the smoke detector at all.
 
If the LED stays lit during an alarm, you can use a phototransistor stuck to the led and covered to let no light in. You'd have to tinker with some resistors to get a good signal, perhaps pair it with a very sensitive solid-state relay. This way you don't tamper with the smoke detector at all.

3 reasons:
- I want to still see my LED
- I want to be able to hide the mess inside the device
- What if there's no LED, but you still have another circuit that provides a small amount of power when the device is in a certain state and that power isn't quite enough to directly drive a relay?

BSR, you should repost your tutorial, but make it for a water softener LED instead of a smoke. Put some warnings in there.
 
Relays aren't bad, but they are so 1995. ;) On optocoupler uses an LED inside, so you could replace an LED in equipment with an optocoupler. You might even be able to put the optocoupler in series with the LED and modify the series resister. (If there is one.)

But not to rain on anyones parade, like was already mentioned, if this involves safety, do not use a homebrew solution. If this is a 120V detector, there is likely an adapter that you can add to the detector with a relay output, BUT if you are hooking this to a monitored panel, even this is not a UL approved solution. These relays are designed to trigger exhaust fans. Instead if you really like the model detector, call the company and see if they make a model designed to connect to a monitored alarm panel.

I know this sounds like overkill, and its very unlikely anything would ever go wrong, but do you really want that on your conscience to save $100?

The same questions always come up when people want to connect their smoke alarms required by the building codes to their monitored alarm. My response is the same, just don't do it. Add additional smoke alarms that are designed to be connected to a monitored alarm and also keep the ones required by code.
 
The problem with the CO stuff is that this company does not make a detector with an alarm output. And they are one of the only companies out there that sell non-UL listed detectors. As I explained in the first post, I don't want UL listed detectors because they are intentionally designed to be crappy to reduce emergency calls.

And, I don't care about modifying this particular safety equipment. I have other CO detectors that will remain unmodified.
 
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