2-Wire system sensor smoke detector to Venus OS (Ras Pi) digital input - let the smoke out of smoke detector

[spk]

In case anyone else happens to decide to use an old, wired smoke head from a Honeywell alarm system (System Sensor 2w-b), here's a way to let the smoke out that you can avoid.

So, the 2 wire protocol has 12-24v input to the smoke detecor head, and a pair of "Remote Annunciator" outputs I thught I could hook a relay to and thus interface to the Venus/Pi. When the detector goes into alarm, it shorts the DC input, and the data sheet mentions its supposed to be limited to 130mA, but....the alarm panel, that I am not using, apparently hosts the current limiting resistor or whatever it uses.

So, while testing the theory, I hooked it it a small 12v supply, hit the test button, measured voltage at the RA terminals, all good. Took it out the the shed and repaed the test on 24v, where I let the smoke out. I had a 2 amp fuse, smallest I had handy, but the tiny wires apparently limited the current under that, but that was enough to overheat something in the smoke head.

Probably the right way to do it is to use resistors in line with it, but what I seem to have working is feeding a 12v relay through the smoke head in series, so it gets around maybe 15v and triggers the relay when the detector goes into alarm. That relay then closes the loop to the Pi digital input and voila I get the email notification from Victron's VRM site (no display on the Pi).

One other note - to reset tho smoke detector you have to power it off and on again, so it's feeding all of this through another relay on a Pi relay hat, so I can remotely interrupt the smoke detector 24v feed. We'll see if that works, but it would have been easier to go with the $20 something smoke detector board with it's own relay that Amazon is shipping me. That's Ionization, though, and this smoke detector is Optoelectronic, so maybe I'll run them both.

 

[n3rdpwr]

Those 2 wire smoke detectors are designed to be tied into a 24v conventional fire alarm system (or in some models 12v intrusion alarm system with 2 wire detection circuits). Those initiating circuits are current limited and designed to be shorted as part of their operation. This is what a pull station or heat detector does, they are essentially a switch that shorts the circuit thus activating the alarm for that zone. The smoke detector doesn't quite short but will draw enough current to trip the alarm on that initiating circuit.

These smoke detectors are listed to work on certain fire alarm panels based on current capacity of the conventional circuits, for example the old school Edwards 6500 has some really strong (higher current) input cards and for certain newer models of smoke detectors, required a special base with an inline resistor in order not to fry the smoke detector when it goes active.

The RA (remote annunciator) terminal on the smoke detector is designed to light up a remote LED and may or may not be up to the task of powering the inductive load of a relay long term. This would vary on the model of the smoke detector (check the spec sheet), some smoke detectors are listed to be used with a relay for shutdown purposes (i.e. shutdown fans, releasing doors, etc...).

I'm sure with some trial a 2 wire smoke detector can be used standalone and looks like you are on the right track with using a current limiting resistor.

The easiest way (that I can think of at the moment) to do this is with a 12/24VDC non-latching 4 wire smoke detector (no need to reset) or if you want to go the AC route, just a simple home smoke detector with the interconnect relay module.

Hopefully this information was useful to you.

 

[Goboatingnow]

In case anyone else happens to decide to use an old, wired smoke head from a Honeywell alarm system (System Sensor 2w-b), here's a way to let the smoke out that you can avoid.

So, the 2 wire protocol has 12-24v input to the smoke detecor head, and a pair of "Remote Annunciator" outputs I thught I could hook a relay to and thus interface to the Venus/Pi. When the detector goes into alarm, it shorts the DC input, and the data sheet mentions its supposed to be limited to 130mA, but....the alarm panel, that I am not using, apparently hosts the current limiting resistor or whatever it uses.

So, while testing the theory, I hooked it it a small 12v supply, hit the test button, measured voltage at the RA terminals, all good. Took it out the the shed and repaed the test on 24v, where I let the smoke out. I had a 2 amp fuse, smallest I had handy, but the tiny wires apparently limited the current under that, but that was enough to overheat something in the smoke head.

Probably the right way to do it is to use resistors in line with it, but what I seem to have working is feeding a 12v relay through the smoke head in series, so it gets around maybe 15v and triggers the relay when the detector goes into alarm. That relay then closes the loop to the Pi digital input and voila I get the email notification from Victron's VRM site (no display on the Pi).

One other note - to reset tho smoke detector you have to power it off and on again, so it's feeding all of this through another relay on a Pi relay hat, so I can remotely interrupt the smoke detector 24v feed. We'll see if that works, but it would have been easier to go with the $20 something smoke detector board with it's own relay that Amazon is shipping me. That's Ionization, though, and this smoke detector is Optoelectronic, so maybe I'll run them both.

Simple constant current supply is the simplest.

 

[curiouscarbon]

thank you for sharing this info!

great idea to include smoke detection?

it turns out smoke detectors can also be smoke generators ?

 

[spk]

Update - the 2-wire smoke detector is working fine hooked to 24v, when hooked to a 12v relay. When the smoke detector activates and shorts, the relay is energized and the current drops low enough not to (apparently) cause any harm. I've tested it with the manual test several times and a can of "test smoke" and it seems to be fine.

I also picked up one of these, https://smile.amazon.com/gp/product/B09NCWCTHJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
which is an ionization type of detector, as opposed to the optoelectric way the other one works, but both are wired up to digital input pins and signal Victron's VRM to notify me.

The Amazon one has a sound alarm to it also, though I doubt anyone will hear it not near the shed.