what is the output voltage on a smoke detector?

[carryyourbooks]

Can someone please verify that it is 9 vdc on the brk9120's?

 

[infinity]

Are you referring to the output on the interconnection conductor?

 

[Dennis Alwon]

My guess is 9V since the battery is 9v

 

[dhalleron]

I'm guessing you want to use a relay on that interconnect.

You might find the interconnect voltage to vary from brand to brand. Some brands transmit data on top of the interconnect voltage. Technically you cannot tie just any relay to that interconnect wire. You must use the proper UL listed relay supplied by the manufacturer. And the instruction sheet for BRK, Kidde and I think Firex says not to use their relay to connect to an alarm panel.

 

[Dennis Alwon]

I have the nmanual for a 9120 and it states

Connect this unit ONLY to other compatible units. Do not connect it to any other type of alarm or auxillary device. Connecting anything else to this unit may damage it or prevent it from operating

 

[infinity]

I have the nmanual for a 9120 and it states

Is that a line voltage alarm?

 

[Dennis Alwon]

Is that a line voltage alarm?

Yes, a standard residential battery backup smoke alarm- 120VAC ionization style

 

[Dennis Alwon]

Here are the specs. Click Here

 

[K8MHZ]

The 9 volt signal from one alarm to the other is very low current. Connecting a relay to it will likely draw too much current, making it not function.

I had to design an interface from the 9 volt signal line to an alarm system. In order to not affect the 9 volt communication line, I had to use transistors to activate a relay that had it's own power source. While going through this, we were working with some fire codes (the alarm company had them) and one of the codes stated that the 9 volts from the alarms could not be used to power a relay. Any relay connected to the 9 volt comm line had to have it's own source of power and also be isolated from the 9 volt signal.

I don't know the exact code, but I built an interface for the alarm panel to their specs and it worked and passed inspection.

 

[K8MHZ]

Another thing I think I had to do was make the interface fail safe, IOW, if the interface failed, even catastrophically, it was required that such a failure not impede the operation of the alarms.

 

[curt swartz]

K8MHZ, why didn't you just use one of the UL listed relays designed to work with the smoke alarms?

 

[renosteinke]

I doubt that smoke alarms 'communicate' in any manner as simple as just supplying power to the horns.

I suspect that they 'communicate' in much the same way as inputs communicate with PLC's. That is, the line is simply checked for the presence of voltage. The PLC then acts according to it's "logic" and sounds a horn- as well as energizing the communication wire. All that line needs, if this is the case, is just enough power to light up a microscopic LED in the next detector.

It's worth noting that there are no standards defining the methods used for communication between the units. The exact method likely differs between both manufacturers and product lines.

 

[K8MHZ]

K8MHZ, why didn't you just use one of the UL listed relays designed to work with the smoke alarms?

This was quite a while ago. If such a relay existed, it wasn't going to be locally available. Also, I like building stuff like that and I had most of the components right on my bench. I could cheaply build and test a prototype and not worry about destroying it.

The house is an up scale second home for a doctor. It's only occupied a few days a month. I know my interface works and what is inside it.

If I had to do it again, I would look for UL listed relay. At least if I didn't like it or the specs I could drop back and home brew one that would make the alarm company happy.

The guy that owned the company that installed the alarm system is a ham radio operator, like me, and both of us like to build stuff. He knew me from both ham radio and other jobs we worked on. When he told me his dilemma and I offered to design and build an interface, he was all for it. If he didn't know me, I could see where he may have not wanted me attempt such a thing.

The guy that takes care of the home is a ham, too. He is the one that has to do the testing. He has to call the alarm company and put the alarm in test mode to push the buttons on the smokes. He is also the one that gets called as the 'keyholder' if any alarm goes off, so he wanted to be assured that the addition of the interface wouldn't be dragging him out of bed for no reason.

But, the simple answer is that I liked making it and I doubted I could get one as fast as I could build one. It was also a challenge to be able to make the device operate as designed and please all involved.

 

[K8MHZ]

I doubt that smoke alarms 'communicate' in any manner as simple as just supplying power to the horns.

I suspect that they 'communicate' in much the same way as inputs communicate with PLC's. That is, the line is simply checked for the presence of voltage. The PLC then acts according to it's "logic" and sounds a horn- as well as energizing the communication wire. All that line needs, if this is the case, is just enough power to light up a microscopic LED in the next detector.

It's worth noting that there are no standards defining the methods used for communication between the units. The exact method likely differs between both manufacturers and product lines.

Wired linked alarms simply put 9 volts on the extra wire to indicate a trip. I haven't seen any difference, but I haven't seen them all. That 9 volt battery is also there to operate the sensors and sound the alarm (it's not a horn, it's a piezo electric device) if the power goes out.