Looking for some help identifying and understanding one of the two components in a circuit I am fixing, should be very simple

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
54
I am just resoldering a 9v battery clip in an old Christmas ornament for a friend. The ornament is apparently designed to be a little smoke detector that fits inside a plastic enclosure that looks like a christmas ornament. The circuit is a piezoelectric buzzer and what I would assume to be the smoke/heat sensing element. In the picture you can see there are two terminals and it apparently seems to be in series with the buzzer(there is one black wire coming from the buzzer that is connected to a terminal of the “sensor.” I connected the positive supply terminal to the other end of the “sensor.” In the middle there is a set of contacts. I don’t recall what this is specifically but it almost seems like it functions like a relay arm but if I close the contacts the alarm buzzes. It looks like the middle of the buzzer may function like a sort of relay too that can magnetically move it when they are lined up in the enclosure. I attached a picture and this looks old but any help would be appreciated. I don’t know how to find a place to start learning about how it works but it doesn’t seem like there is much to it.
 

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MrChips

Joined Oct 2, 2009
30,792
I am just resoldering a 9v battery clip in an old Christmas ornament for a friend. The ornament is apparently designed to be a little smoke detector that fits inside a plastic enclosure that looks like a christmas ornament. The circuit is a piezoelectric buzzer and what I would assume to be the smoke/heat sensing element. In the picture you can see there are two terminals and it apparently seems to be in series with the buzzer(there is one black wire coming from the buzzer that is connected to a terminal of the “sensor.” I connected the positive supply terminal to the other end of the “sensor.” In the middle there is a set of contacts. I don’t recall what this is specifically but it almost seems like it functions like a relay arm but if I close the contacts the alarm buzzes. It looks like the middle of the buzzer may function like a sort of relay too that can magnetically move it when they are lined up in the enclosure. I attached a picture and this looks old but any help would be appreciated. I don’t know how to find a place to start learning about how it works but it doesn’t seem like there is much to it.
What appears to be contacts could be a bimetallic strip. When heated, the strip would bend towards the contact and close the circuit. You can try applying a little heat from a cigarette lighter and see if it closes the contact.
 

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
54
What appears to be contacts could be a bimetallic strip. When heated, the strip would bend towards the contact and close the circuit. You can try applying a little heat from a cigarette lighter and see if it closes the contact.
Thank you, that was extremely helpful and the lighter worked.
 

wayneh

Joined Sep 9, 2010
17,498
Since the relay closes based on temperature, this is apparently a fire detector and not a smoke detector. Cute, but don't rely on it!

My daughter and I were settling down to watch a movie one Christmas when my 30-yr old CO detector went off. My first thought was that it finally failed. But I knew it would be idiotic to ignore the alarm and dug out a new detector we hadn't yet put in place. The new one that actually shows the level instead of just making noise.

Long story short, the heat exchanger in the furnace had failed and was pumping out CO into the house. A 30-yr old detector may very well have saved the lives of my daughter and me.

Point is, have working detectors in your home. Doodoo really does happen.
 

wayneh

Joined Sep 9, 2010
17,498
Yes, you were lucky.
I've been told they are only supposed to last about 10 years.

Do you remember what the reading was?
I remember it varied a lot depending on where we placed it and whether the furnace was running or not. The level would drop fairly quickly when not running. I don't remember specifics much but I know at one point I saw triple digits for the ppm readout. That would be really bad over the period of a few hours. We may have been fine if we turned off the furnace for the night but chose instead to go elsewhere for the night.
 

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
54
Since the relay closes based on temperature, this is apparently a fire detector and not a smoke detector. Cute, but don't rely on it!

My daughter and I were settling down to watch a movie one Christmas when my 30-yr old CO detector went off. My first thought was that it finally failed. But I knew it would be idiotic to ignore the alarm and dug out a new detector we hadn't yet put in place. The new one that actually shows the level instead of just making noise.

Long story short, the heat exchanger in the furnace had failed and was pumping out CO into the house. A 30-yr old detector may very well have saved the lives of my daughter and me.

Point is, have working detectors in your home. Doodoo really does happen.
It’s funny you say that, it’s not mine but a friend’s, and there was actually a fire and the enclosure is melted but they still hang it on their tree for a laugh. I wanted to fix it for them but was worried it may buzz without any fire. I agree it does seem like a heat detector more than a smoke detector.
 

MrChips

Joined Oct 2, 2009
30,792
It’s funny you say that, it’s not mine but a friend’s, and there was actually a fire and the enclosure is melted but they still hang it on their tree for a laugh. I wanted to fix it for them but was worried it may buzz without any fire. I agree it does seem like a heat detector more than a smoke detector.
I agree that it is a fire detector thought it is rather dubious on its usefulness. The tree would already be up in flames for it to sound the alarm.
 
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