Electronics question...

[Uh_Clem]

I have a problem with a smoke alarm in my kitchen. More accurately I'd call it a "cooking" alarm in that every time I cook anything it starts screeching. So I do what most people I know do and open the thing up and take the battery out until I'm done cooking. Well this evening it started beeping every minute or so to let me know the battery was dying so I went to replace it and realized I don't have any 9 volt batteries. I have a ton of AA's and AAA's though since most other electronic devices use those two sizes of betteries. It's sitting on the kitchen table right now silent because I don't have the right size battery. So that gets me wondering, is there something about smoke alarms that requires that they use 9 volt batteries? Wouldn't it make more sense to design them to use a more common battery?

edit: spelling

[Rob Lister]

Originally posted by Uh_Clem
I have a problem with a smoke alarm in my kitchen. More accurately I'd call it a "cooking" alarm in that every time I cook anything it starts screeching. So I do what most people I know do and open the thing up and take the battery out until I'm done cooking. Well this evening it started beeping every minute or so to let me know the battery was dying so I went to replace it and realized I don't have any 9 volt batteries. I have a ton of AA's and AAA's though since most other electronic devices use those two sizes of betteries. It's sitting on the kitchen table right now silent because I don't have the right size battery. So that gets me wondering, is there something about smoke alarms that requires that they use 9 volt batteries? Wouldn't it make more sense to design them to use a more common battery?

edit: spelling

Back in my day, the 9v battery was the most common. Every new-fangled pocket-sized transistor radio used them. I can still smell the freshly baked plastic...ah, memories.

Anyway, they are designed to use a 9v but I can't think of any component necessity within the smoke-detectors that would require such a high voltage. Which is really to say that I'm sure they could design it around a couple of AA or AAAs if they wanted to. I like that they don't because I can't keep AA or AAAs in my TV changers because the kids steal them for use in their gameboys and such. Pisses me off to no end but at least a dead TV changer won't kill everyone.

[shecky]

Originally posted by Uh_Clem
Wouldn't it make more sense to design them to use a more common battery?

9V batteries are pretty darn common around here. :confused: You should be able to find a decent alkaline 9V in any supermarket.

[Beanbag]

It could be for a couple of reasons. It may just be that the smoke sensor requires a voltage in the 7-9 volt range to operate. There is no reason why you couldn't string together 6 AA cells to get that voltage. I suspect the choice of the 9-volt cell has something to do with wanting a nice, small, round housing to mount unobtrousively on the ceiling.

I've got AC-powered smoke detectors in my house, but they have a 9-volt battery as a backup in case the power goes out (which it might in an electrical fire).

I know the "original" smoke detector sensors in the 70's used a small heater coil as part of the ionizing process; therefore, they took a great deal more power to operate. They weren't battery operated. You plugged them into the wall. Modern units have a small chunk of radioactive material (usually americium) that ionizes the air in the test chamber, and changes in conductivity of the ionized air is what the sensor uses to detect smoke.

Regards;
Beanbag

[Bruce]

9V batteries are rechargable and have a longer shelf life than AA's. As with Beanbag, where I live, all new houses are required to have hard-wired smoke detectors. The electricity from your house is keeping the 9V batteries constantly charged, and the battery is there for back-up. 9V batteries can last 5-7 years under these conditions.

[MRC_Hans]

9v batteries are not intrinsically rechargeable, but just like all other batteries, they come both as rechargeable and non-rechargeable (primary). I have a very compact model of smoke alarm, and it uses two lithium cells (=6v), so apparantly the 3V of two AA cells is not enough, for some technical reason.

Rechageable batteries are not recommended for the sort of long-term use represented by smoke alarms, clocks etc. because they have a much higher self-discharge than primary batteries, and the lifetime if a primary battery is so long that there is no saving in using a rechargeable.

As for the cooking problem, never place a smoke alarm in the kitchen. The best places are corridors, stairwells, and bedrooms (if you get false alarms in the bedroom, cut down on the onions ;)). The best types of smoke alarms can be coupled together, so that if one is activated, they all go off.

Hans

[Soapy Sam]

1. Aesthetics. It has to be small and neat or people remove it.

2.Battery life. If you have to change batteries often, people either don't replace them, or they actually remove them to put in something far more important such as the TV remote or the walkman.

[TillEulenspiegel]

Originally posted by Beanbag
I know the "original" smoke detector sensors in the 70's used a small heater coil as part of the ionizing process; therefore, they took a great deal more power to operate. They weren't battery operated. You plugged them into the wall. Modern units have a small chunk of radioactive material (usually americium) that ionizes the air in the test chamber, and changes in conductivity of the ionized air is what the sensor uses to detect smoke.

Regards;
Beanbag

Huh , I've never seen that. I worked for the inventor of smoke alarms ( BRK corp.-->Pittway corp, ---> First Alert corp.) in the early 70's when I was 15 and lied that i was 18. The scheme was simple, an ionization chamber with a piece of Americium 243 at the center with a grid all around the inside. The Am243 ionized incoming smoke particles and they in turn would be attracted to the grid which was attached to an IGMOSFET. The system was 9V supplied. The bias resistor (IIRC) was some 1.2 Tetra Ohms and the bias current was in Pico A . Never saw a heater mech.


Edit to add: Sorry didn't address the question . There were two reasons for the 9V batt. 1) and most important was the advent of MOS ( metal oxide semiconductors) technology, this type of device absorbs a very small amount of current quiescently so instead of regular chips and transistors eating Ma to stay "on" the low drain meant You could use a 9V instead of stacked double AAs ( more AH). The second was to make it small as was pointed out here.

[Rob Lister]

Originally posted by TillEulenspiegel
Huh , I've never seen that. ...

I think the device to which he was referring was actually a photodetection system. Air would pass through a t-shaped tube. A phototransistor was positioned in the bottom of the tube and a light source at one end of the crossed T. Smoke entering the top of the tube would defract the light and trigger the photocell.

[ctgPi]

Originally posted by Beanbag
It could be for a couple of reasons. It may just be that the smoke sensor requires a voltage in the 7-9 volt range to operate. There is no reason why you couldn't string together 6 AA cells to get that voltage. I suspect the choice of the 9-volt cell has something to do with wanting a nice, small, round housing to mount unobtrousively on the ceiling.


I have once blatantly disregarded manufacturer's safety instructions and disassembled some spent batteries I had lying around but had never bothered to trash. The 9V battery I had dissassembled was composed of six neatly wrapped AAA cells strung together, so the "convenience" explanation is rather plausible.

[TillEulenspiegel]

I have once blatantly disregarded manufacturer's safety instructions and disassembled some spent batteries I had lying around but had never bothered to trash. The 9V battery I had dissassembled was composed of six neatly wrapped AAA cells strung together, so the "convenience" explanation is rather plausible.

Well the cells are 1.5V each but compare them with an aa cell or a aaa cell. The greater mass gives a higher AH output. The 9V cell is a coupla 100 Ma cells with less life for high drain applications as VS an aa cell which when new and stacked ( to 9V) can carry > 1 AH. That's why small motor applications use stacked configurations Like radio controled cars ,Etc. ( as well as many other applications). That's why the adaptation of MOS technology was so important.

[Beanbag]

Originally posted by TillEulenspiegel
Huh , I've never seen that. I worked for the inventor of smoke alarms ( BRK corp.-->Pittway corp, ---> First Alert corp.) in the early 70's when I was 15 and lied that i was 18. The scheme was simple, an ionization chamber with a piece of Americium 243 at the center with a grid all around the inside. The Am243 ionized incoming smoke particles and they in turn would be attracted to the grid which was attached to an IGMOSFET. The system was 9V supplied. The bias resistor (IIRC) was some 1.2 Tetra Ohms and the bias current was in Pico A . Never saw a heater mech.


IIRC, there was a set of gas and smoke detection sensors put out by Taguchi (sp?) that had a heating element built into the sensor. It might not have been an ionizing detector; it --MAY-- have used thermal conductivity. Like I said, it was back in the 70's, when I was waking up from the 60's. As I recall, the complete unit was a boxy number that had an AC line cord you plugged in. It took a few minutes to warm up and settle in. I think they went with that approach because they didn't want the hassle of radioactive materials at that time.

Regards;
Beanbag

[Beanbag]

I used to pick apart the 9v rectangular cells myself. The cheap ones from Radio Shack originally were a stack of 6 flat, rectangular cells stacked one atop the other. They were waxed to keep the moisture in. I discovered the joys of creating large stacks of the cells to make higher voltage batteries. That was back when Radio Shack had the free battery-of-the-month card, and I had a couple dozen. I'd hit each store within bicycle distance for a single 9v battery each over a couple of days, making sure I had a different sales person each time at each store. The largest stack was about a foot long and put out slightly less than 100 volts. I used it for charging a large electrolytic capacitor for vaporizing fine copper wire cut out of lamp cord. Surprised I didn't die of copper poisoning from all the vaporized copper I must have inhaled.

Ah, the joys of a mis-spent youth.

Regards;
Beanbag