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Electronics => Projects, Designs, and Technical Stuff => Topic started by: 8086 on March 30, 2013, 02:58:07 am

Title: Interesting connection inside a cheapo smoke alarm
Post by: 8086 on March 30, 2013, 02:58:07 am

So I bought a cheap smoke alarm as the battery had died in the one I had. I only bought it for the 9V battery, which was the cheapest option in the store, so I had a smoke alarm spare. Teardown time!

I found an interesting design choice near the radioactive source/detector. There's a metal shield that connects directly to a pin on the DIP device next to it, without using the PCB at all. Both go through a hole drilled in the board and there appears to be something like a guard ring around the pins.

I hadn't seen this before so I thought it might be of interest to some of you.

http://imgur.com/a/GRGeW (http://imgur.com/a/GRGeW)

Title: Re: Interesting connection inside a cheapo smoke alarm
Post by: NiHaoMike on March 30, 2013, 03:50:30 am

That's a high impedance connection. For it to work, leakage must be kept to a minimum.

BTW, it's pretty easy to wire it to always be in an "alarm" state so you can use it as a nice buzzer. (Or just wire the piezo to a microcontroller.) Also, if you attach the radioactive element to a cheap webcam board, you can get a nice random number generator.

Title: Re: Interesting connection inside a cheapo smoke alarm
Post by: TerraHertz on March 30, 2013, 03:53:58 am

Cute.
For detection of ultra low current from that front electrode, to avoid surface creep leakage on the board, they eliminated the board!
You see an equivalent scheme in other sensitive circuits, except using a teflon post to support a solder tag that has the critical circuit component legs attached.
This way sure is cheaper!

Title: Re: Interesting connection inside a cheapo smoke alarm
Post by: amyk on March 30, 2013, 08:06:38 am

Did they really try to scrape the part# off that easily identifiable (http://media.digikey.com/pdf/Data%20Sheets/Allegro%20PDFs/A5364.pdf) IC? Guard ring and the rest of the circuit look identical to the datasheet.

Title: Re: Interesting connection inside a cheapo smoke alarm
Post by: codeboy2k on March 30, 2013, 10:36:55 am

Just don't go trying to scrape out that radioactive stuff more than necessary for just 1 random number generator.

This guy was trying to make a reactor in his childhood, and as an adult was caught stealing smoke detectors.
Likely he wanted to try again.

from: http://www.foxnews.com/story/0,2933,299362,00.html (http://www.foxnews.com/story/0,2933,299362,00.html)

(http://i.imgur.com/msWCNL0.jpg)