Canned Ravioli Saves the Day
erco
Posts: 20,256
Had a project deadline today, so I had to finish it last night. All the individual pieces were working on the bench, and was quite a collection of cheap parts I've touted here before. Dollar store speaker amp, MP3 player, tilt/impact switches, 10-cent buzzer, etc. Project is a toy which plays various MP3 sounds and flashes LEDs based on user & switch input. I made an auto-off circuit using the cutest little 10-cent reed relay. The 510-ohm coil only pulls a few mA from the 3xAAA cells, so it's powered directly from a micro pin. After 30 seconds of inactivity, the micro switches the relay off, providing 100% isolation, zero battery drain. Works GREAT in the lab, on the bench. 
But when I assembled the module for the final installation around midnight, it would turn off anytime there was a loud sound file playing. Electrical noise? Added filter caps. No. Added decoupling caps. No.
Wait, I also have an electromechanical buzzer on the PCB which I use to send occasional confirmation beeps & tones. That could be the source of some EMI. Disconnect buzzer. No improvement.
Reduce amplifier volume, problem goes away. But that's not an option.
Power drain? Is the micro resetting from low power when the speaker/amp is drawing lots of current? Battery wires too small or routed wrong? Rewired. No. Did it get even worse? The LEDs aren't even connected at this point, it will need even more power!
Sweating now, it's 1 AM. Added a transistor switch to drive the relay. No improvement.
The reed relay was opening up for mysterious reasons. But I'm the relay guy, relays usually like me. Not tonight though...
Stared at the assembly for a while. In the assembled form, the circular PCB are screwed to 3 bosses inside the speaker housing. Makes for a beautiful, compact prototype, not that anyone would see it inside the toy. I've mentioned this speaker here before, it has a huge permanent magnet speaker inside. Hey wait, that huge permanent magnet is very close to the PCB in the final installation, could that be causing the problem?
2 AM, let's add a quick (insulated) metal shield between the speaker and PCB. No steel handy. Recycling's gone, I cut a circle out of my aluminum Diet Coke can. No improvement. Pulled the ferrous pop top off a can of Chef Boy R Dee ravioli and installed it. 100% success. Turns out that sensitive little reed relay was being affected by speaker magnetism at high volumes. DOH!
What's funny is that this was a second-gen circuit. I had made a nearly identical board previously which worked perfectly. The relay was an afterthought on that board, and I mounted it at 90 degrees for pin access. Apparently the reed contacts were less susceptible in that 90-degree configuration.
So my story ends happily. Finished the job at 3 AM and delivered it to a very happy client at 10:30 who will never suspect that there's a can lid inside.
Now it's time for some sleep. Then I have to build another one. Maybe two.
Note to self: hoard more ravioli.
But when I assembled the module for the final installation around midnight, it would turn off anytime there was a loud sound file playing. Electrical noise? Added filter caps. No. Added decoupling caps. No.
Wait, I also have an electromechanical buzzer on the PCB which I use to send occasional confirmation beeps & tones. That could be the source of some EMI. Disconnect buzzer. No improvement.
Reduce amplifier volume, problem goes away. But that's not an option.
Power drain? Is the micro resetting from low power when the speaker/amp is drawing lots of current? Battery wires too small or routed wrong? Rewired. No. Did it get even worse? The LEDs aren't even connected at this point, it will need even more power!
Sweating now, it's 1 AM. Added a transistor switch to drive the relay. No improvement.
The reed relay was opening up for mysterious reasons. But I'm the relay guy, relays usually like me. Not tonight though...
Stared at the assembly for a while. In the assembled form, the circular PCB are screwed to 3 bosses inside the speaker housing. Makes for a beautiful, compact prototype, not that anyone would see it inside the toy. I've mentioned this speaker here before, it has a huge permanent magnet speaker inside. Hey wait, that huge permanent magnet is very close to the PCB in the final installation, could that be causing the problem?
2 AM, let's add a quick (insulated) metal shield between the speaker and PCB. No steel handy. Recycling's gone, I cut a circle out of my aluminum Diet Coke can. No improvement. Pulled the ferrous pop top off a can of Chef Boy R Dee ravioli and installed it. 100% success. Turns out that sensitive little reed relay was being affected by speaker magnetism at high volumes. DOH!
What's funny is that this was a second-gen circuit. I had made a nearly identical board previously which worked perfectly. The relay was an afterthought on that board, and I mounted it at 90 degrees for pin access. Apparently the reed contacts were less susceptible in that 90-degree configuration.
So my story ends happily. Finished the job at 3 AM and delivered it to a very happy client at 10:30 who will never suspect that there's a can lid inside.
Now it's time for some sleep. Then I have to build another one. Maybe two.
Note to self: hoard more ravioli.
This discussion has been closed.
Comments
Can't live without canned Ravioli....
What matters is that all of his usual audience, his robots are one-week away from here, and are visiting designers in two locations in preparation to visiting Maker Faire.
This should give the tinfoil hat wearers something to think about.
How did you cut that perfboard so neatly?
The original and the best.
https://www.britishcornershop.co.uk/heinz-ravioli-in-tomato-sauce
The Heinz Spaghetti Bolognese is excellent as well.
https://www.britishcornershop.co.uk/heinz-ravioli-in-tomato-sauce
All to be eaten on toast of course. One can per serving.
The great thing about these is that you can use the cans to make long range antennas for your WIFI and other wireless links.
But then, sometimes, there is a desperate longing for something a bit different. Like my mum used to give me
My mom made ravioli from a can. Which is why I haven't had it in 50 years. If I live that long, I plan not to have it for another 50 years.
Erco, you need to test if you get the same effect with metal lids from soup cans, steamed vegetables, chili, and yes -- even Spam. The unique shape of the Spam can might provide additional signal rejection. You could be on to something!
This could usher in a whole new age of advertising opportunities... "free antenna with every can of soup", " 'high-quality' RF filter included", "Free Manhattan-style breadboard; just remove lid!"
Speaking of which, that's another candidate to test!
Put your mobile phone into a die cast aluminium project box like this one:
Put the lid on and secure it tightly with the six screws. Note how the lid is a good tight fit, you could not slide a cigarette paper in there. Especially with the recess in the lid.
Now get your friend to call your phone.
Well, what do you know, the phone gets the call and rings! (Most likely, except perhaps not in the USA where the cellular service is so poor)
When I tried this experiment a year ago I was amazed. And puzzled, how on Earth is this possible?
After a chat with an antenna designer I understood what was happening and could take steps to stop the phone receiving the call whilst in the box.
Anyone here like to suggest what was happening and how I fixed it ?
Sorry no. Certainly it can be, it's a conductor after all. It's time for you to review your high school science classes re: Faraday Cage. Perhaps start here: https://en.wikipedia.org/wiki/Faraday_cage Despite your incorrect assertion you are getting close. But the change/addition is very minor. You might not even see it from the outside of the box.
The answer lies in having some idea as to how those "mobile waves" are getting in.
Anyone who has such a metal box should try the experiment. It's fascinating.
In any case, radio waves are complex.. so I won't try to guess how you modified the setup.
Thing is it's all to do with the wavelength of the electromagnetic radiation you are trying to block. Basically when the gaps between the conductors of your cage start to get small with respect the wavelength, the waves are reflected rather than passing through the holes. A chicken wire box may block the long waves of medium wave broadcasts but it won't stop the little short waves of your mobile phone.
Actually we should not think about the holes. The holes don't do anything. We should think about the conductor around the hole that is creating an antenna that can absorb and re-radiate, reflect, the incoming waves.
In the extreme X-rays and gamma rays will pass through your "solid" aluminium box. Their wave length is so short that as far as they are concerned it's full of big holes. The distinction between Faraday Cage and Farday Shield is a bit silly. All conductors have holes. Unless you specify the wave length.
Radio waves are indeed slippery things. However there was noting special in what I did to block the phone waves. It's been standard practice in RF shielding for decades of course. Only it was the first time I experimented with it. Well, since my long forgotten classes in micro-waves and such back in uni.
That box has six screws that we can assume short the lid to the box in 6 places. But what about in between? It's not sure the lid and box are in electrical contact all along the edges between the screws.
But I thought, those gaps are incredibly thin, much smaller than the wavelength of mobile waves (about 10cm). You can't even see light through them due to the recess. How can anything radiate through that?
My antenna designer friend put's it like this: When the mobile waves hit the box they create eddy currents in the skin of the aluminium. Normally those eddy currents would just re-radiate the waves. Reflection.
But those eddy currents circling around near those thin slots between the screws continue their circling through the slot and into the inside surface of the box. From their they re-radiate, but this time inside the box!
The fix, as Dave hints, is to create more short circuits between the lid and box such that the slot length is short wrt the wavelength. This way the eddy currents get shorted out and the "loops" formed re-radiate on the outside of the box rather than the inside.
Spring metal fingers is a way that it is often done. Often seen with metal can covers over high frequency components on circuits.
My solution? Tin foil. I put little bits of folded tin foil half way along each slot and screwed the lid down. Thus making the slots half as long. No more phone ringing!
As for wrapping a phone in tin foil that is an eye opening experience as well. Casually wrapping foil around a phone will not do it. There will be little gaps as above. I had to really try to get enough tin foil wrapped tight enough to kill the phone.
I strongly suspect we have nothing to worry about. Unless erco's robots return to where he is with a New York City sized ego......
Any links or references to what you say ?
It comes in his famous book, "Cuckoo's Egg".
I have not read the book, but I assume it tells the story told in the video link above. About catching a KGB hacker/spy from Germany in 1986. You say "according to" as if Clifford's story were some wild made up story or conspiracy theory. As far as I can tell it is true.
I saw Ciff Stoll at the Maker Faire in the summer. Really wanted to say hello, but at that moment I could not remember his name so I was to shy to do so.