EEVblog #673 - Mailbag

[dentaku]

At least that was a good example of why solar panels should be put in a box before bubble wrapping them because you're not as likely to bend the box while struggling with the tape.

[timelessbeing]

+1 for the µRuler campaign.

[Laertes]

Love the 2-Minute Teardowns™.
Also, another +1 for the uRuler campaign!

I wonder why the checkpoint thing is so powerful. To my knowledge, there's a lot of different systems for theft detection RFIDs in frequency ranges between 10kHz and microwave bands, but almost all of them are dumb as a rock and the sender/detector units could easily be built fully analog - that is some serious overkill hardware they have there. And it's not like they manufacture hundreds of it, it's probably more like hundreds of thousands, so why not optimize for cost? Am I underestimating modern anti-theft RFIDs?

[NiHaoMike]

Could be for a library or other place that rents stuff, where you have to check if all leaving items are in fact authorized to leave, as opposed to removing or disabling the tags on purchased items.

[EEVblog]

Re the ferrous battery correction on the Sanwa VOM...
D'arsonval meter movements can have an external magnetic field and hence the meter accuracy can be degraded with nearby magnetic objects.

Interesting, thanks.

[alho]

Love the 2-Minute Teardowns™.
Also, another +1 for the uRuler campaign!

I wonder why the checkpoint thing is so powerful. To my knowledge, there's a lot of different systems for theft detection RFIDs in frequency ranges between 10kHz and microwave bands, but almost all of them are dumb as a rock and the sender/detector units could easily be built fully analog - that is some serious overkill hardware they have there. And it's not like they manufacture hundreds of it, it's probably more like hundreds of thousands, so why not optimize for cost? Am I underestimating modern anti-theft RFIDs?

The simplest anti-theft tag that I'v seen is few metal strips stacked together, so the tags are dumb as a rock. Security gate has transmitting coil that makes tag resonate and second coil detects it. The tags are passive so transmitter must be powerful and receiver  sensitive and the gate has to detect different types of tags at different frequencies. Large market might have 10 gates at cash registers so 10 powerful transmitters and 10 sensitive receivers very close together.  The gates cant take much space (few inches wide) which limits shape of antennas and shielding. Store owner wont accept many false alarms, once a day is probably too many. Does that hardware still look like overkill? Just imagine what kind of problems signal reflections from store structures must cause (has to work in any shape store that gates are sold to) or gates at next door store made by different manufacturer.   

[wilgil]

Does it bother anyone else that there appears to be a DSO upside down on the shelf behind Dave?

[miguelvp]

Does it bother anyone else that there appears to be a DSO upside down on the shelf behind Dave?

Nah, it looks fine down under.

Since he build that new bench and the new mailbag format that scope is been that way since ever. He knows but I guess its to attract attention for us OCD people

[HKJ]

I wonder why the checkpoint thing is so powerful. To my knowledge, there's a lot of different systems for theft detection RFIDs in frequency ranges between 10kHz and microwave bands, but almost all of them are dumb as a rock and the sender/detector units could easily be built fully analog - that is some serious overkill hardware they have there. And it's not like they manufacture hundreds of it, it's probably more like hundreds of thousands, so why not optimize for cost? Am I underestimating modern anti-theft RFIDs?

There is a couple of different security types, not only RFID (That needs a cpu, because it uses digital communication).
Some of the old analog types needs a rather powerful sending coil (A CRT within a few meters of a gate would be affected) and the receiver would either be a couple of analog filters or a DSP. I have seen this type used in libraries, because the security can be activated again.
Another type of security is a resonance circuit, where the gate scans for the resonance circuit. These tags are either removed or burned out, before the goods can be moved through the gate.

[Laertes]

Love the 2-Minute Teardowns™.
Also, another +1 for the uRuler campaign!

I wonder why the checkpoint thing is so powerful. To my knowledge, there's a lot of different systems for theft detection RFIDs in frequency ranges between 10kHz and microwave bands, but almost all of them are dumb as a rock and the sender/detector units could easily be built fully analog - that is some serious overkill hardware they have there. And it's not like they manufacture hundreds of it, it's probably more like hundreds of thousands, so why not optimize for cost? Am I underestimating modern anti-theft RFIDs?

The simplest anti-theft tag that I'v seen is few metal strips stacked together, so the tags are dumb as a rock. Security gate has transmitting coil that makes tag resonate and second coil detects it. The tags are passive so transmitter must be powerful and receiver  sensitive and the gate has to detect different types of tags at different frequencies. Large market might have 10 gates at cash registers so 10 powerful transmitters and 10 sensitive receivers very close together.  The gates cant take much space (few inches wide) which limits shape of antennas and shielding. Store owner wont accept many false alarms, once a day is probably too many. Does that hardware still look like overkill? Just imagine what kind of problems signal reflections from store structures must cause (has to work in any shape store that gates are sold to) or gates at next door store made by different manufacturer.   

Hmm...I would have thought the amount of power these resonant systems put out isn't all that great. Because of the size of the coil(usually something in the area of a coil wound a couple of times around the entire detector, maybe 1.2-1.6m tall) and the very short range it's designed for(distance sender-detector ~1m) they should be able to get away with very little power, shouldn't they? Would also avoid all the problems involved with stuff like interference from the shop next door...

[SeanB]

The simple strips are actually a very complex system of a magnetic strip sandwiched between a mumetal strip, that is magnetised and when the external field is coupled with it it saturates and makes a harmonic of the excitation frequency, which is detected by the receive coil and then is processed by a DSP to get the signal out. Degauss the strip and it no longer generates the harmonics, and it can then have a magnetic field applied to it to reset it to operate again.

The drivers use a large strip aluminium coil, typically 1in wide strip, wound into a coil spaced with foam tape. Around 20 turns about 2ft wide and 5ft high, and this is driven with a high power audio signal of around 15kHz, generated by the DSP and amplified with a class D amplifier, and then filtered by brute force LC filters to drive with a very low distortion sine wave, low distortion as you are detecting harmonics in the presence of a very strong excitation. The receive side uses a few bandpass filters that remove the transmit signal and amplify to feed the DSP that then does the heavy lifting. The systems switch between each coil acting as transmitter and receiver alternately, and use 2 or more frequencies at the same time to provide tbetter discrimination. Processor there is pretty simple, mostly generating the control signals, making the PWM transmit drive and doing comms to the built in counters and displays, along with driving external communications.

Power is around 150VA, with massive amounts of capacitors to provide very low ripple on the supply rails.

Yes, I did take one apart............. Still got a lot of the boards knocking around.

However the one Dave got is more likely a controller for an access gate, like you get in large buildings where you clock through gated with a pass, and where you need a card to use things like lifts and to enter offices. Thus the need for a Lotus database back end, and the smaller contactless card driver, along with the motor drive systems for things like gates, semaphore lights and to interface to things like door releases and door position detectors and most likely also interfaces for alarm inputs and outputs, and with most likely inputs for occupancy sensors, temperature and light sensors and more outputs to control lighting, air damper valves and possibly even automatic blinds or window shades. Having all that on board having such a high power processor is starting to look like it might be needed.

[Laertes]

Power is around 150VA, with massive amounts of capacitors to provide very low ripple on the supply rails.

Holy shitsnacks, that's a lot more power then I thought...any chance you'd do a video on or post some photos of the boards out of the system you took apart?

[GBoos]

As to that resistor, I'm guessing it's rated MUCH higher than 500V - these kinds of resistors were/are used for amongst other things for high voltage dividers. I'd look for the error somewhere else.

Yes, this are High-Voltage Resistors made by Ohmcraft
http://www.ohmcraft.com/
Series CR KOBRA.
The big one is made for 32kV and 12W of max. Power,
the small one is made for 16kV and 7W max. Power.
BTW, 500V @ 20GOhm makes 12,5µW Power.

[jerry507]

Dave, I happened to be listening to some Amp Hour episodes on a flight overseas and one of them mentioned Chris trying out layout on a plane. Your thought was that you're so tied to the internet that it's not worth bothering. If that's the case, what is the inherent downside of a program using the cloud? Obviously a bad program in other respects will ruin the experience, but given two identical programs where one is "cloud based" and the other stores files to your local hard disk is there any reason not to like this? Seems a bit contradictory.

[Rasz]

Dave, I happened to be listening to some Amp Hour episodes on a flight overseas and one of them mentioned Chris trying out layout on a plane. Your thought was that you're so tied to the internet that it's not worth bothering. If that's the case, what is the inherent downside of a program using the cloud? Obviously a bad program in other respects will ruin the experience, but given two identical programs where one is "cloud based" and the other stores files to your local hard disk is there any reason not to like this? Seems a bit contradictory.

its lovely, especially if you want chinese/competition with friends at nsa/cia/Altium to have your company design before your company does
its ok as long as you are also ok with mounting Dropcams (cloud based webcam) in your bathroom

[SeanB]

Power is around 150VA, with massive amounts of capacitors to provide very low ripple on the supply rails.

Holy shitsnacks, that's a lot more power then I thought...any chance you'd do a video on or post some photos of the boards out of the system you took apart?

Found the only board still around, I stripped the caps out of it though, to reuse on a UPS to lower battery ESR. This was the power amplifier board, with the 2 class D power amplifiers on the left. Another board had the massive LC output filters and the huge film capacitors that were used to make a pure sine wave drive for the coils.

[rs20]

At least that was a good example of why solar panels should be put in a box before bubble wrapping them because you're not as likely to bend the box while struggling with the tape.

Reading the letter first would have helped. As would cutting the tape. Speaking from experience, and not hindsight.

I was starting to think that amount of bubble wrap was a joke.

Sorry, I normally try to restrain myself, but:

The amount of bubble wrap was a joke. That was just an absolutely stupid way to package that cell. What breaks a solar cell? Even moderate bending forces. What does nothing to dissipate bending forces? Bubble wrap. What is likely to cause lots of bending forces during opening? Craploads of tape that achieves nothing useful. What protects a solar cell? A nice sturdy box and a little bit of foam/bubble sheet with just enough tape to hold the lid shut. If you think for half a second about what breaks a solar cell, and were to go out of your way to package it in the worst possible way, the sender of that package succeeded. Dave should feel no guilt whatsoever, this is a valuable lesson for people that think that bubble wrap is just a magic potion that doesn't require thought to use. Facepalm at anyone who defends the sender.