Recent Posts

11

Beginners / Re: Help finding PCB design flaw

« Last post by ataradov on Today at 11:24:08 pm »

First of all, stop powering things from USB power supplies. Use a lab power supply with a low current limit.

Then attach your actual schematic. The schematic in the archive does not seem to match the board. In the Schematic D1 just goes to micro-USB connector. On the board it goes to both.

Wait, schematic is for a completely different board.

12

Beginners / Re: Polystyrene cap percentage replacement ?

« Last post by algorithm on Today at 11:22:43 pm »

So somethings ive done. Replaced the 4 electrolytics, removed all diodes and transistors to test out of circuit. Removed all 062 op-amps to test. (1 062 was bad) removed lots of resistors. Im just fighting a losing battle it seems. I just removed 2 resistors correlated with the questionable ranges rotary traces... Color code as follows: black, gray, white, gray. 2nd resistor:gray, white, gray, gold... (Laughs) i would have assumed they were both the same but black cant be tolerance band. But wtf are they supposed to be? I cant even find an 8g ohm resistor for sale, or 8.9g. the resistors test 900ohms and 90ohms. So are the bands misprinted? Times like these...

13

Metrology / Re: 0.1 Hz to 10 Hz Passive Filter

« Last post by Alex Nikitin on Today at 11:18:24 pm »

Some further results with a better voltmeter. The noise floor with the filter is now about four times better (~500nV p-p at NPLC1) and a 9V battery doesn't add to it. The noise measured from the Fluke 731B 10V output is about 1.5uV or 0.15ppm p-p. Good enough for me!

Cheers

Alex

14

Projects, Designs, and Technical Stuff / Re: role of optocoupler for biasing drain of amplifiers

« Last post by moffy on Today at 11:12:37 pm »

There you go, an isolated 15V supply for the MOSFET driver, just what was needed.
Think of the MOSFET as a resistor with one terminal connected to the +24V. When the driver is 'off' the resistance is very high, when the driver is 'on' the resistance is very low, since the driver is referenced to the source of the MOSFET and it now has a floating supply it is happy.

P.S. They could have used a P channel MOSFET and saved quite a bit in complexity and cost.

15

Beginners / Re: Polystyrene cap percentage replacement ?

« Last post by algorithm on Today at 11:11:01 pm »

You stated on the highest UF scale it shows some capacitance with the leads open. Like how much 1UF on the 10,000UF scale? How bad is it percentage wise of full scale? Auto-Zero may have never been able to fully zero on that scale? Perhaps it is nearly at the threshold of not being able to auto-zero on all the capacitance scales and the main bridge needs re-balancing therefore that one scale itself is not faulty?

its range is 200uf. On display i see between 100uf, to 47uf to 15uf. Seems to vary everytime i try to test something out of circuit and put it back in... Last time it was 99uf and before that it was 47uf. I hoped those readings woukd point out the part obviously but it didnt for me.

16

Projects, Designs, and Technical Stuff / Re: Suggestions for secure authentication solution?

« Last post by jbb on Today at 11:08:02 pm »

Getting a pre-cooked authentication IC may be the best way forward here.  Although it's intended for heavier duty, using a TLS library may be a good way forward here.

I had some thoughts about anti-removal detection:

• you could send a heartbeat message through the secure channel
• this implies that the host & nodes will be powered on continuously - are they battery powered? If not, what about power cuts?
• make sure the client really wants this feature. It's going to be a pain, with false tamper detections and trips to fetch the disconnected node and bring it back to the shack for re-pairing. Given some of your very cool installation work is installed 10 meters above the ground, such trips could be a giant pain in the backside.

Regarding cryptography:

• If you're prepared to 100% trust that there aren't any eavesdroppers on the wire, you can do quite basic pairing.
• If you want to authenticate that the Node is an authentic MikesSecureStuff Node, it will need some kind of certificate inside. This will need secure loading at the factory (see below) but should allow secure pairing over an untrusted wire (yay!)

Building a Node @ factory

• Generates an asymmetric key pair e.g. Elliptic Curve Cryptography (ECC) public and private key
• Sends the public key to a Hardware Security Module (HSM) computer in the factory (this may be air-gapped and placed in a secure cage), which makes a certificate to say 'the above private key is indeed a genuine MikesElectricStuff widget'
• Stores the public key, private key and certificate in secure EEPROM
• Runs a TLS server, ready for the Host to connect

Building a Host @ factory

• Gets loaded with the a number of trusted HSM certificates in the factory
• Optionally, generate an asymmetric key pair and get a certificate (same procedure as for node)
• Stores the (optional) public and (optional) private key and list of HSM certificates in secure EEPROM or something
• Runs a TLS client for each Node connection.  When connecting to the Node, the TLS library will verify the Node certificate against the list of HSM certificates.

17

Beginners / Re: Polystyrene cap percentage replacement ?

« Last post by algorithm on Today at 11:07:19 pm »

> Upon probing a leg of a resistor it seemingly zeroed !
> The resistor was a 562k. Took it out of circuit and it tested good.
> Further inspection revealed that resistor traced directly to a 470pf polystyrene cap.
> I pulled both identical caps from board to have comparison.
> The first and 1 connected to said resistor reads 458pf.
> The other measured 462pf.

But should be short-circuit. Not guilty.

i wish i knew

18

Projects, Designs, and Technical Stuff / Re: Suggestions for secure authentication solution?

« Last post by NorthGuy on Today at 11:07:12 pm »

I don't think you need a secure chip. Simple security algorithms can be implemented on nearly any MCU.

You need to start from the list of attacks that you want to prevent, such as:

- when pairing, you want your node to somehow authenticate itself to the host so that the host know this is one of your nodes.

The attacker may -

  - sniff the pairing process and use it to create his own nodes
  - sniff communications of the paired node and retrieve something that can be then used for pairing a new node
  - physically open the node and retrieve the key which then can be used for pairing
  - pretend to be a host and engage into a fake pairing process with unpaired node to figure out how to pair
  - pretend to be a node and try to pair with the host, possibly many times every second
  - etc etc

The more you think of the better.

Then you get to the next step:

- host needs to authenticate itself to a node (whether paired or not)

An attacker can ...

And so on until you cover all the things that you want to secure.

Then you design security protocol which make all of these attacks impossible, or at least as difficult as you can.

If such process sounds difficult, this is because it is. No amount of algorithms or security chips will help if you leave at least one of the attack venues open. That is how real things get compromised.

That said, the probability of actual attack may be very low. Therefore, you can provide "fake" security, which is enough to scare amateurs even though it has no chance in case of real attack. Bit if there's no real attacks, what difference does it make?

19

Projects, Designs, and Technical Stuff / Re: Need a simple circuit for ADC to SPiDF

« Last post by moffy on Today at 11:05:35 pm »

I was looking for something similar and found this a USB soundcard with ADC/DAC and SPDIF in and out.
https://www.ebay.com.au/itm/226035836430?itmmeta=01HVRZAVY445SG3T1SYBF3STJ2&hash=item34a0c9260e:g:QIUAAOSwqNNkJ7zY&itmprp=enc%3AAQAJAAAAwIQopQvyaCI6RBnUhEBnGfg8n9VZW18s%2Fg%2BL8uD1ZYE%2FL6Go0RmyyizeEeXwkvCt1IbtvaOQRu3c6RfGfGYAIJlsCQBtP%2FBvIHvjIN3p5TroP%2Ft4E6k9AwHKoghljY8GigMdZb7iDI%2B2mZO%2FdzabDXfmyxiefO39%2F3%2Bf66hoPXrVcnFeVOBTY43JOOflHm3SwRKTH6U8EHlLSmTLd0QsAgA6qmGqNtgW6lsIr1iOUFczObsUDUryHWJu5rYAmvL5ZQ%3D%3D%7Ctkp%3ABk9SR5S_q5_eYw

That's a deal.  I also found a Wondom device today that I think will do it for even less: https://store.sure-electronics.com/product/609  I think this board has a full USB compliant audio interface and outputs SPIDF and toslink as well as audio on a 3.5mm jack.

I've used a lot of their products and tested some as well.  The interface with SigmaStudio (always get that wrong) by Analog Devices makes them very powerful.  You can put noise reduction and dsp based crossovers in powered speakers for cheap.  I use their boards with my synthesizers for crossovers to subs.

The reason I need SPIDF is that I already have high quality cabling running from a closet to two remote locations.  I cant pull USB cables or ethernet up there without a major hassle. I wish I had ethernet in those two locations but I don't.  So I am going to put the Pandora box into a closet and run SPIDF over the existing cables using a line driver I built many moons ago.  The alternatives are just not practical in my current state of health.

I would have thought there was a way to do it using parts I have on hand.  I have ADCs, etc, STM and arduino boards, all kinds of stuff.  I saw where someone used a parallax propeller but it looked like a purpose-built device.

Thanks again.

Jerry

It is hard to justify some DIY when you can buy reasonable products that are cheaper than buying the components let alone the cost and time of making the PCB. Must say that Wondom price is exceptional. Hope your health improves it is a struggle when you aren't well.

20

Beginners / Re: Polystyrene cap percentage replacement ?

« Last post by algorithm on Today at 11:05:12 pm »

well im not exactly proud to admit but used a tc1, I would have used the lcr meter (the patient under operation) though yeah hehe. Though with the tc1, its fairly accurate, and I had the 2nd cap measuring 462pf. I performed multiple tests on each and there was no variation respectively.

So you have more trust in that thing than in 2% capacitors?  It does not even have accuracy specs AFAIK.

well its just all i have that can test less than 1nf... My fluke only reads to 1nf. But after replacing a film cap on a obscure german industrial machines circuit, the tc1 could differ from my fluke by like 2nf. So if that polystyrene cap was 2nf more.