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31  Using Arduino / General Electronics / Using a Window Comparator... on: April 29, 2014, 01:01:38 pm
Hi guys and gals,

I am considering the use of a dual comparator like the TS9002 (which is similar to the MAX923 but costs hobbyists 5x less) to help me with a LiPo backup battery backpack I'm thinking of developing. I have no experience with either of these chips and so I wonder if anyone else has used them in a similar capacity. The TSM9002 has two schmitt-trigger comparators and a voltage reference with which one can set hysteresis.  

The overarching idea behind all this is keeping it simple. I know there are really cool solutions out there (I like the LTC4081 in particular) which will do a better job of extracting any and all power, optimize efficiency, etc. but at the expense of simplicity, $$$,  and the potential for EMI. Moreover, this rig is supposed to keep the MCU running for only short bits at a time, no more than a minute @ 200mA or so.

Anyhow, the proposed power circuit consists of a wall-wart that puts out 5V nominal which is fed through a polyfuse and a Schottky diode to feed the main Bus. A trio of 3.3V LDO voltage regulators with dropout voltages ranging from 220mV to 150mV then turn that main bus power into well-regulated 3.3V power for my sensors and the main MCU. Thus, the main bus voltage should be about 4.7V-5V, as long as the wall-wart is running as usual. The backup battery (a single cell 100mAh Lipo, 3.7V nominal) is charged from the wall-wart using a separate power connection with a MCP7381T trickle charger.

I'd be using the window comparator to detect whether an external power supply has been disconnected (i.e. VPowerSupply drops below 3.9V) and if the Lipo battery still has enough potential (i.e. VBattery>3.6V). Next, I am considering using a NAND gate combined with a P-Channel MOSFET since I'm switching on the 'high' side of the circuit to connect and disconnect the battery from the main bus.  Once the external power supply jumps over 3.9V, the battery would get disconnected from the bus and the MCP7381T charger can refill it. Similarly, if the battery voltage drops below 3.6V, it's lights-out to protect the battery pack.  

The window comparator circuit would be very similar to the example given in the TS9002 and MAX923 datasheets except that instead of using a series of three resistors to detect over and under-voltage on a single supply I'd be querying the status of two separate power supplies using four resistors altogether. Conceptually, does it seem like I am on the right track?
32  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 04, 2014, 12:10:15 pm
No no, I meant insulated, not isolated. my bad. I will go back and correct that. I had meant to use this power switch tail that uses a triac to turn on and off the power supply to the DVR. Does that make more sense? Apologies again for the typo. 
33  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 04, 2014, 08:57:00 am
Oh dear!

So, do you think a reed switch (or parallel banks thereof) listed at 0.5A capacity would be able to handle the kickbacks and inrushes that a 50W switchmode power supply can generate? Or is the Mosfet approach the most likely to produce clean breaks regardless of where in the cycle the power is switched off?
34  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 04, 2014, 08:31:33 am
Hi Grumpy_Mike!

Thank you for your insights, I really appreciate them. So it seems that using a Triac-based, zero-crossing power tail may work very well in terms of keeping humans isolated insulated from mains voltages while also affording us the ability to measure impacts on the basis of 1/2 wave (i.e. 8.3ms) increments. I imagine that the sags experienced by the equipment must be longer than 3 cycles to cause the reported problems. I'll circle back when I have more data. Thank you again.
35  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 03, 2014, 01:31:01 pm
Hi and thanks to you both for the answers. The equipment we're looking to testing is consumer electronica like DVRs that have a fairly moderate power draw (<50W) but which can react somewhat sensitively to very short power outages or even sags. You can imagine how happy home users might be if their DVR omits recording the latest Game of Thrones episode just because there was a short hiccup in the line power supply.

However, I am not aware of any industry standards re: how long a power supply is supposed to last (i.e. be able to ride through an incoming hiccup, high or low). Based on published specifications, I presume that most power supplies are tested on a continuous +/- 20% nominal VAC basis but I would really appreciate any resources you can recommend.

I'd like to explore three scenarios, i.e. testing on a
1) Continuous brownout basis (i.e. at what sag voltage does the DVR stop recording)
2) Intermittent brownout (i.e. how low can you go with a 0.5ms, 1ms, 1.5ms, etc. brownout without affecting the recording)
3) Complete dropout basis (i.e. all the way to 0VAC) at 0.5ms, 1ms, etc.

I haven't considered spikes, and I would be very appreciative of any industry norms you could point me to. I saw an interesting circuit that used a 18V transformer as a buck or boost converter to explore the behavior of the attached equipment at +/- 15% of nominal voltage using a DPDT switch to switch back and forth.
36  Using Arduino / General Electronics / Staying in phase and measuring sags... on: April 02, 2014, 03:59:39 pm
Hi everyone,

I have a question how to best measure the impact of short, sudden sags on input voltage and wonder what the best way is to measure such impacts in a reproducible and scientific manner. One instance of such sags is when the upstream network of your line power is perturbed and the T&D network switches your home to a different circuit.

An easy way to measure 'sag resistance' is to simply hook up the equipment to a traditional variac and play the 'how low can you go' limbo game re: reliable unit operation. However, that type of operation is more of a brownout scenario, not a sudden sag that only lasts a couple of ms. Presumably, a power supply should be able to tolerate a lower input voltage on a few ms basis than on a continuous basis.  

But, assuming I would want to explore such lower voltages on the basis of a few ms at a time, I am somewhat stumped on how to do it. For instance, let's say I'd take two variacs, set one at reference and the other at a sag voltage. That way, both output voltages have the nearly same phase change due to the variac and hence should align almost perfectly. Then, how to flip flop between them?

One option is a reed switch like this one, that has a very fast release time and more than adequate contact rating. Said reeds release / switch on a 0.5ms basis.  Simple enough, but even 0.5ms w/o power is likely enough to cause some sort of measurement error. 

Doing the flip flop with Triacs also doesn't seem easy to this non-EE due to their habit of not 'turning off' until the current reverses. I suppose one could use zero-crossing detecting Triacs and only flip and flop on alternative wave cycles but that limits the time resolution to a mere 8.3ms. Not the end of the world and the folks at the power switch tail site offer a complete zero cross switching solution.

Then there is this charge-coupled MOSFET circuit that should allow very fast switching between sources on a somewhat arbitrary basis.  You'd need four of these, with two sets attached to each variac. With some OR'ing, upstream the output would then be based on one MCU pin being high or low... But, unlike the Triac solution, there doesn't seem to be a fully-built solution.

Or should we invest in a $$$ arbitrary waveform generator like this one? Decisions, decisions...
37  Community / Bar Sport / Re: Your latest purchase on: March 09, 2014, 04:20:38 pm
We all think he's a prepper.  Actually most of us in this area are to some degree.

What a silly rabbit. A real prepper never builds near other people. Makes it too obvious what you're up to. smiley Given the quality of gas these days, I hope you cycle through your stuff quickly enough before it decomposes. smiley-grin But the location certainly explains the use of plastic sheathing for diapering the containers.

I guess I'm a bit of an optimist or like living like the proverbial grasshopper in a urban environment without nuclear self defense weapons or secret underground vaults. That said, it later occurred to me that there might be perfectly legitimate reasons for the underground containers - such as growing mushrooms, wine storage, or similar purposes. Of course, there could be more 'recreational' things growing down there also. The electrical bill will tell the story...
38  Using Arduino / Storage / Re: Why SD libraries fail with resistor voltage dividers on: March 08, 2014, 09:07:38 pm
FWIW, I incorporated some ESD protection into my SD card-bearing card, all thanks to this discussion. Many thanks to everyone that contributed to this long series of interesting articles. Speaking of which, do you think that a device like the PESD series of unidirectional TVS diodes is suitable for micro SD cards? I'd presumably use the 3.3V model? Ditto for SIM card protection in a GPRS board?
39  Community / Bar Sport / Re: Your latest purchase on: March 08, 2014, 09:02:07 pm
So, the garage is all finished now and looks pretty cool.  All of us are wondering what he is going to do with the containers buried under it. 

I bet he's a prepper… trying to get it all in w/o anyone noticing. Same thing happened all over the US in the 60's… many folk started installing nuclear shelters on the sly, hoping that none of their neighbors would notice and come knocking in case of a nuclear attack. Your neighbor evidently didn't want anyone to know about the storage underneath the garage… especially the planning department in town, since every muni employee would then also know where to go in an emergency...

Your neighbors approach sounds hallways workable assuming the soil drains very well and the water table is pretty low. Shipping containers are pretty tough and cheap… but metal only goes so far in terms of resisting water, regardless of how many layers of plastic diapers your neighbor applied. A better approach is spraying an foundation-sealing elastomer on the outside of said containers and then applying several inches of XPS, which won't absorb water and insulates well.
40  Using Arduino / Storage / Re: Why SD libraries fail with resistor voltage dividers on: March 04, 2014, 03:13:55 pm
I have found using the TXB series of voltage translators from TI for SPI communications to be painless. Ditto for the BSS138 approach popularized by rocket number nine.  The BSS138 approach adds a few mosfets to the resistor divider components and you're done (with a slightly different circuit layout). No magic, no worries re: capacitance, measurements, etc.  The TXB approach is even more convenient, all-in-one, necessitating at most two external decoupling capacitors.

The only benefit re: using the voltage divider approach that I can think of is being able to breadboard it. However, given that Adafruit is offering a lovely TXB breakout board that is breadboardable, I don't see the point from a time vs. money value point of view.  

These days, any SD card board that uses the voltage divider approach of level shifting is a sign of a cheapskate/ignorant designer IMO - a sign which is ignored at your peril. Even iTead uses the BSS138 approach in various schematics I've had a look at where voltages had to be translated. There is also the TXS series, for Open-Drain And Push-Pull Applications.  Or, switch to a 3.3V-based infrastructure like the Due or Teensy 3 series and be done with it. I went that route.

Kudos to FatLib16 for his many excellent libraries and the research posted here.  I am going have to take a closer look at all the TVS references that I am totally ignorant to. Some of those chips are extremely tailored - the semtch Eclamp in particular.
41  Using Arduino / Sensors / Re: Silicon Labs Si7013, Si7020, and Si7021 humidity sensors on: February 20, 2014, 03:29:06 pm
Ok, looked at pinouts, commands, etc. I hope someone can compare these also to the HTU21D or the SHT21 to verify my findings:

  • I2C Device address: same (0x40)
  • Pinouts: Identical
  • External package dimensions, pin locations, etc: Virtually identical (within 0.1mm)
  • Command registers: Very similar - All the measurement commands are the same, ditto on reset, read and write RH and T.
  • Where the SiLabs units seem to differ is adding command 0xE0 to read the temperature from a previous RH measurement, and the commands to read the electronic ID/Firmware revision.
  • The only major difference is that 'OTP Reload' bit is in the user register byte not implemented ('reserved') in the SiLabs units, whereas it is implemented but not recommended for use on the HTU21D series. Not a great loss then.
  • CRC implementation: Same format, same initialization (0x00)
  • RH and Temp calculation formulas: same

The only slightly mystifying information is the so-called Power up time that the SiLabs units mention but the HTU units do not.  I suspect that no-holder master calls will require a power-up between calls, requiring a longer net conversion time than suggested in the SiLabs table, at least at first glance. If the power-up time is added to each conversion time, the resultant total time to make a RH reading is about the same.

What is interesting about the powerup time is how it seems to vary by temperature. Either way, this sensor seems to be good for at least one reading per second on a no-hold basis (i.e. good enough for my purposes), making these units likely compatible with the Sensiron Arduino library. I look forward to my shipment!
42  Community / Bar Sport / Re: Your latest purchase on: February 18, 2014, 09:59:03 pm
Silicon Labs Si7020 I2C humidity and temperature sensor (see writeup in sensor section for longer description). Looks like a nifty replacement for the SHT21, and HTU21D series of DFN6-sized sensors (i.e. 3x3mm) at a great price point ($4 ea with dust shield!). I hope they stick around, and stay at this price point unlike their competitors (the HTU21D is in and out of stock constantly, and the price at Digikey raised 50% to $12.50 ea, a real headache!)

So I bought 5 with the dust shield.  smiley
43  Using Arduino / Sensors / Silicon Labs Si7013, Si7020, and Si7021 humidity sensors on: February 18, 2014, 04:54:33 pm
Hi everyone,

Today, I tried to restock my supply of HTU-21D's... but the stock has currently run out in NA. Looking for an alternative, I stumbled across this series of Humidity sensors from Silicon labs. They are the same footprint as the SHT-21 and HTU-21D (DFN-6, i.e. 3x3mm), use the same voltage range (1.9-3.8V), etc. So they are easiest to use with 3.3V systems, like the Due, Pro, or Teensy series.

I haven't looked over the data sheets in detail re: the programming, but many of the offered resolution ranges are similar and the unit allows operation with or without the host CPU being kept blocked. On the Si7013 you can even set the heater intensity (up to 94mA current draw!!!).

FWIW, the si7013 is a strange and interesting creature, i.e. a I2C temp and humidity sensor that additionally features an accessible differential ADC for external NTC temperature measurements, complete with registers for calibration, slope, and offset. Or measure anything else of interest, 0-Vdd for unbuffered and 0.5-Vdd for buffered operation (1.9V < Vdd < 3.8V). Could make a nifty little ADC for things like photoresistors in a external weather sensor system, for example. The ADC offers up to 16 bits of resolution but it's relatively slow (i.e. about 12ms per conversion).

Lest I come off as some sort of marketing drone, I wonder if anyone here has used one of these sensors? They seem like a good substitute for the HTU and SHT series, especially ever since the HTU prices went up 50% at Digikey (from about $8 to now $12+). By comparison, the Si7021 (+/- 2% RH and 0.4*C accuracy) costs about $4 in single quantities and the slightly less accurate si7020 ("only" +/-3%RH) is about 10% less. Both sensors can even be bought with factory-installed PTFE dust shields (add 10% to base price - and look for the  -GM1 version instead of -GM model, for example).

The only downside of these chips to hobbyists like us is that a DFN-6 is hard to solder by hand and pretty much requires a reflow solution and a custom-made PCB. Granted, someone like Sparkfun or their affiliates might sell the PCB, but these sensors are definitely not as easy to integrate as the DHT-22, for example. In particular, the Si7013 seems like a bad candidate for hobby use (even if you are experienced with reflow soldering the HTU21D or SHT21 series) because Silicon Labs are cramming 5 pins per side into two sides of a 3mm square package, resulting in 0.5mm OC pin spacing... in a package where you can't observe the pins when the chip is installed.... IMHO, a perfect candidate for blue smoke unless you have a well-calibrated pick and place machine!

Anyhow, I've bought 5 of Si7020's with the PTFE dust cover and I'll report back when I have them re: their compatibility with the Sensiron library. At first glance, it looks like some modifications may be required but setting options, CRC, and so on are similar. Thus, a fork may be possible, something made easier by the fact that the Silicon Labs units can be queried and will reply with the SKU for the model as well as the revision of the firmware on them (!!!).
44  Using Arduino / Sensors / Re: Question regarding Sensiron SHT Library on: December 18, 2013, 03:17:09 pm
where did you source the HTU21 sensor from?., as I recall. It's how I found about the sensor in the first place. Cheers.
45  Community / Bar Sport / Re: Your latest purchase on: December 04, 2013, 11:14:02 am
Still looking for a reasonably-priced signal generator whose output frequencies do not have to exceed 60Hz
You can buy AD9850-based devices for about $5 each, with roughly 10mHz resolution up to 40MHz.

Thank you, AWOL for the suggestion. There is a plethora out there, next step will be finding one that can handle up to +/-10Vpp and so on. Some seem to offer on-board amplifiers for that. Thanks again.
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