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16  Using Arduino / General Electronics / Re: Which regulator for 30V DC to 5V DC on: May 01, 2014, 08:43:23 am
The high input voltage to me suggests the use of a switchmode DC-DC power supply. Using linear regulators on a high input like that means a lot of heat dissipation since a linear regulator would literally convert 83% of the incoming power into heat, i.e. about 20 Watts (!!!). A good switchmode power supply will likely achieve around 80+% of efficiency so it'll not only be much more efficient, it'll also run much cooler!

One drop-in solution is the dimension engineering line of power supplies. They have a fixed 5V model, and adjustable-output ones too.  Their higher-capacity line (3A-capable) tolerates up to 35V input. I have had good luck with these power supplies.

I would always double-check to make sure that whatever DC-DC solution you're looking at can handle the very high input voltage. Few modules can, though Crossroads linked to a great set at Pololu.
17  Using Arduino / General Electronics / Re: INTERFACING 4N35 WITH ARDUINO on: May 01, 2014, 06:16:46 am
Allow me to quibble a bit,

There are specific linear opto-couplers made for measuring mains or higher voltages (Avago makes them, among others). However, their implementation is not trivial - for example,  they require dual power supplies (one on the high side, one on the logic side) but they do offer very nice linearity.

For this application, a transformer makes a lot more sense. The folk at Open Energy Monitor have some nice tutorials how to measure mains voltages relatively safely using transformers and the like. There is also a nice code base to wade through.
18  Using Arduino / Sensors / Re: Silicon Labs Si7013, Si7020, and Si7021 humidity sensors on: April 30, 2014, 04:45:17 pm
Apologies, I had gotten started with fobbing the latest set of boards and then got sidetracked by several months of intense work unrelated to this effort.

I will try to restart the reflow processes, etc. to get the boards made. Requires free time, unfortunately.
19  Using Arduino / General Electronics / Re: Tip 147 heat sink on: April 30, 2014, 03:10:28 pm
Have a look at the data sheets and determine how much heat the chip is going to have to dissipate based on your measurements (i.e. use  a DMM or similar device to determine the maximum wiper motor current). Next, see how many *C the device will heat for every watt of heat that has to be dissipated. Determine the maximum ambient temperature and the maximum temperature you want the TIP 147 to reach, now you know the allowable delta-T of heat rise. Then use a chart that the manufacturer of the TIP hopefully provides to see how big of a heat sink they recommend for a given Watt dissipation and allowable heat rise.

Googling brings up articles like this one at Digikey, this one at Sparkfun, or this one at Texas Instruments.
20  Using Arduino / General Electronics / Re: Heating Problem: Powering Arduino Mega 2560 with 5V - 10A Transformer on: April 30, 2014, 02:58:16 pm
Two things,

As others have noted, the Arduino cannot drive 100 LEDs without the help of external components. Well, not without likely exceeding the limits of the Arduino. Ditto for the voltage regulator, it's not meant for large loads. That said, Neopixels/WS2812 units don't require a lot of power on command/control pins, just on the power pins. So, you should be able to connect 100 these LEDs to the Mega without issues, but only as long as they do not derive their power from the mega board. At the very least, make it a separate connection to the common power supply.

Furthermore, I would suggest having a deeper look at all the excellent resources that Adafruit has published re: using the WS2812 or NeoPixel as they like to call it. In particular, look into how they want you to hook up the LEDs.

If memory serves, using two power supplies is encouraged, i.e. one for the Arduino, one for the LEDs, with a common ground.  That way, if the LED power supply goes down due to overheating or the power quality suffers due to transients, it won't be as likely to affect your Mega. Given the cost of a Mega, I'd also investigate the use of a buffer chip to make it that much harder to smoke the Mega. has some good tutorials in this regard since their MCU runs on only 3.3V and therefore pretty much needs a buffer chip.

I would also verify the output voltage of your external power supply, just to be sure that you don't release the magic smoke from the NeoPixels. Lastly, verify the input voltage range for the voltage regulator / barrel jack on the Mega. IIRC, it prefers something like 9V, not 5V. At 5V input, the regulator should be going into dropout, doesn't seem like a good idea.
21  Using Arduino / General Electronics / Re: Using a Window Comparator... on: April 30, 2014, 01:05:29 pm
Attached is a board with a P-Channel MOSFET since I'm trying to switch the high side. The board is proposed to use a 74AHC1G00 NAND gate, along with a AO3415 P-channel MOSFET to switch the high side.

The comparator chip contains a reference and I'd like to think that two measurements on a separate set of chips would be OK. That is, if INA+ exceeds 1.182V + VHB, the OUTA pin will toggle positive, just as if INB- drops below 1.182V - VHB that the OUTB pin will toggle positive. VHB has been set to 1%, i.e. 0.006V. Then, if both OUTA and OUTB are positive, the NAND gate will sink the current from the MOSFET and connect VBATT to VBUS.

That is, if I got it right? Do the pullups and the current limiting resistors look OK?
22  Development / Other Software Development / Re: GPS shield with Ardunio on: April 30, 2014, 10:24:17 am
That depends on a couple of factors that we don't know the answer to.

Your PC almost certainly does not receive a GPS signal to correct its time. Most PCs connect to a NTP server from time to time to correct their internal clocks. That saves the PC manufacturer money since they can then use less expensive / accurate real time clocks inside the PC to keep time. On the downside, you need a internet connection and many PCs have to be set up to connect to a NTP in the control panel settings for that to work.

As for the GPS time output, it's set to Greenwich Mean Time (GMT) or Coordinated Universal time (UTC). Simply add and subtract the delta from your location in terms of time zones. Usually, time zones increment in hours, though some areas of the world (Nepal, Bhutan, etc.) deviate from that usual increment.

Also, remember that GMT does not shift for summer and winter time (if that's observed in your area).

What I find hilarious is that every car I've driven with an OEM GPS navigation system does not use the GPS data to set/update the 'vehicle time' based on its location (to determine the time zone) and GMT. If an Arduino enthusiast managed to code a GPS clock that sets its time zone based on its location, summer/winter time data, etc., it's hard to believe that car manufacturers lack the resources to do the same. Presumably, a patent protects the idea, hence the lack of implementation.
23  Using Arduino / General Electronics / Re: "Supplying voltage via the 5V or 3.3V pins [...]" to what? on: April 30, 2014, 10:10:26 am
@ Lefty: Ditto. The jumper pins are the most foolproof method, though not as convenient, and convenience / ease of use is an understandable reason for the Arduino hardware team to choose the path they did. I used that approach for home-made boards for a long time, pretty foolproof and up to 4 external power sources can be selected at will using a "+"-shaped pin header + jumper shunt, with the center pin being the one that supplies the board and the ends of each arm being a potential power source.

Another approach is to simply dump all external power via Schottky diodes into a common power rail. That works as long as every component is OK with the incoming power varying somewhat (4.7-5V). The Arduino can certainly handle that and many 5V logic chips too but there are potential issues with Analog measurements unless you implement an external reference of one kind or the other (Zener diode + resistor into AREF, for example).  

Some of my home-made boards are 3.3V only, making a 4.7-5V common-bus approach very easy to implement - i.e. the downstream LDO doesn't mind one bit if the incoming voltage drops to 4.7V, that means less heat to dissipate.
24  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 30, 2014, 09:57:20 am
Thank you, thank you, thank you.

Needless to say, you are an amazing resource and thank you again for sharing that information. More reading!
25  Using Arduino / General Electronics / Re: Reflow oven (aka Toaster oven) modification on: April 29, 2014, 07:29:34 pm
... and one more thing... does it necessarily make sense to go through all this effort when you can buy a certified reflow oven on Amazon/eBay for about $285, delivered? Consider that the materials alone (oven, insulation, heat sink, reflow shield, K-thermocouple, etc.) probably came out to about $125.

Sometimes the journey is just as much fun as the destination.  smiley
26  Using Arduino / General Electronics / Re: Reflow oven (aka Toaster oven) modification on: April 29, 2014, 07:14:59 pm
A couple of observations...

I happen to use a heavily-modified toaster oven with a rocket scream PID controller. As best as I can tell, the unit ramps up as desired, i.e. it follows the reflow pattern recommended for my Kester 'green' solder paste (i.e. no Lead). The only part that is likely 'non-compliant' so far is the cooldown phase, i.e. I have to figure out how far to open the door so that the temperature drops as intended, hopefully without cracking a ceramic cap. If you'd like to see some pictures, see this thread.

As for the construction process, I disassembled the oven and removed all controls. A ebay-sourced SCR (2x20A) is perfect for this formerly-'Black and Decker' $30 toaster oven since the four quartz-tubes are OEM-limited by a fat diode. The SCR achieves the same thing, but with control and I used the second channel for the convection fan. On the front face, I mounted a leftover plastic enclosure that I salvaged from a Neuton mower whose motor had gone bad. It holds the Arduino 2009 and the Rocketscream shield perfectly. I stuffed the (formerly empty) voids between the inner and outer oven cavity with 2000*F rated insulation.

Inside, I further reduced the mass 'visible' to the quartz lamps by stuffing the same insulation between the lamps, filling the rear of the cavity where no PCB would ever go, etc. The unit still gets hot during operation, but nowhere as hot as it would get without the benefit of insulation. I even insulated the door window, leaving just a little peep-hole to limit heat loss through the glass as well. I presume this is why the oven has no issues ramping up as needed - there is 'spare' capacity and the quartz elements are not on 100% of the time. At full power, it draws around 1340 Watts, IIRC.

If anyone in the Boston area is interested in looking at the thing or would like some leftover insulation, I'd be happy to share.
27  Using Arduino / General Electronics / Re: Staying in phase and measuring sags... on: April 29, 2014, 02:27:29 pm
Thank you all again. The test is challenging and I really need to get my hands on some data first to see how one would be test the condition. If this turns into a project, I'll be back with better information!
28  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?
29  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. 
30  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?
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