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1936  Using Arduino / Project Guidance / Re: Epileptic Siezure Alarm on: October 22, 2012, 07:50:36 pm
There are detectors for use whilst a person is in bed and hence "normally at rest" but the device required is for use during a person's normal working day.

Do you know how these detectors work, i.e. what do they sense and key on? Or can you give a link to the maker of these detectors? I know they don't cover the scenario you're interested in, but still may be a starting point.
1937  Using Arduino / Sensors / Re: MAX31855 thermocouple problems on: October 22, 2012, 05:34:45 pm
The datasheet does not recommend any capacitor across the TC leads, what's with that? I have some MAX31855s but have not tried them yet. Had very good luck with its predecessor, MAX6675.

Define "wacky"?
1938  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 22, 2012, 05:27:42 pm
You could make the ground leg of the voltage divider be an Arduino output pin.  Make it low to assert ground, take your A2D measurement, then make it an input and write a 0 to it, to turn off the pull-up.  It should not sink much current in this state.

Good idea! The test current would be limited to 20mA, which might be a bit of a light load, but it could work.
1939  Community / Bar Sport / Re: Surface Mounted Processors (Good or Bad Design?) on: October 22, 2012, 06:18:36 am
Using the DIP package makes a lot of sense on a prototyping board (read Arduino) for the reasons you list. Once I have a project fairly well settled, I'll often make a custom PC board. At that point, if space is a consideration, then a surface mount part may be the way to go.
1940  Using Arduino / General Electronics / Re: Mains Voltage and Resistors. on: October 22, 2012, 06:06:13 am
It depends. A resistive voltage divider only provides a constant voltage if the load that it is connected to draws a constant current. It has to be designed with a specific load in mind. If whatever the Arduino is doing results in it using a relatively constant current then it could work. But it might not work well for another Arduino project that has different current requirements.

It's a poor design for a power supply. So poor that it can hardly be called a power supply. Even a Zener diode would be preferable. Attempting such stunts will result in your EE friends laughing at you.
1941  Using Arduino / General Electronics / Re: Mains Voltage and Resistors. on: October 22, 2012, 05:44:35 am
To calculate the peak value of a sine wave, multiply the RMS value by square root of two. This will be the voltage that the filter capacitor will charge up to under no-load conditions. 240V * √2 = 339V.

Resistive voltage dividers hardly ever make good power supplies.

A primary risk in powering anything from mains is the shock hazard that can result if a person comes in contact with the circuit while standing on a grounded surface, etc. The transformer used in typical step-down AC-to-DC supplies provides isolation from the mains.

The so-called All American Five tube radios were notorious in that they had no transformer isolation, and if the plug was not polarized, then the chassis could be connected to the hot side of the mains.
1942  Using Arduino / Networking, Protocols, and Devices / Re: XBEE for theatre on: October 21, 2012, 03:39:03 pm
What is the part number on the bottom of the XBees?

Should be something like XB24-Z7WIT-004
1943  Using Arduino / General Electronics / Re: 24 or 26 AWG cable for my arduino UNO ?? on: October 21, 2012, 11:36:33 am
I picked up a bunch of new breadboards when I started messing with microcontrollers and I'm a bit surprised but I have very, very few problems with them. That could well change once they have 100,000 miles on them.

I've got about a half mile of CAT-5 cable around, so I use a lot of that (24 ga).
1944  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 21, 2012, 11:20:29 am
You got it! Glad to help.

I do use the same 5V wall warts to run circuits directly and they work fine, but to do the battery backup, and accomplish the switching between the battery and the wall wart, I figured it was simplest if both just fed the boost circuit through the diodes.
1945  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 21, 2012, 06:55:28 am
To simply detect mains fail, I'd connect to the anode of the diode, i.e. the side closest to the wall wart. Left side of D1 in the attached schematic. Then all that is needed is to test for presence or absence of a voltage at the analog input.

The Arduino has a built-in 5V linear regulator.
Its input is connected to the power jack and to the VIN pin.
Its output is connected to the 5V pin.
The Arduino can be powered either by:
(a) Connecting 7-12V to the VIN pin or to the power jack, or
(b) Connecting a 5V source to the 5V pin as in the schematic below. This bypasses the regulator.
1946  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 20, 2012, 06:05:26 pm
this should measure the voltage, with a voltage divider the voltage gets 0.18*Voltage.
Alternatively it could be mounted between each powersupply and the correspooding diode. Then this goes to A0 respectively A1 to know what the battery supplies and what the power adaptor supplies.

Yes, I get that, but to what end?
1947  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 20, 2012, 03:12:39 pm
The VIN pin is the same as the power jack. The output of the infamous boost converter would then feed the Arduino's 5V pin instead of VIN, thus bypassing the built-in linear regulator.

Regarding "next idea", what is it trying to accomplish?
1948  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 20, 2012, 02:16:40 pm
Yes, that will work, but using the Arduino's linear regulator (via the VIN input) is inefficient, only around 56% efficient, meaning that 44% of the power supplied by the battery is wasted as heat when the mains are down. That's like wasting over 2½ of the six AA cells. The larger the input voltage, the more is wasted with linear regulators. OTOH, switching regulators like the MCP1640 boost converter are upwards of 90% efficient. If a linear regulator cannot be avoided, to get the most out of the battery, use a low-dropout (LDO) type and minimize the difference between the battery voltage and the output voltage. But this is a double-edged sword, reducing the voltage difference reduces the region within which the regulator will function properly, and hence reduces the run time in backup mode on the battery.

Pure and simple, switching regulators win hands-down in this scenario.
1949  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 20, 2012, 12:22:47 pm
This is not clear for me, that means, always the higher power supply is taken?
I think, that would result that the sum of each voltage is supplied then, that might be too much.
And what is the sense of the diodes, then?

Correct. Higher of the two. Not the sum. Either one or the other. Output from the boost converter is always 5V.

The diodes, as I explained, cause the higher of the two power sources to be selected, and at the same time, prevent current from flowing back into the lower of the two. In addition, the small forward voltage drop guarantees the input to the boost converter is never more than 5V. This is a requirement of the MCP1640, it will not regulate if VIN > VOUT.

Quote
Some diodes like this,

Probably would work, but are overkill. 5A or even 3A is not needed. As shown by the schematic, I'm using 1N5818, these are very common and can handle 1A.

Quote
I am thinking of something like this now

That is not going to work exactly the same, there is a subtlety there. 6 fresh AA cells can supply more than 9.5V. So assuming the typical 9V regulated wall-wart delivers less than that, then the battery will supply the circuit initially, even with the wall wart connected. It will continue to do so until the battery is partly exhausted, and the battery voltage drops below that of the wall wart. This does not occur with a 5V wall wart and 3xAA cells.
1950  Using Arduino / Project Guidance / Re: rechargable batteries for uninterruptible power supply on: October 20, 2012, 08:58:12 am
The picture you drew is in fact the way my circuit works, it just uses less than 9V. It's actually quite simple, but a little explanation is probably in order. It's basically just a boost converter (MCP1640) that will boost a voltage less than 5V up to 5V. J1 is input from a 5V regulated wall wart. It's important that it not be more than 5V, so it should be a regulated (usually a switching-type) wall wart. TB1 is a terminal block where the batteries are connected, either 2xAA or 3xAA alkaline cells. It is important that the battery voltage (TB1) be less than the wall-wart voltage (J1).

Diodes work as you say, current can only flow in one direction. When the mains are supplying power, the voltage at J1 is greater than the battery voltage at TB1. This causes D2 to be reverse-biased and prevents current from flowing out of the battery. D1 is forward-biased so current flows through D1 to the boost circuit. When the mains drop, then D2 is forward-biased and current flows from the battery to the boost circuit. D1 is then reverse-biased which prevents any current from flowing back into the wall-wart.

All diodes drop some amount of voltage, I'm using Schottky diodes to minimize the voltage (and hence power) loss, and also to guarantee that the input voltage to the boost converter is less than 5V (for the output to be regulated, the input voltage must be < the output voltage).

The rest of the circuit is straight from the datasheet. If there are no power outages, the AA cells should basically last as long as their shelf life.
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