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5746  Using Arduino / Project Guidance / Re: Clock / timing project on: October 19, 2012, 08:15:45 pm
Well I'd start with part 1 naturally smiley-wink

OK, my first question is what accuracy of timekeeping do you want? (in ppm, seconds/day or whatever).

Secondly have you some idea of what kind of display, how big a unit, how long it has to run from on battery power, budget constraints?

For battery power I'd suggest a 2 or 4 line LCD display isn't a bad option.  If you can spend a bit more money I've used the TextStar LCD display module a few times, its dead easy to use (but no backlight)

For wall-clock levels of accuracy the simplest step is to get an Arduino with a proper quartz crystal oscillator, rather than a ceramic resonator (I've has to resolder my Uno to do this, alas).  I think the Leonardo is guaranteed to have crystal, but not sure any others these days have (some of the better clones might - check out seeedstudios)

For really good time accuracy the Chronodot is something I've seen used, less than 1 minute a year drift.  Otherwise there are various other less accurate RTC boards usually using the DS1307.

For 200 timestamps you'll be able to store that in 4 or 5 bytes each (depending on whether the year matters), or perhaps 8 bytes in a less complressed form - so 1.6kB of storage - that might be cramped on the Uno.
5747  Using Arduino / Microcontrollers / Re: Atmega 328P power source supply problem on: October 19, 2012, 07:58:46 pm

In theory a 328P should run down to 1.8V but you need to drop the clock frequency down. Try setting it to 1MHz internal clock and see what happens (use "fuse bits" for this).

You'll also have to set the brown-out-detection voltage in the fuse bits too, it will brown-out at 2.7V by default with normal Arduino bootloader IIRC.  Since current consumption is less at lower clock frequencies just reducing the clock speed may allow running from button cells (if nothing else takes many mA).
5748  Using Arduino / Microcontrollers / Re: atmega memory jumps on: October 19, 2012, 07:53:46 pm
Only if that memory is flash memory - the AVR microcontrollers are Havard architecture, not von-Neumann.  And writing flash is (despite the name) slow.   Its probably faster to use an interpreter.
5749  Using Arduino / Programming Questions / Re: using a button to start the circuit on: October 19, 2012, 07:51:35 pm
All assuming your button is wired up appropriately smiley-wink
5750  Using Arduino / Programming Questions / Re: using a button to start the circuit on: October 19, 2012, 07:50:13 pm
if (pin12= HIGH);
{// put my sketch here}

Really? There are three obvious errors in one line of code there (some kind of record), = instead of ==, pin12 instead of digitalRead (pin12), and the blank statement after the if (the ';')  Don't you mean

 while (digitalRead (pin12) == LOW)
  my_stuff () ;

The sketch is always running, you have to wait for the event you are interested in.
5751  Using Arduino / Sensors / Re: ACU>RITE Temp Sensor # 606TX on: October 19, 2012, 07:35:37 pm
Do you know what the modulation type is for sure?
5752  Using Arduino / Motors, Mechanics, and Power / Re: brushless motors ,esc , quadcopters on: October 19, 2012, 07:26:30 pm
Why not arm them all at the same time - perhaps they will time-out if not armed soon after power-up?
5753  Using Arduino / General Electronics / Re: Noob question: Understanding power on: October 19, 2012, 07:23:35 pm
One observation:  powering the servos from the +5V Arduino rail could lead to damage to the chips on the Arduino - its a risk I certainly wouldn't take, unless I had a spare board and didn't care about the expense.  Servos and motors produce nasty voltage and current spikes and generally you never share motor power with logic if you want reliability.

Secondly you really should get a multimeter and measure the current consumption of one of your servos (while continuously moving) to get a better estimate of the power supply / batteries you will need.  For now I'd suggest 4xAA battery pack for the servos only, share a common ground and run Arduino from USB.  At a later point you'll need to work out how to power the Arduino from a separate rail to the servos (whether via a LDO voltage regulator and / or filter circuit, or DC->DC converter, or separate battery).

If I connect 6 servos in parallel and each need 1A, would I need to produce 6A from the power source or would they share it like in voltage and 1A is all that is needed from the power source?
If they are active at the same time, 6A, if only one is moving at once, perhaps 2 or 3A (they take some current when parked).
If I need to produce 6A, is that dangerous when the voltage is somewhere in the region of 9v-12v?  I only ask as this link and one on wikipedia claims that above 2,000mA (2A) can produce some quite nasty results, but I'm not sure if it's only when a certain voltage is present (my guess is that is the case, but wanted to be sure before frying myself)
Unless you put wires on your tongue, 9 or 12V is perfectly safe.  Large currents can lead to _heating_ issues, but not shocks.

If I connect 4 AA batteries in series, my understanding is this will increase the voltage but the current will be the same as 1 AA battery.  So if 1 AA battery is 1.5v and 1800mA/hr, it would be 6volts and 1.8A maximum for 1 hour.  If you do it in parallel, it increases the max current but not the voltage.  So in this case it would be 1.5v and 7.2A maximum for 1 hour

Firstly units. Current is measured in amps, charge (capacity) is measured in coulombs or ampere-hours (Ah).  amps/hour is a rate of change of current, completely wrong here.

A measure of capacity (1800mAh, not mA/h) says nothing about how much current can safely flow.  You need to know the maximum discharge rate for that - anyway lets assume these hypothetical batteries have a maximum discharge rate of 1.8A, then yes in series the rate is unchanged, in parallel it adds.

Different battery chemistries have different properties (for instance most NiMH rechargeables would only be happy discharging at the hour rate or so, whereas a LiPoly battery might be rated to fully discharge in 120 seconds!).

Lastly if you buy some 2Ah batteries, assume their capacity will be down to 1Ah before too long - in the real world rechargeable batteries perform quite poorly without careful management and manufacturers bend the truth to breaking point.  And over-discharging many kinds of rechargeables completely knackers(*) them, especially when in series as one cell may get reverse-charged.

(*) capacity reduced by large factors.
5754  Using Arduino / General Electronics / Re: Nichrome wire on: October 19, 2012, 06:59:05 pm
I'd strongly advise against a home-made mains-voltage heater - where are you going to get all the exotic ceramics you need for insulation (mica?, alumina sheets or paste, etc etc)?

You need to be aware of the temperature dependence of resistance of nichrome - fortunately much less than many metals, but there is a dependence you need to be aware of. 

Most soldering irons these days are low voltage for safety and convenience of temperature-control (solid state), but they are a heating element in ceramic insulation inside a metal tube usually I believe.
5755  Using Arduino / Displays / Re: UNO + TFT Shield = Oscilloscope! on: October 19, 2012, 04:16:36 pm
Cool - but cooler with a trigger input?  Add a pot to one analog input to set the trigger level - not that much more work?
5756  Using Arduino / Programming Questions / Re: More SPRINTF woes! on: October 19, 2012, 04:09:43 pm
You need to tell sprintf if a variable is long.

    const char *mask = "Value is 0x%08lX\r\n";
5757  Using Arduino / Programming Questions / Re: arduino uno R3 as I2C master on: October 19, 2012, 04:03:35 pm
First hit in google for "hmc6352 arduino" I got:

That is specifically for the sparkfun module though, which I think has own pullup resistors.
I understand Wire library is supposed to be for the purpose of I2C communications, but there are no functions there for clock generating.
The library does everything for you, don't worry about it.
5758  Using Arduino / Project Guidance / Re: Output from PortB & PortD at the same time on Uno r3 on: October 19, 2012, 03:45:13 pm
How exactly?  a fraction of a microsecond is achievable with:
  PORTB = bval ;
  PORTD = dval ;
for instance if bval and dval are byte variables (I've measured it as 63ns, one clock cycle)
5759  Using Arduino / Sensors / Re: Medium-range RF signaling device on: October 19, 2012, 03:32:01 pm
Longer range suggests longer wavelength, in general, so an ISM device in 433MHz band (or whatever is appropriate to your territory) will fare better than anything in 2.4GHz band for same power, for instance.  And you must have proper antenna to get good range.
5760  Using Arduino / Sensors / Re: Piezo Vibration sensor problem on: October 19, 2012, 03:26:46 pm
Indeed - piezo elements behave approximately as current sources, so the voltage pretty much can be whatever you want - the smaller the load resistor the lower the voltage.  Try 1M, 100k, 10k and you'll get some indication of what sort of value is best for your application.
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