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3481  Using Arduino / Sensors / Re: Wind speed and direction idea on: July 07, 2013, 01:14:53 pm
Any turbulence at all will likely defeat you.  The Pitot tube pressure sensing method (tube pointing into the wind) works
at aircraft speeds, but Its sounds unlikely to have much success at low speeds.

The main problem is that pressure goes up as the square of the windspeed, so low speeds mean very small pressures.
Ultrasonic anemometers are used these days, and the time difference in ultrasonic pulse propagation is
linear in windspeed.

A google search revealed a bernouilli effect pressure system for measuring extreme winds - so clearly for high winds
pressure is a workable method.   It might be an interesting experiment to see how much signal to noise you
get for normal windspeeds using something like pitot tubes or bernouilli venturi...
3482  Using Arduino / Sensors / Re: max effective resistance for pull down resistor? on: July 07, 2013, 12:55:22 pm
If you move to high-resistances you will need to protect against electro-magnetic pick-up in the
cabling, which in practice just means adding a few nF of capacitance between the input pin and ground
at the Arduino end.
3483  Using Arduino / Motors, Mechanics, and Power / Re: Arduino not supplying enough power?? on: July 07, 2013, 12:51:40 pm
A voltage regulator is what you need.   Most 9V batteries are unable to supply suitable current BTW - use a pack of AA cells.
3484  Using Arduino / Motors, Mechanics, and Power / Re: DC 5V 28YBJ-48 Stepper Motor Question on: July 07, 2013, 12:47:54 pm

Now, i'm totally new at this, and from the test examples i could find this thing is really slow.
Is that what the 1/64 stands for?

And if that's true, than what's a good motor to drive a robot?

That motor is for controlling air-vanes in car air-conditioning systems, it is neither fast,
efficient or powerful.

What sort of robot? How big/heavy/fast?  Is this for traction? - stepper motors are not efficient for traction, they are
for positional control.
3485  Using Arduino / Motors, Mechanics, and Power / Re: Best drive sequence for max torque on bipolar stepper ? on: July 07, 2013, 12:45:17 pm
You don't need to worry about how the poles are arranged, its not relevant, they are arranged correctly.

The best sequence for high torque is full-step drive, where both windings are always on.
We'll call the windings A and B and each winding can either be active one way (+), off (0) or
active the other direction (-).

You want something like

A+  B+
A-   B+
A-  B-
A+  B-
3486  Using Arduino / Motors, Mechanics, and Power / Re: 12 volt relay at 24 volts at 20 amps on: July 07, 2013, 12:37:53 pm
If you switch 20A there will be a spark!  Its a question of how quickly it quenches and how well
the contacts withstand the heat (relay and switch contacts use special heat-resistant alloys to
reduce the damage).
3487  Using Arduino / General Electronics / Re: rtc crystal accuracy on: July 07, 2013, 12:33:58 pm
The cheap 32768Hz watch crystals are designed for use in watches and assume a fairly
constant temperature is maintained by the wearer of the watch!

As the last poster said for better accuracy you will need a temperature-compensated crystal oscillator
(TCXO for short).  These take more power to run than RTC's though.  Some RTC modules have temperature
compensation built-in (while retaining the low power requirement), I think if you Google "chronodot" you'll
find an example of one.
3488  Using Arduino / Motors, Mechanics, and Power / Re: 12 volt relay at 24 volts at 20 amps on: July 06, 2013, 04:23:10 am
It probably OK - twice the voltage means double the arcing power available for the same current,
but you are using 1/3 the rated max current, so that should compensate I think (although its not
really that simple).

The isolation between the contacts is 500V so there's no problem per se with the voltage, just
the arcing/sparking damage as the contacts open.

DC is always more demanding to switch than AC since there are no zero-crossings to help
quench an arc.

Switching large DC voltages at high power requires oil-filled relays or high-pressure inert gas
switch gear due to this difficulty - once an arc is struck it takes a much lower voltage to keep
it going (so long as there is enough power to maintain it - a spark is a low-resistance channel
of ionised gas).
3489  Using Arduino / Motors, Mechanics, and Power / Re: My DC motor works but it won´t move the wheel... on: July 06, 2013, 04:16:02 am
The Adafruit motor shield loses 1.5 to 2.5V or so, so you'll need 7V or so to power it and
drive a 4.5V motor.  This is the case for any motor controller using darlington output
stages (and is why they are not the best for low voltage motors - a MOSFET H-bridge
would be a better choice).

The lack of voltage is probably the main reason you aren't seeing much motion.

3490  Using Arduino / Programming Questions / Re: Need a S-I-M-P-L-E example of using PinChangeInt or ooPinChangeInt on: July 05, 2013, 05:05:32 pm
pin 15 on the Uno is the same as A1.

The example shows attachInterrupt being called for two pins, check with the library sources but I suspect
you just call it for any and all pins you want, and the library handles dispatching to your handlers - that would
be the obvious way to do it.
3491  Using Arduino / General Electronics / Re: Most common (single rail) Op-Amp used with Arduino? on: July 05, 2013, 04:00:33 pm
It is sloppy practice to use an Op Amp where you need a comparator. Op Amps may draw excess current. That does not mean you can't (the biggest limitation is slower switching times), but in practice, there is no one all-purpose Op Amp.


Also good comparators are dirt cheap anyway....

What makes a comparator good is not the same as what makes an op-amp good (some op-amps
draw significant current from their inputs when they are separated by a volt or more, since this is
the unusual case for op-amps)
3492  Using Arduino / Sensors / Re: Too much variations in temperature? on: July 05, 2013, 03:53:05 pm
If there's a breeze you will get rapid fluctuations since breezes stir the air (warm air high in room, cool air lower
in room).

But firstly try testing the sensor in a known constant/slowly varying temperature environment if your can (if its a probe
place it in a glass of water)
3493  Using Arduino / Sensors / Re: a module that can receive and transmit at the same time? on: July 05, 2013, 03:46:33 pm
Mobile phone technology used to be full-duplex analog audio, and this was one of the reasons why the first
mobile phones were large and cumbersome.  I can't remember if the move to GSM did away with the need to be duplex
(still uses separate uplink and downlink channels). 

Unless you need proper duplex its much easier to avoid it.  Its like shouting at the top of your voice whilst
simultaneously listening to the quietest noise you can imagine.  The ratio of transmit to receive power can be
fifteen orders of magnitude or more.
3494  Using Arduino / Sensors / Re: Sensing a contact between two point on: July 05, 2013, 03:37:49 pm
How would the medium being oil be any different from the case of air?
3495  Using Arduino / Programming Questions / Re: Need a S-I-M-P-L-E example of using PinChangeInt or ooPinChangeInt on: July 05, 2013, 03:33:39 pm
So what confuses you about the example here: http://playground.arduino.cc//Main/PinChangeIntExample
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