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31  Using Arduino / Motors, Mechanics, and Power / Re: Question about stepper motor and l298n (for this particular project) on: July 23, 2014, 11:26:40 am
That system doesn't have any position feedback, its open-loop and will perform
differently with different loads even.  Having said that that heavy gearmotor will
be less sensitive to load than some DC motors.

If you want definite position control with a stepper motor you have two options,
either pick a high resistance motor that can be driven from 12V via a L298 or
similar dual-H-bridge, or choose a low-resistance motor that is suitable for
chopper drive (such as via an A4988).

High resistance typically means 40 ohms or thereabouts, typically labelled
something like "12V 0.25A"
low resistance would be 1 to 3 ohms or so, perhaps labelled "1.2A 2 ohm"

High impedance motors are often unipolar and easy to drive with an ULN2803
or similar, low impedance motors are always bipolar (4 wire), and with too
high a current rating you may struggle to keep the driver cool.  High impedance
voltage-driven motors will run slower in general.

Occasionally you see medium impedance motors, around 10 ohm, which are
possible to drive from 6V or so via H-bridge (or via a chopper drive), which
gives more options.
32  Using Arduino / General Electronics / Re: ADC false readings on: July 23, 2014, 11:11:12 am
The ADC on the Due has a bandwidth of MHz, so there is a lot more noise bandwidth.

What bandwidth do you actually want to sample the signal on?  Make sure you LPF
the signal going into the ADC pin (or sample frequently digitally LPF it afterwards).

You will find the ADC is noisy on the Due as it shares power with the digital
parts of the chip (alas).
33  Using Arduino / General Electronics / Re: Arduino Due: Minimum reading of "20" on analog inputs on some PCBs on: July 23, 2014, 11:05:24 am
You might have to consider a ZIF socket and S.O.T. of the SAM chip, although that
complicates manufacturing as you'd have to process waffle trays of them yourself.

You might want to investigate parts from other suppliers should there be bad batches
around.

Have you correlated performance against date/batch codes on the chip?
34  Using Arduino / General Electronics / Re: measuring curren from a chip... on: July 23, 2014, 10:49:28 am
As I said earlier:
Quote
7805 quiescent current is 3mA
35  Using Arduino / Motors, Mechanics, and Power / Re: Arduino Motor Shield Help on: July 22, 2014, 12:05:08 pm
The motor shield is for driving DC motors, not bipolar stepper motors.  If you want
a powerful fast stepper motor you need a current-controlled chopper-drive controller
with a high voltage supply driving a low-impedance motor.

If you only need a slow stepper then a high-impedance motor with fixed-voltage
dual-H-bridge controller is enough.

Perhaps its time to say what you are actually trying to achieve?
36  Using Arduino / General Electronics / Re: Mega2560 max current question........... on: July 22, 2014, 12:00:15 pm
You need to connect all of the power/ground pins anyway, or the chip isn't guaranteed
to work, since they all require proper decoupling right next to the pins.  This is generally true
of all logic chips where multiple power / ground pins are there to provide a low-inductance
path the the PCB and provide signal return paths for nearby digital pins.

37  Using Arduino / General Electronics / Re: Which transistor to control 12V (350mA) Solenoid Valve? on: July 22, 2014, 11:55:14 am
the 1N4007 isn't really an overkill. It is used as free-wheeling diode and it should protect against the inductive current when switching off the power on the coil. The current that is produced in this moment is much higher then the original source current. Here you can have 100 - 200 V for a very short moment. That's why it is always a good idea to use a 1N4007 for that purpose.

No, the current at that point is exactly the original source current, since inductances
resist change in current.  The voltage is generated to maintain the current, should there
be no easy route for the current the voltage will force a route.

The important property of the diode is fast turn-on.  Most diodes are reasonably
fast at turn on (turn off is another matter), but larger diodes are slower, so 1N4001
is probably better than 1N4007.  For 350mA you could even use a 1N4148 which has
a repetitive peak forward current rating of 450mA (although that's rather close for
comfort).  Even a fairly slow diode is likely to limit the voltage spike to 10V rather than
1000V, so in general they just work.
38  Using Arduino / General Electronics / Re: measuring curren from a chip... on: July 22, 2014, 11:45:54 am
Place the multimeter (current mode) across a switch in the supply lead.  Once all is working
open the switch and the meter measures the current.  If you need to switch
current range, close the switch, change the range, open the switch - that way the
chip won't reset or power-cycle due to range-changes.

This matters when measuring small currents as the low-current range on the meter
uses a large shunt resistance which is unable to carry enough current to reboot
the processor, but which is needed to measure tiny sleep-mode currents.

The shunt resistor on ground of 7805 makes no sense to me - the ground pin
must be at ground potential or the output voltage could be too high.

A 7805 will totally dominate the power consumption of the board in sleep mode,
there are special low-quiescent-current regulator chips available - look for the
magic word "micropower" in datasheets.

Microcontrollers in sleep mode are typically 5 to 0.1 uA, 7805 quiescent
current is 3mA, about 1000 times bigger.
39  Using Arduino / General Electronics / Re: Power Supply questions on: July 22, 2014, 11:37:19 am
I have a couple questions regarding it: the power supply has a +V and -V as well as ground. does it matter what V i use?

I think it's just weird labeling and "V-" really means "GND".
Looking at the spec sheet:
-V is the negative terminal of the 12V supply,
+V is the positive terminal of the 12V supply.
PE is the protective earth connection.
There is no "ground" connection, you are free to choose ground.

Note that this is an isolated supply, but the output terminals mustn't go too far from
earth (the input-output isolation is tested to 3kV, but output-earth isolation is only
tested to 500V)

The leakage current is under 1mA, which means the outputs could be floating
at mains voltage levels if not tied to PE somewhere - recommended unless you have
a genuine reason not to.
Quote
The first thing you should do when it arrives is probe it with a multimeter.
Definitely - your multimeter is invaluable all the time, check everything
you can and fewer expensive mistakes will happen.  In particular you can
verify voltage and polarity.
40  Using Arduino / Programming Questions / Re: Significant difference between millis() and micros() on: July 22, 2014, 11:26:20 am
Do you concede that the issue is how long the code takes to convert
strings to bit-patterns on the LCD, not any difference in behaviour of
micros() and millis() which both work as expected.

You must compare apples to apples, not apples to oranges, to compare
the two functions, conflating the code being timed with the code printing
out the results of the timing has been the confusion and you are still doing it.
41  Using Arduino / Programming Questions / Re: How can do something like this: IF HIGH && > 3000ms on: July 22, 2014, 11:21:57 am
If you want time to enter the equation, you need it to literally
enter the equation...

Code:
const int xbeePin20 = 7;       // pin 7
int led = 12;

void setup() {
  Serial.begin(9600);                // For testing
   pinMode(xbeePin20, INPUT);         
}

unsigned long went_high = 0L ;
boolean pin20_oldstate = false ;

void loop()
{
  boolean value20 = digitalRead(xbeePin20); // Pin 7
  if (value20 != pin20_oldstate) // detect changes
  {
    if (value20)
      went_high = millis () ; // capture rising edge timestamp
    pin20_oldstate = value20 ;
  } 

  if (digitalRead (xbeePin20) && millis () - went_high > 3000)
    digitalWrite(led, HIGH); // LED 12
  else
    digitalWrite (led, LOW) ; // must turn it off too
}
So you need a variable to record timestamp, and then compare current time
to that timestamp if appropriate.

Note the idiom of detecting a pin state change - use a boolean variable to record
the previous state.  Then the timestamp only gets set when the pin transitions HIGH.

The actual test turning on the LED will repeatedly fire, but that's OK because
repeatedly turning an LED on isn't a problem.  I've added code to turn it off too,
that's probably needed somewhere!
42  Using Arduino / Motors, Mechanics, and Power / Re: DC Motor with External Power on: July 22, 2014, 11:10:45 am
A small 9V battery is useless for powering motors.  You need battery pack that can provide
enough current (several amps peak will be expected).  This means good quality branded
AA rechargables or alkaline, LiPo or SLA.

PP3 9V battery - perhaps 100mA
AA cell, cheap - perhaps 500mA
AA cell, quality - 1A to 2A
LiPo/SLA - 10A+, fuse required.
43  Using Arduino / Motors, Mechanics, and Power / Re: How to Control a Motor that has Been Taken Out of a RC Car with the Arduino on: July 22, 2014, 11:00:53 am
But first you need to find out how much current the motor takes - motor
drivers like the L293 and L298 have very limited current capability.

Some more information about the car and or motor?  Measure the motor
resistance?
44  Using Arduino / Motors, Mechanics, and Power / Re: Use computer PSU for high-amperage LED RGBW on: July 22, 2014, 10:58:55 am
More to the point you expect us to answer questions about an unknown
power supply - can you not at least say what it is?
45  Using Arduino / Motors, Mechanics, and Power / Re: Minimizing power consumption of an Uno? on: July 22, 2014, 10:53:49 am
The USB->serial converter chip is also taking current, if you want micro-power
behaviour the Uno is the wrong board.  A bare-bones board or similar is needed.
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