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5866  Using Arduino / Motors, Mechanics, and Power / Re: How to convert 12 V 2.5 amp battery to 12V 800mA battery? on: April 22, 2013, 06:41:58 pm
Don't not over charge the battery like MarkT said, Outofoptions has it right. The battery capacity (amp hours Ah) is how many amps the battery can put out for one hour before it goes dead. Therefor a 12v 2.5Ah battery will last one hour if you pull 2.5A of continuous current from it, but if you're only pulling 800mA form it, it will last 37.5 hours. I hope this helps.

Actually no, most measured capacities are measured at the 10 hour rate - a 2.5Ah battery will put out 0.25A for 10 hours.  At
higher discharge rates capacity typically decreases for most battery chemistries.
5867  Using Arduino / General Electronics / Re: Linear Motion on: April 22, 2013, 06:38:33 pm
Indeed, they are a clever technology - but the device you are looking for is a "electronic scale"
or "linear encoder" - they use several possible technologies from capacitance, optical moire fringes, some
use a magnetic method.
5868  Using Arduino / Programming Questions / Re: Arduno and Thermister, not getting readings I should be? on: April 22, 2013, 06:32:14 pm
You need to measure its resistance with a multimeter at a few different temperatures to characterize its
response curve.

Choosing a fixed resistor for the bridge circuit depends on the resistance of the thermistor - you get
most resolution if they are about equal at the temperatures you are interested in.
5869  Using Arduino / General Electronics / Re: Opto couplers and relays - please clarify on: April 22, 2013, 02:26:43 pm
Typical switch cycles will be more like 100,000 - basically they wear out eventually, whereas an opto coupler
will happily switch at 100 kHz for decades or more.
5870  Using Arduino / Programming Questions / Re: ADC problem, where is the voltage come form? on: April 22, 2013, 02:24:08 pm
  pinMode(sensorPin, INPUT);  // << INTPUT or OUTPUT
This is NOT affecting the analog pin at all. It ONLY affects digital pins.
I think what you are trying to get at is that when using pinMode() or digitalWrite () on an analog pin you
must use its proper full pin number (the #defines A0, A1, A2 etc are valid, as are 14..19) - you mustn't
use the bare analog pin numbers (0..5) for such calls.

I didn't plug any external Vcc to it
Then you should not be reading it, or should have NO expectation about what the function will return.
Indeed a floating input is exactly that, floating around picking up any nearby signals via capacitive coupling,
or even static electricity from nearby insulating objects.
5871  Using Arduino / Programming Questions / Re: Arduino MEGA Serial Communication using TX2 and RX0 on: April 22, 2013, 02:19:12 pm
You have no decoupling anywhere - not difficult to see why you don't need the IR receiver to pick up any light if
its comparator catches glitches from the supply rail - decouple all chips, especially comparators (they are extremely
sensitive).   You may have to add a little capacitance on the comparator inputs to prevent false triggering too.
5872  Using Arduino / Programming Questions / Re: Arduino.h : No such file or directory on: April 22, 2013, 02:11:36 pm
Searching these forums for "Arduino.h" will find lots of threads on this issue I think.
5873  Using Arduino / Programming Questions / Re: digital input pins are too sensitive on the arduino uno on: April 22, 2013, 02:10:17 pm
You mustn't let an input "float" (open-circuit) if you want a defined input value, input pins have effectively infinite resistance
and will sense ambient electronic noise quite happily.

So you never connect just a switch to a pin, you always have a resistor pulling one way and a switch the other.  Fortunately
all Arduino pins have built-in pull-up resistors (about 30k ohms) that can be activated by calling
  digitalWrite (pin, HIGH) ;

when the pin is an INPUT (this is specific to AVR microcontrollers such as in the Uno and Mega).

Or you can use an external resistor (which can be wired to pull-up or pull-down).
5874  Using Arduino / Motors, Mechanics, and Power / Re: DIY Autopilot on: April 22, 2013, 02:03:42 pm
Here are some belts/pulleys that are too small for you I think:, but the thing to search for is "timing belt" and "toothed belt" I think.

For motor a stepper motor might be good (no holes to let splashes in like a DC motor!).  Some sort of indexing scheme might be
needed, at least at power up.  You'll need to investigate how much torque will be needed, and you'll need a manual override
option as steppers hold position strongly.   Continuous rotation servo is another possibility - but first you'll need to get an
idea of the forces / torques needed.  And find a supplier for large pulleys for toothed belts (unusual item) - or even gear wheel
 (nylon gears would be corrosion-proof).
5875  Using Arduino / Programming Questions / Re: Variable Delay: the big problem on: April 22, 2013, 01:48:30 pm
You are declaring variables like vm twice - did you mean to?

[ to be more precise you are declaring two variables vm, one global used by semaforo(), and one
local to loop() which you print to the Serial stream.  The variables are completely distinct.
5876  Using Arduino / Programming Questions / Re: How to output an Infrared LED? on: April 22, 2013, 01:42:07 pm
Depends on whether there is a lot of background light - if so then you may need to modulate the
LED and use a band-pass filtered receiver to ignore the background.  This is how remote controls
work, there are examples if you search.

If background light not an issue then just pass the design current continuously through it with a
series resistor.  If its a high current LED then a transistor may be needed - if its 20mA then you
can drive from an OUTPUT pin without the transistor (but don't forget the current limit resistor).
5877  Using Arduino / General Electronics / Re: SMA connector wiring on: April 22, 2013, 01:37:35 pm
Assuming its a decent piece of coax (polythene or teflon dielectric)...  But anything will be better than the coiled wire smiley
5878  Using Arduino / Motors, Mechanics, and Power / Re: How to convert 12 V 2.5 amp battery to 12V 800mA battery? on: April 22, 2013, 01:31:35 pm
You mean 2.5Ah not 2.5A and 800mAh not 800mA ?  Capacity (charge) is not current.

The easiest way to reduce the capacity of a lead-acid battery is to over-discharge (just once will wreck it).  smiley-wink
5879  Using Arduino / Motors, Mechanics, and Power / Re: help with dc motors on: April 22, 2013, 01:28:51 pm
The main reason to filter the noise is to increase the sensitivity of the RC receiver.
5880  Using Arduino / General Electronics / Re: Opto couplers and relays - please clarify on: April 22, 2013, 01:22:22 pm
Looking at that Make example... it's not a very good example. They tell you to connect the relay coils directly to the Arduino which is not a good practice.

Using relays:
Using mosfets:

Much worse than that, the circuit is more or less guaranteed to destroy the Arduino - where is the free-wheel
diode across the relay coil?  That is an essential component.  They are (or happen to be) using the on-board LED and resistor
that is attached to pin 13 as a snubber network, which is outrageous really...
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