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3481  Using Arduino / Motors, Mechanics, and Power / Re: L298 non zero output voltage on: July 02, 2013, 11:03:43 am
Since the device has darlington output stages it will have quite a large leakage current
(compared to single transistor output stage), but still effectively zero when we talk
about driving a motor.

The chip designers didn't care about the unladen output voltage of a motor driver,
its not a useful parameter, you might even be the first to try measuring it!
3482  Using Arduino / Motors, Mechanics, and Power / Re: distance sensor for motor rotation on: July 02, 2013, 10:57:07 am
What kind of distance sensor ?

What kind of motor?

Do you have a motor controller board or shield?

In general vague questions will only get a vague answer - anything is possible with electronics
(subject to the laws of physics) after all!
3483  Using Arduino / Motors, Mechanics, and Power / Re: Managed to damage a power supply but can I fix it? on: July 02, 2013, 10:54:38 am
Frankly any semiconductor involved in the damaged supply rail is a candidate,
replacing them one-by-one till it works (starting with the cheapest) is a possible

Might be easier to get another supply that has current limiting outputs (bomb-proof).
3484  Using Arduino / Motors, Mechanics, and Power / Re: Motor solution. on: July 02, 2013, 10:52:02 am
Certainly a big servo like that cannot perform on an inadequate power supply - fix this first.

But secondly you have given it a large torque load to handle - your robot arm would be far far easier
to drive if each section was counter-balanced (even partially).  The current setup needs a lot of torque
just to raise the arm, let alone any load.

The servo motor is a 10kg torque, should I change it into 15kg torque so that it can raise up easily?

kg is not a unit of torque.  I suspect you mean 10kgf-cm or some such unit - without knowing for sure I can't
tell if the change would be useful.  You should always try to work in S.I. units for torque, its so much more
convenient and simple.

You need to work out the torque needed to drive the arm comfortably (a rough estimate in N-m is
10 x M x L, where M is the total mass of the moving arm in kg and L is the length in m.)

For instance a 0.4m long arm weghing 0.5kg would work well from a 2N-m servo.
3485  Using Arduino / Motors, Mechanics, and Power / Re: Driving 4 small geared motors on: July 02, 2013, 10:40:28 am

How would I go about using a 7.4v 2200Mah battery with this?
An off-topic note about units - the case for symbols matters:  volt is "V", not "v", milli is "m" not "M" (which means mega), ampere is "A" not "a".

Also 2200mAh would normally be written 2.2Ah (except by manufacturers of lithium and NiMH batteries
who seem to fetishize the use of mAh!   lead-acid batteries and cells are marked in Ah though.

This industry-wide oddness occurs in at least one other area -  capacitors are rarely marked
in nF and never marked in mF (pF and uF are used even when this makes the numbers huge or

I suspect its something to do with marketing departments obsession with using larger numbers than the
competition, assuming a customer will be drawn to "10000mAh D-cell" in preference to "10Ah D-cell".

The capacitor thing is more mysterious though, possibly related to the US's backwardness in adopting
S.I. (I believe pF used to be called uuF originally).  nanofarads have sort of become mainstream now
but millifarads haven't, which is even odder (milliohms and millihenries are commonplace)

[ When I write u as in uF I really mean a greek lowercase letter mu, of course ]
3486  Using Arduino / Motors, Mechanics, and Power / Re: Driving 4 small geared motors on: July 02, 2013, 10:21:09 am
The output current is rated up to 1.2A per channel (or up to 3.2A for a short, single pulse).
A fuse is useful.

I would say for LiPo and lead-acid batteries (both capable of currents high enough to set fire to
wiring) a fuse (or over-current protection circuit) is essential.   Place fuse on +ve wire right next to
the battery, then any wiring downstream is protected should there be an accidental short.
3487  Using Arduino / General Electronics / Re: Powering nRF24L01 (3.3v) with Arduino output pin (5v) on: July 01, 2013, 03:48:59 pm
Use a PNP transistor or p-MOSFET to switch the 3.3V supply to the RF module? 
3488  Using Arduino / Motors, Mechanics, and Power / Re: Gears vs Belts on: July 01, 2013, 03:44:10 pm
Torque is gram-cm, not kg-cm.
So many misunderstandings of S.I. in one glib statement!

Firstly you don't mix names and symbols, so g-cm or gram-centimetre, but never gram-cm.

Secondly the kg is a unit of mass, not force.  The "kilogram-force" or kgf would be a suitable
force unit. In SI the kilogram is the base unit, not the gram (historical reasons).

Thirdly in S.I. the newton is always the prefered unit of force, and metre the prefered unit of
length, so newton-metre or N-m is the unit of torque.  And never N-cm (you can say cN-m though smiley-wink.

Now a newton is a joule/metre, so newton-metres are simply joules, but that's forgetting the
rotation, hence joules/radian is actually a more logical unit but for some reason N-m is used rather
than J/rad.

Anyway at least no-one's tried to use "kg/m" or "pounds/foot" or some such real howler smiley
3489  Using Arduino / Motors, Mechanics, and Power / Re: Nema 17 and 4cm fan on: July 01, 2013, 03:19:47 pm
Well nema 17 means 1.7 inch that's 4.3cm, no wonder the distance between holes doesn't match.
2 digit NEMA numbers refer to face mounting plate sizes in tenths of an inch I believe,
3-digit NEMA numbers are different (I think its side-mount plate to axis distance or something
like that)

Mounting holes are probably not covered by the NEMA classification since it covered cylindrical
and square motors...
3490  Using Arduino / General Electronics / Re: Strange behaviour with rx pin on Standalone atmega328 on: July 01, 2013, 02:55:43 pm
Pull up is only needed because the bootloader tries to read from the RX pin after reset (it times
out if the line is idle)  Often you can get away without a pull up (this is due to luck though)

Some bootloaders may even turn on the internal pull-up on the RX line?  I have a vague recollection
about this, no doubt entirely wrong!
3491  Using Arduino / General Electronics / Re: Simple question about resistor for LED on: July 01, 2013, 02:49:36 pm
The forward voltages are closely related to the energy of photons of light of the relevant colour,
which can be calculated from the wavelength w (in nm) as 1240/w (in volts).  This leads to values
of about 1.9, 2.45 and 2.6 for 'normal' hues of red, green, blue.

Measuring an RGB LED I have gave 1.8, 2.6 and 2.6V - rough agreement (the real situation is more
complicated, the band-gap voltage of the semiconductor material is also very important - here I believe
both the green and blue LEDs are gallium nitride, hence the same forward voltage (the colour can be tuned
by clever design that affects the quantum mechnics of the junction).
3492  Using Arduino / Motors, Mechanics, and Power / Re: Motor Control Shield and Vin on: June 30, 2013, 02:11:02 pm
If one set got hot and the other didn't then one set was shorted out.
3493  Using Arduino / General Electronics / Re: optoisolator question on: June 30, 2013, 02:08:01 pm
There is no abs max rating for pulse output from the pin, you use 40mA as the abs max if you
want to play safe.  100ms is a long time for a single output pad on a microcontroller (pad die
area perhaps 0.001 sq mm), probably reaches thermal equilibrium in a couple of ms.
3494  Using Arduino / Programming Questions / Re: how are routines executed on: June 30, 2013, 01:58:10 pm
Shifting details from code to data, writing programs as systems instead of dialogs has worked well for me. Program = code + data. I prefer having less code in that equation. 

Absolutely - the corollary to that is the observation that its harder to put bugs in data, and easier to spot them,
so reliability is increased, not just maintainability and flexibility...

Of course a source-file is data to a compiler...
3495  Using Arduino / Programming Questions / Re: program wont return to loop from interrupt. on: June 30, 2013, 01:47:17 pm
And don't use serial print inside and interrupt routine...

Specifically you cannot use Serial operations in an ISR or the system could hang waiting for
a serial interrupt to fire (which it can't because you are inside another ISR and interrupts are
disabled during an ISR).

Similarly you cannot use delay() in an ISR (nor should you want to, ISR's need to be quick
and light-weight).
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