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5761  Using Arduino / Programming Questions / Re: Shrinking a UTFT sketch on: March 08, 2013, 12:07:23 am
You have edited  libraries/UTFT/memorysaver.h  to reduce memory usage I presume?
5762  Using Arduino / Motors, Mechanics, and Power / Re: 2N2222 transistor as a switch on: March 06, 2013, 08:44:43 pm
No a transistor is not a switch - it switches, but its not that simple (MOSFETs are conceptually a bit more like
switches incidentally).

For an NPN transistor to conduct the base voltage has to be above the emitter voltage.  The Arduino can
only generate 5V, so the emitter in your circuit has to be less than 5V (about 0.6 to 0.7V below in practice).

The circuit configuration you have is called an emitter-follower, and it not used for switching, its used for
buffering analog signals typically.

You need a "common-emitter" configuration.  emitter to ground, collector to load, base via resistor to
Arduino - many examples all over this and other sites - so long as there is enough current through
the base resistor it rises 0.7V above the emitter and allows conduction.

For a transistor to be a switch it needs to go from hard-off to hard-on under control - hard-on is called
"saturated" and requires more base current than you might naively suppose (if the transistor has a gain
of 100, saturation might require the base current to be > 1/20th of collector current).  In saturation the
collector voltage falls to very close to ground (below the base voltage in fact).
5763  Using Arduino / General Electronics / Re: Digital pots like AD5206 on: March 06, 2013, 08:36:07 pm
The switches being CMOS analog switches (a.k.a. transmission gates), not little mechanical switches!
5764  Using Arduino / General Electronics / Re: P=V*I applied on DC resistor circuits. on: March 06, 2013, 08:33:19 pm
I use a couple of 10 ohm power resistors (25W each) bolted to a large piece of 6mm thick aluminium plate
as a dummy-load for testing things (like motor drivers, solar panels) - in parallel they give me 5 ohms, in series 20,
or just one of them gives 10.

If I put my 30V 3A power supply across one of them and crank up to full voltage the resistor gets hot
very quickly as its dissipating 30 x 3 = 90W  (I don't do this for long!).

The equations make perfect sense once you have hands-on experience like this.  Another important
realisation is that P = V x I  combines with V = I x R to yield  P = I x I x R

Power is proportional to the square of current - this means that high currents can be a real problem
to handle - 10A is a hundred times more "heat generating" than 1A for instance (which is why fuses are
very handy - they stop the wiring catching fire).
5765  Using Arduino / General Electronics / Re: Breadboard series/parallel? on: March 06, 2013, 08:22:15 pm

Having your two devices powered in series would mean that you had the ground pin of the first device feeding into the positive power input on the second device.  Kinda like batteries.

... and isn't useful here.  Components taking a 5V supply are all connected in parallel to the 5V rails.  Since the supply acts
to keep the rails a constant 5V apart no device should be aware of the others (unless the supply is overloaded).
5766  Using Arduino / General Electronics / Re: Can someone verify these two circuits are good before I fry something? on: March 06, 2013, 08:17:11 pm
Your link is broken - can't tell if its a logic level MOSFET or not.

The diode across the relay should be a standard rectifier diode, not a zener.

I presume the 5V in the first circuit represents an Arduino output pin?  You need a 150 ohm resistor perhaps to
limit transient gate current if its a big MOSFET.

The voltage divider in the second circuit is rather high current - use 10k and 39k and you won't have to worry about
calculating resistor heat dissipation.
5767  Using Arduino / Project Guidance / Re: calculating my power needs on: March 05, 2013, 10:08:14 am
Firstly we need to find out about these motors, the datasheets are not easily found(!)

I found M42SP-7 but not M42SP-7T.  The -7 has at least two possible winding variants anyway.
I found M42SP-6NK but not  M42SP-6NKA.  The -6NK also has multiple variants.
M42SP-6TE I couldn't find.

What is clear is that these are all bipolar motors and the nominal voltage figure is irrelevant
as is usual in bipolars (well you don't want to exceed the inter-winding voltage breakdown).

The current figure seems to be a peak (not RMS) value.  Check the resistance values with
a multimeter - hopefully these agree with the label.

You want some decent performance out of these motors so you absolutely do not want to
build a step-down switching supply, you want to go with the highest voltage supply you have
and use a bipolar chopper mode driver (such as the Pololu A4988 stepper drivers).

Given the info you have the the total power needed to energize one winding in each motor is
about 14 watts.  Assuming 80% efficiency this means about 17.5W needed which would be
about 0.55A at 32V - that 32V 0.94A supply sounds upto the job (for driving the motors slowly
at least).  For faster speeds more power is drawn and I've read somewhere you want a supply
capable of about 50% or the motor winding peak value (summed over the motors).  That would
mean more like 1.2A.  Depends how many of the motors you drive fast simultaneously.
5768  Using Arduino / Project Guidance / Re: Record Changes in Height of Book Stack - Best Sensor? on: March 05, 2013, 09:40:37 am
Ah, that suggests an approach.

Rather than having to fish around for a set of springs that are just right, use a stepper? motor and screwthread drive
to raise and lower the platform so that the top of the stack is level with some dual light-beam detector.   By counting the
steps needed each time is changes you can count the number of books.

By using a screwthread drive the motor can be powered down between uses.  When both beams are blocked or neither
are blocked (for more than a few seconds), you power up motor and step till just the bottom beam is blocked, simple!

An alternative with less heavyweight engineering is to use one light beam detector than is motor driven.  To measure
the pile of books it just goes to the bottom and scans upwards till the beam gets through - even a simple DC motor can be
used if the speed is nice and constant - just time the motor travel!
5769  Using Arduino / Project Guidance / Re: Kid ski helmet led light on: March 05, 2013, 09:30:44 am
The safety of crash helmets is dependent on the structure not being altered - adding anything on the top that could
catch against a hard object sounds very dodgy to me, instantly splitting the protective shell...  Also I can't see how
you can attach without weakening it - glues are out (polycarbonate is utterly weakened by many solvents), and
drilling holes is obviously a no-no.

Tying something soft on seems the only safe method.

If it wasn't for the glue/solvent issue LED strips would be attractive - perhaps these could be tied on rather than
stuck on?
5770  Using Arduino / Project Guidance / Re: 20 bit output Arduino Duemilanove on: March 05, 2013, 09:25:32 am
shiftOut takes a byte so you'll need to call it repeatedly for each set of 8 bits.  Only after
shifting out to all the 595's should you pulse the latch pins.
5771  Using Arduino / Sensors / Re: Which gyroscop for an auto balanced skateboard? on: March 05, 2013, 09:21:07 am
A standard 6DoF IMU would be a good choice, can then maintain orientation with DCM or quarternion method - there are
many forums about this, mainly on quadcopters, but the principle is the same - combine short-term gyro input and long-term accelerometer
input to estimate orientation.

For a simple one-axis balancing robot a single axis gyro and two-axis accelerometer is enough, but i think these skateboards
use tilt in the other axis as a control input for rounding bends?
5772  Using Arduino / General Electronics / Re: Digital pots like AD5206 on: March 05, 2013, 09:00:53 am
For any particular device you need to check the datasheet but in my experience all digital pots are switched resistor networks, so
no trouble with any analog signal that is within the supply rails and within the bandwidth of the device.  That particular pot
has quite high stray capacitance on its analog terminals, note, so it doesn't stay linear much past the audio range, especially the 100k
5773  Using Arduino / General Electronics / Re: What are chips like the MEGA2560 used for in real-world applications? on: March 05, 2013, 08:52:05 am
I see the applications for the low-end 8 bit controllers that Atmel makes and the ARM processors too, but I don't understand the niche for the ones in the middle like the 2560.  It's fairly expensive and has a small number of pins and raw compute power and memory compared to ARM.  What is it used for?  It seems the only slam-dunk application would be legacy low volume apps where compatibility with existing code is paramount.  What sort of products do we know the 2560 is in except for the Arduino Mega?  And does the go for XMEGA and AVR32 as well which seem to fall in the same boat?

Its not as expensive as you think - commercial products will be buying multiples of 10,000 and the price breaks for high
volume are substantial - qtys of 500+ already about half unit price...  Typically PCB board area (and associated manufacturing
costs and enclosure) is more expensive than the chips on it unless exotic devices are used.

Some of the market will be for gadgets developed on smaller AVR processors, but which have become more featureful in
successive versions and the developers move to a microcontroller that has more memory which is code-compatible with
previous versions.  The cost of re-engineering the hardware and software to switch between vendors is not trivial and
carries plenty of risk compared with staying with tried/trusted tech - especially in a competitive market where keeping
up with market developments is a race with time.

Also when a range of similar products is developed it is desirable to base all of them on the same software base - so using a range
of uControllers from the same vendor pays off - the cheaper/less featureful units can shave cost by using cheaper versions.  The more
advanced units in the product range will demand more memory, I/O, interfaces etc...

The uControllers vendor that can provide a wide range of software-compatible controllers will be favoured in the long run.
5774  General Category / General Discussion / Re: String Overflow on: March 05, 2013, 08:32:19 am
Its not C!, some flavour of Basic?
5775  Using Arduino / Motors, Mechanics, and Power / Re: Solenoid valve. on: March 04, 2013, 06:25:32 pm
You will need to find a 12V solenoid valve or stick with using a mains-rated relay to control it.

In theory a solenoid valve could have its coil re-wound, but I suspect taking one apart will ruin it...
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