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5791  Using Arduino / General Electronics / Re: L293D alternative on: June 28, 2013, 05:30:08 am
You'll need to follow the traces on the reverse side of the board and need to know which of the connectors
on the board went to which part of the RC car before being able to determine the role that chip played.
It's quite likely its a microcontroller pre-programmed specific to that toy - there is a lot of discrete transistor
circuitry on that board which may or may not be RF receiver circuitry (can't see any obvious quartz crystal).

"Reverse engineer" = "detective work".

Absolutely no a-priori reason to suspect that that chip is a motor driver (there are several power devices on
heatsinks already taken from the board that are much more obvious candidates for this role)
5792  Using Arduino / General Electronics / Re: a question about capacitors in a DC current ! on: June 28, 2013, 05:20:59 am
First you need to make a distinction between steady-state DC (nothing ever changes), with
practical DC circuits (which change with time so are not purely DC - at the very least
they are switched on and off).

No practical circuit is pure DC anyway...

So capacitors are not just "something to do with AC" - they store charge, specifically
equal amounts of +ve and -ve charge on the two plates, and larger capacitors store
more charge for a given voltage difference.

Charge can be stored for several reasons - one is decoupling - you want to provide a
constant supply voltage to a varying load over timescales of milliseconds down to
nanoseconds - most capacitors on digital logic circuits are there precisely to do this
(power supplies are too slow and too far from the loads (the logic chips) to perform
this task, since logic loads change in a few nanoseconds, and at that timescale all
the wiring on your circuit board has very significant impedance due to stray inductance.

Logic circuits are, in effect, very high frequency AC environments.

You mention an RC car - the remote signal is a radio transmission - that's AC.  The demodulated
signal from the receiver is a pulse train using PWM - that's AC.   The brushless motor powering
the wheels is inherently an AC device that requires rapid high current switching between the
windings.  (if its a brushed motor then it's power is controlled using high-power PWM, again
this is not DC).   

No interesting circuit is ever purely DC!
5793  Using Arduino / Motors, Mechanics, and Power / Re: 300m cable: a power transmission plea for help! on: June 26, 2013, 05:06:47 am
This sounds like a lethal device.   You need to consult experts in marine electrical safety and ROVs before even
contemplating this.
5794  Using Arduino / Motors, Mechanics, and Power / Re: Control stepper motor with 2 pins - 1 direction only on: June 26, 2013, 05:02:28 am
Have you looked into using L298 based shield?

I imagine wire it up like Figure 6, where each coil would replace the motor.
Then drive the coils in parallel so they are both pulsed to turn the motor in the same direction with a High Pulse on In1 / In3, and Low Pulse on In2 / In4.

You don't understand stepper motors - you have to cycle through at least 4 phases in seuqence to achieve
rotation, driving both coils in parallel won't achieve this.
5795  Using Arduino / Motors, Mechanics, and Power / Re: Stepper motor with tcp packets on: June 26, 2013, 04:59:07 am
That codes rather complex to follow - can you derive a small sketch exhibiting the same wrong behaviour
that is easy to follow?

BTW which stepper motor, which driver, how is it powered, what's the moment-of-inertia of your propellor?
If you don't know the last figure its entirely possible you have inadequate torque for the application or
you are trying to accelerate too fast.
5796  Using Arduino / General Electronics / Re: PS2 Keyboard Interference on: June 26, 2013, 04:53:56 am
Firstly if you use CAT5 make sure each signal wire is twisted with ground (essential for proper performance
over long runs).  Connect ground to multiple wires if necessary, one pair can be signal+ground, one can be

Secondly the ferrite is killing common-mode signals caused by the keybaord and its cable acting as a radio
antenna.  It needs to be mounted close to the Arduino end of the cable to have any effect.  All the wires
from the keyboard should run through it together.

If your cable is long you can see crosstalk and other signal problems - adding 100pF caps between signal and ground
at the Arduino end can help tame fast edges (although they shouldn't confuse a serial link).  Adding 1k pull-up
resistors to each signal line at their destination end is also a wise precaution (doesn't PS2 require pullups anyway?). 
100 ohm or so resistors in series with signal lines can help tame reflected signals too.
5797  Using Arduino / General Electronics / Re: Making a transformer as an experiment on: June 26, 2013, 04:47:36 am
with an oscillating DC current...
In that case, it wouldn't be DC.  smiley-wink  You can run a square-wave through a transformer, but you normally don't get a square wave out of the secondary.

That's precisely what you should get, the same voltage waveform out, since the same flux change links
the two windings (for an efficient transformer).

Of course an air-cored transformer will not be efficient since the flux linkage is far from 100% (except for
bifilar wound transformers).

Once you load the secondary you may get distortion of the waveform - that usually indicates that the
efficiency is varying between the frequencies of the various signal harmonics.  A good pulse transformer
will keep waveshape when terminated correctly across a wide frequency range.

Without using a ferrite core you are unlikely to make a reasonable wideband transformer.
5798  Using Arduino / General Electronics / Re: Relays constantly on using a multiplexer. on: June 26, 2013, 04:41:30 am

So i wonder if looping thru the 8 channels very fast
Could keep the relays on constantly?

Relays may  not tolerate high frequency AC drive, you'll get significant iron losses
(relay cores are soft iron rather than silicon steel laminate).

Furthermore you'd have to drive them at much higher voltage to get the right
average current, and your multiplexer wouldn't take that.

The correct approach is to use something like a 74HC595 shift register (which latches
output states) and have a drive circuit for each relay (presumably NPN transistor
and flyback diode.  ULN2803 provides 8-channels of such drive, so a 595 and 2803
together would be a common choice.  Check the voltage and current specs of your
relays of course.
5799  Using Arduino / General Electronics / Re: How much current flows through a MOSFET when off? on: June 26, 2013, 04:34:54 am
10uA is far less than the self-discharge rate of most batteries - you can ignore FET leakage
at room temperature.

In the diagram the gate current limiting resistor is shown as 10 ohms - no use for the Arduino,
150 ohms would be a more suitable value (keeps Arduino pin current within spec - less than
5800  Using Arduino / Motors, Mechanics, and Power / Re: Stepper Motor Burning Up: Extreemly hot to touch on: June 24, 2013, 02:14:04 pm
Looking at the datasheet it says operating temp range 0..50C, allowable temp rise 70C - that means it could
run as hot as 120C if ambient conditions were 50C, or 90C if ambient=20C.  (bottom two rows).
5801  Using Arduino / Motors, Mechanics, and Power / Re: Question: Driving a heating element on: June 24, 2013, 02:09:12 pm
Then they almost certainly overheated - full plastic body doesn't conduct a lot of heat away in a hurry...
5802  Using Arduino / General Electronics / Re: Soldering wire onto MOSFET on: June 24, 2013, 02:06:20 pm
If you solder to the heatsink you may overheat the chip.  And you won't be able to use it as a heatsink.  use
a nut/bolt and solder tag if you want.

You can drill out PCB holes larger.

You can mount veropins or similar on the board and solder wires to them.
5803  Using Arduino / Programming Questions / Re: Local Variables Losing Values on: June 24, 2013, 02:03:46 pm
A variable doesn't have to be defined static to be global in extent - it just has to be defined outside of
any function.

static keyword hides the global variable from other source files - makes the scope to be file-local.  normal
global variables have global scope (your whole program) as well as indefinite extent (live forever).

"extent" means the time duration for which a variable exists, "scope" means the subset of the program
code from which it can be named.  local scope is not the issue here, its local extent.
5804  Using Arduino / Programming Questions / Re: DS1307 RTClib on: June 24, 2013, 01:55:20 pm
day of week is also on the chip, but you have to set it and interpret it yourself - I can't remember if it
cycles 1,2,3,4,5,6,7,1,2,3,... or 0,1,2,3,4,5,6,0,1,2...

The 8 active bytes are


5805  Using Arduino / Programming Questions / Re: serial monitor returns extras calculations on: June 24, 2013, 01:51:53 pm
You sent a digit '1', 0x31 followed by a newline character 0x0A.  you subtract '0' from each, so '1' becomes 1,
newline becomes -0x26 (hex, ie -38 decimal).

The serial monitor sends a line at a time with a newline character terminator.  Your Arduino code isn't
checking for newlines and spaces.
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