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5866  Using Arduino / Programming Questions / Re: Problem with analogRead() on: February 10, 2013, 04:10:54 pm
Add a resistor (about 10K) from pin 7 to Arduino +5 volts.

edit: as AWOL said

Since the button is connected to +3.5V that won't work - use a pull-down to ground.  3.5V will consistently
read as HIGH when the button is pressed.   You need the resistor to pull the pin back down to 0V when the
button is released.

Unconnected inputs on CMOS chips are like radio antennas, they pick up any nearby signal very easily
since they are almost perfect voltage sensors with infinite resistance (well thousands of megohms or more).

Why 3.5V?  Perhaps you mean 3.3?
5867  Using Arduino / General Electronics / Re: Detecting when a LiPo battery connected trough a step-up board has low power. on: February 10, 2013, 04:06:52 pm
I'd include the resistor, something like 1k to 10k is fine, just to prevent the risk of accidental short(*) on the traces of the Arduino
grounding the LiPo - massive currents would then flow, vaporising pcb traces.  LiPo's can put out 50 to 100A or more if abused...

(*) Accidentally put the board down on top of something metal for instance.
5868  Using Arduino / General Electronics / Re: what the flux? on: February 10, 2013, 08:31:02 am
Any chlorinated solvent should be used with plenty of ventilation...  Or avoided(!)
5869  Using Arduino / General Electronics / Re: Piezo Sensors + LED Strip = Weird readings on: February 10, 2013, 08:28:46 am
There may also be electromagnetic induction causing interference from the LED circuit to the piezo.  PWMing a lot of LEDs
means a lot of EMI unless the circuit is shielded - each time the LEDs are switched on and off the rate-of-change of current
can be 10^8 amps/second or so - this will induce voltages in nearby circuits (as Faraday discovered).

Move the LEDs away from the piezo element.

Don't have open loops - these are bad news as the strength of interference is directly proportional to loop area -
always run your wires in a tight bundle, preferably twisted together (but separate bundles for LEDs and piezo,
of course!).

For instance the two wires from the MOSFET to the LEDs should be a twisted pair.  The wires from MOSFET to the 12V supply
should be a twisted pair.  There should be decoupling capacitors on the 12V rail right next to the MOSFET (at least 10uF,
100 would be better), to reduce the radiation back up the 12V supply wires.  Even though the MOSFET isn't directly
connected to the +12V rail you should run the 12V wire up to it, then away again to the LEDs.
5870  Using Arduino / General Electronics / Re: Too many shift registers? on: February 10, 2013, 08:20:39 am
For LEDs which are relatively high current loads switched at logic speeds (5ns perhaps) I'd use 0.1uF + 10uF per chip,
and 100uF+ for the whole board.

Switching many LEDs simultaneously could be causing a rate-of-change of supply current of the order of 10^8 amps/second,
way beyond what a voltage regulator on a remote board can handle, so in a microsecond or so those 0.1uF capacitors are
drained and the circuit malfunctions - before the voltage regulator has time to adjust to the new load (which might take many

The extra decoupling should hold the supply voltage up for long enough.
5871  Using Arduino / General Electronics / Re: Reflow oven (aka Toaster oven) modification on: February 10, 2013, 08:04:47 am
I use a bright torch to watch through the window - its a 12W LED in fact!  Intense point source lighting helps bring out
the shiny beads of solder as they melt.  Of course an LED isn't compatible with high temperature, but a small 12V halogen bulb
would be, there are holders for them with glass-fibre insulated wires too.

Some pictures of the conversion to top-side elements are in posting #44 here:
5872  Using Arduino / General Electronics / Re: Struggling to understand the basics of a 7 segment display common anode on: February 10, 2013, 07:58:51 am
Incidentally the 3V3 rail isn't a good choice for LEDs since

a) Low current only, might not be able to drive many segments at once
b) A bit too low a voltage for blue and white LEDs - may only be able
   to light up dimly...
5873  Using Arduino / Programming Questions / Re: Port Manipulation problem on: February 10, 2013, 07:53:52 am
Humans find binary constants hard to read, hexadecimal notation is easier once you are used to it IMO,
since it groups in sets of 4 bits its harder to slip up by one bit (you can slip up by 4 bits though, but only
with long values in practice).  Its also far far easier to memorize a hex number, even temporarily.
5874  Using Arduino / Programming Questions / Re: Delay question on: February 10, 2013, 07:38:57 am
Which Arduino board?  There is no way to shutdown the USB chip on boards such as Uno and Mega AFAIK,
so putting the microcontroller into a sleep mode won't drastically reduce current consumption.  Usually you'd
need an real-time-clock chip if going that route as running the microcontroller's 16MHz clock is a large part
of the current consumption.

Its worth searching these forums carefully there have been several lengthy threads on this sort of thing.

5875  Using Arduino / Motors, Mechanics, and Power / Re: Adjusting two BigEasy drivers causing jumpy steppers. on: February 09, 2013, 05:54:52 pm
More decoupling required?  Are you running the cables for the motors close to any other cabling?  I presume
10A is more than enough.
5876  Using Arduino / Motors, Mechanics, and Power / Re: Wall Wart amperage question. on: February 09, 2013, 05:53:47 pm
The latter outcome should not happen if its a genuine UL-certified part (and not a cheap knock-off).  A general
purpose PSU is more likely to be built to take abuse than one designed for a single piece of equipment I suspect.
5877  Using Arduino / General Electronics / Re: Battery choice/source help? on: February 09, 2013, 05:49:54 pm

my question(s) are as follows:

1.) How much current can the CR2032 give?  I have looked and not only do I see different numbers.. I usually only see it for mAh...  not mA?  Im thinking I need about
1mA? 2mA?  Certainly very little indeed.

I understand that these CR2032's are mostly for high energy, low power projects...
Strictly micro-power applications only, like calculators (a few tens of uA)

and I guess Im needing the opposite... or at least just more current.
For an SDcard you need a regulated 3.3V and perhaps upto 150mA - SDcards can be power hungry, different cards take
different currents, and more when writing.

2.) The space/area I have to work with.. really is only suited for a 'coin cell' type of battery...  it can be a little 'bigger' (23mm diameter I think I have room for?.. maybe a bit more if needed)...  Is there anything out there that can work for my needs?
Do you need the capacity of an SDcard?  An EEPROM might be less power hungry.  You will really struggle to find anything for that
size, but the best power density is LiPo, the best voltage match might be LiFePO4.  Well having said that 23mm diam, but how long?

3.) Is there a value/sepc on these datasheets Im supposed to be looking for/at to see if the battery can DELIVER the current I need?
When you can find a datasheet for a battery it will have discharge curves - these are perhaps the best info.  The datasheet only
covers one type of cell from one manufacturer.

Secondary question... about my PSU (benchtop Mastech HY1803DL)

I set the voltage to +3v
I set the current to be 'about' .10-.11 Amps  (100-110mA)

when I connect it to my project..and power it on..

I see the current LCD screen 'jump'.. to about .06A....... when I press the button and trigger an audio clip to play.. I see the current LCD screen on my PSU jump to about .08A

Im curious as to what my current LCD screen is 'reflecting' (telling me).. is this the current 'draw' of my project?  (kinda cool/nice if it is...nice way to 'tell')

Yes, you're powering up the SDcard at that point perhaps, and perhaps an audio amp, so the current will rise.  80mA is actually quite
modest in fact
5878  Using Arduino / General Electronics / Re: Power Mosfet on: February 09, 2013, 05:39:14 pm
See this thread - the device mentioned in reply#3 only needs logic level drive and is designed for ignition coils:,141276.0.html
5879  Using Arduino / Project Guidance / Re: How large of a stepper motor? on: February 08, 2013, 09:40:15 pm
That motor driver should be able to handle the motor in bipolar-series mode I think, but you'll want to limit the current
with PWM otherwise it'll be 2A instead of the rated 1.4A. (Assuming a 12V supply)

You won't get as much top speed from it though (for that a high-voltage constant-current bipolar stepper motor driver
 and parallel-mode is the answer).
5880  Using Arduino / General Electronics / Re: Understanding ACS7xx Current Sensor on: February 08, 2013, 09:24:32 pm
retrolefty, a resistor of 1.2mOhm is between pin 2 and 3. So at 30A there is 36 mV between 2 and 3.

Well I would reword that as "the maximum resistance between current terminals is specified as 1.2mOhm" - the resistance
is incidental to the operation of the device since it's a hall-effect sensor - some of that resistance may be the pins, the solder on the
pins, there will be some from the PCB traces too (for 1oz copper board the ohms-per-square is about 0.5mOhm).

Or put another way there will be at least tens of millivolts everywhere at those current levels, and 30A continuous wouldn't be
a sensible application for these sensors.
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