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16  Using Arduino / General Electronics / Re: Resistor value on: September 28, 2014, 03:31:34 am
is my calculations correct?
Yes the calculation is correct. However your reasoning is not.

You do not need that much current through Q5. This transistor only drives a 1K load plus the base currents of Q1 & Q2. There is no way you want to drive so much current through Q5.
17  Using Arduino / General Electronics / Re: Resistor value on: September 28, 2014, 02:12:52 am
Which means, to get the transistor to fully saturated, Ib must be 40mA?

Only if your transistor is drawing 10A, which is unlikely.
18  Using Arduino / General Electronics / Re: Darlington Array with Bipolar Stepper Motor on: September 28, 2014, 01:19:31 am
I am using this motor for an extruder for a 3d printer. Will the l293d be powerful enough for that (or i still need more power)? It does not need high speed, only torque.
It depends entirley on what motor you are using. If it is like you linked to the answer is still no because you have to arrange the supply voltages to be such that your motor has 4.5V on it when passed through the chip.
19  Using Arduino / Project Guidance / Re: servo not working on external power supply on: September 27, 2014, 05:32:03 pm
if you use the #include function to include a library, then try and use the Arduino without the computer, does this affect its ability to include that library?
No. The library is compiled and stored in the arduino.

Yes the power supply is fine,
Yes but is the ground of this power supply connected to the ground of the arduino.
20  Using Arduino / Motors, Mechanics, and Power / Re: motor wheel stops when placed on ground on: September 27, 2014, 03:03:16 pm
If the motors are rated as 9V motors then as you loose 2 to 3V in the driver you are not powering the motor with enough voltage. You need to add a few more AA batteries so that the motor "sees" the voltage it is expecting.
21  Using Arduino / General Electronics / Re: 10mm Ultra Bright LED with no resistor? on: September 27, 2014, 02:56:35 pm
So I tried using the 3.3v output from Arduino to power the LED with no limiting resistor.
That might seem to work as a one off but remember the spec said that the forward voltage is:-
It's specs say it is 20ma and 3.2v Typical and 3.8v Max.
So with some LEDs you will be within the current spec, some ( like you found ) below it and others it might not light at all.

Bottom line is you do not have enough voltage available to drive this LED reliably. Note also the forward voltage changes with the age of the LED and also the ambient temperature. So while you might get away with it sometimes it is bad practice.
22  Using Arduino / Programming Questions / Re: Uploading different sketches with Arduino on: September 27, 2014, 12:56:25 pm
Basically you can't.

You acheave the same effect by writing your sjpketch with two functions then in the loop function you look at the switch and decide what one of the two functions to call.
23  Using Arduino / Project Guidance / Re: Window controlled by water sensor on: September 27, 2014, 11:44:55 am
Unless that relay can operate at 5V and less than 40mA you will damage your arduino.
If it will you still need the reverse bias diode across the coil or again you will damage components.
24  Using Arduino / Project Guidance / Re: Detect the position of a modelrailroad turnout on: September 27, 2014, 11:36:21 am
Many many years ago when I did this I just used a very small solenoid to change them.

In English these are known as "points" not a "turnout".
25  Using Arduino / Project Guidance / Re: setting threshold via buttons ? on: September 27, 2014, 11:32:32 am
Ok i fixed it now BUT it now when i press Up it continues to increment non-stop !

Look at the change state example in the IDE:-
File -> Examples -> 02Digital -> ChangeStateDetector
26  Using Arduino / General Electronics / Re: Darlington Array with Bipolar Stepper Motor on: September 27, 2014, 11:28:44 am
Grump mike- what ic should I use if this one is a bad choice?

It is a bad choice because your motor is only 600mA at 8 ohms. That means only 4.5V to drive it. If you do that you will get poor performance. So you need a driver that incorporates a chopping regulator so that the motor can be driven as fast as it is capable of going.

Use something like this:-
27  Using Arduino / Displays / Re: 59 - 09 in LCD and RTC on: September 27, 2014, 11:22:29 am
then post your way of making zeros so we have the ultimate way and learn from our errors
OK here:-

void displayTime(){

void aPrint(int number){
  if(number < 10) lcd.print(0);
28  Using Arduino / Project Guidance / Re: Window controlled by water sensor on: September 27, 2014, 04:01:08 am
.Sorry i couldn't come up wit h the shematic.
Why not, just draw it with a pencil and paper and then post a photo of your drawing. You could be making all sorts of other mistakes which seems likely given the track record of this post.
29  Using Arduino / Project Guidance / Re: proximity sensor only activated by certain things on: September 27, 2014, 03:30:33 am
It is the int bit I front of readTag that defines it.
So outside the loop you have int readTag
And inside the loop just have readTag =
No int in front of it.
This is a very fundamental missunderstanding of variables you have here.
30  Using Arduino / General Electronics / Re: Resistor value on: September 27, 2014, 03:24:35 am
The load current is determined by the supply voltage and that 1K resistor by using ohms law.

Plus the current into the base of the complementary transistors which is determined by what load the motor draws.

Do you want a link to ohms law?
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