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196  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 05:19:56 pm
I did not leave out hFE/beta, I was discussing driving the base hard so the device goes deep into saturation.
You want the current to be limited by the load resister and not the gain of the device

It is not necessary to drive a 2N2222 that hard to get it to saturate with a load of, say, 20mA. It saturates with 150mA of collector current with just 15mA of base current. Again, why use a transistor driver at all if you are going to use that much current from the Arduino?
197  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 05:10:36 pm
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They look right to me. The base is in the middle where I expect it and the plastic part would fit in the standard TO-18 triangular PCB layout without having to cross the legs.
I think the "round" part of the PN2222 body and the "curved" part of the 2N2222 pin
arrangement point in opposite directions, so if your pcb layout looks as follows, you're
up the creek [of course, I may be upside down over here],
http://jeelabs.org/wp-content/uploads/2011/01/screen_shot_2011_01_03_at_120850.png

The silkscreen may be misleading. All goes to reinforce the fact that you need to look at the data sheet.
198  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 04:45:36 pm
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When the NPN emitter is grounded Vbe is ~0.7V and base resistor is easy to calculate:
(5V - 0.7V)/20ma = Rbase

I would think that if you were going to drive the base with 20mA, you might as well leave out the 2N2222.  smiley
199  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 04:07:40 pm
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So, there is actually a difference if the LED is in the collector side or the emitter side ??

Absolutely. When using an NPN transistor as a switch, it is more common to place the load on the collector side and drive the transistor into saturation so as to dissipate the least amount of power in the transistor. The way you have it, there is a couple of volts across the CE junction, not a problem really in this case because of the capabilities of the 2N2222 but something to think about.
200  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 03:59:31 pm
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The link goes to a CircuitLab page that makes it easier to evaluate.
That's even more confusing. If you didn't know any better, you'd thing the 5V supply
on the collector was upside down, ;-). Possibly confusing to a noobee.
I'm not crazy about that schematic capture browser app.
Also, this is not good, pinouts between NP2222 plastic case and 2N2222 metal case
are backwards, oof! Murphy strikes again.
http://www.stanford.edu/class/ee133/datasheets/2n2222.pdf
http://www.jameco.com/Jameco/Products/ProdDS/38237ST.pdf
http://www.fairchildsemi.com/ds/PN/PN2222A.pdf

They look right to me. The base is in the middle where I expect it and the plastic part would fit in the standard TO-18 triangular PCB layout without having to cross the legs.

I congratulate OP for going through the design process. The first circuit is more common though, in my opinion.
201  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 02:00:37 pm
Yeah, but with that freaking fritzing program you can't tell that - have to go look up the parts and see which pins are B, E, C - which was totally obviouse in the original post.

The link goes to a CircuitLab page that makes it easier to evaluate.
202  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 01:21:16 pm
I'm curious why OP chose to switch the high side instead of using the original circuit. With this setup, the transistor is operating in the linear region and will never reach saturation.
203  Using Arduino / General Electronics / Re: transistor base resistor on: February 07, 2013, 11:13:39 pm
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Variations in transistor beta values from device to device. Changes in beta value Vs ambient temperature, etc. Most designs using transistor as switches don't try and arrive at the 'obtimum' base current to use, but some factor well above that calculated value.

Fortunately, there was a pretty good data sheet available for this device and we were able to design for those variables. With all due respect, I don't think you can speak for most designs. And it's 'optimum".  smiley

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Design as you wish, I'm just telling you what I have seen as general practice in real world circuits. One should go through the base current calculations using datasheet specs to make sure the device will 'always' be saturated under all possible causes of variation, therefore making sure to use more then the minimum amount required to reach saturation level.

Your experience with what is "general practice in real world circuits" is very different than mine. Belt and suspenders techniques do not always produce a better or more reliable circuit. I think if you look at the data sheet of the part OP is using and then look at the recommendations given, you will see that more than the minimum was used, but not 5 or 10 times more.
204  Using Arduino / General Electronics / Re: transistor base resistor on: February 07, 2013, 07:18:35 pm
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So using more then the a specific theoretical base current to reach saturation is not a bad thing and in fact will ensure the transistor will still be in saturation even for reasons the external circuitry cannot control directly

Not a bad thing but an unnecessary thing. Please elaborate on some of the "reasons the external circuitry cannot control directly".

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So using more then the a specific theoretical base current to reach saturation...

Not theoretical but fairly well characterized on the data sheet, in this case.

205  Using Arduino / General Electronics / Re: transistor base resistor on: February 07, 2013, 06:37:17 pm
When you're using a transistor as an on/off switch the resistor is mainly there to stop more than 40mA coming out of the Arduino pin and damaging it. Trying to calculate an 'optimum' value is pointlessly difficult as it depends on things like temperature. The only harm in using a smaller resistor is letting a few more electrons pass through it and wasting a tiny amount of power power.

330 Ohms should work with any transistor (15mA-ish from Arduino pin), 1k Ohms is commonly used too.


OP is trying to learn good design techniques and your approach doesn't support that goal.
206  Using Arduino / General Electronics / Re: transistor base resistor on: February 07, 2013, 06:11:23 pm
So firstly lets look at the output current - about 5mA - because when the transistor is on 5V will be across R1 so 5mA will
flow through it to the collector.

[When a transistor is fully on ("saturated") the voltage from collector to emitter will be a fraction of a volt, perhaps
 0.1V]

If the transistor has a current gain of about 100, then 50uA to the base will be about enough to allow 5mA to
flow from the collector - however this isn't going to saturate the transistor - you want several times more
current to ensure this - so lets say 200uA.

The base voltage will be about 0.7V or so, so 4.3V across the base resistor should allow about 200uA to
flow - meaning a base resistor value of 22k would be OK.  The value isn't critical so long as the transistor
is saturated 10k would work, 2k2 would work...

The current gain of a bipolar transistor is never a precise value and it does vary with collector current and
collector-emitter voltage (which is why you need more base current to saturate).

I did an analysis very similar to yours but used the only value I could find on the data sheet for a base-emitter current that would guarantee saturation at 10mA or less, 1mA. Using that base current and the data sheet saturated base-emitter voltage for 5mA of Ic, 0.5V, I calculated a base resistance of around 4K7. I wouldn't argue with your value range.
207  Community / Bar Sport / Re: KIckstarter of the week - USB micro led cable! on: February 06, 2013, 11:02:35 pm
Get ready for the Samsung Galaxy IIII (Yes Samsung Galaxy 4) set to hit the shelves in April smiley



Yes, I heard it will come with a charging cord with a LED on both ends!  smiley
208  Community / Bar Sport / Re: KIckstarter of the week - USB micro led cable! on: February 06, 2013, 07:12:55 pm
Yea, a trivial kind of dumb product. That of course means the inventor will become a millionaire next month once the iPhone rat pack discovers it as the next 'big thing' in the way of garnish for their baby.

Except it won't fit any of the iPhones. The Lightning connector on the latest iDevices makes the problem moot for them anyway.
209  Using Arduino / Audio / Re: Mems microphone lavalier style ( tie/shirt clip microphones ) on: February 06, 2013, 06:33:09 pm
i know it's not a easy task, For the RF problem, are you saying that existing rf setup like xbee couldn't do the job or bigger range module like 5.8ghz seen in A/V transmission from remote to rc plane are not "robust" enough ?


Xbee is designed for packets, not a stream of data. Don't know about the other. Is it analog or digital?
210  Using Arduino / Project Guidance / Re: Looking for enclosure with a slot for dip switches? on: February 06, 2013, 04:31:47 pm
What if instead of a dip switch I have a tiny push button and LED.  The user could push and hold the button till the LED blinked once, or twice, or three times etc. and then release and that would be the selected setting.  I would read their inputs on the button and write something to eeprom and store their selection.  That might be even more slick and would only require drilling some holes for a button and LED which can be made to look very clean and factory.

Probably an interface hotrodders could identify with. Be sure you consider a way to bail out and cancel any changes.

Right.  And something as important as spraying nitrous in a motor they'd need sound confirmation of their settings.  Hence the very simple and easy to operate dip switches.  Not only are they easy to set but easy to reference and confirm a setting.  I'll see what I can come up with and use the led/switch method as a fall back for now.

I think the screwdriver adjust DIP switch is the easiest to use method you've mentioned. You don't even have to be able to see it, just count the clicks after homing to zero.
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