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181  Using Arduino / General Electronics / Re: Transistor Gain/Saturation question on: February 14, 2013, 12:04:26 pm
Absolutely, sorry about that! And most of all thank you!

No problem! You're welcome!
182  Using Arduino / Sensors / Re: Vehicle Detection Sensor- Any Recommendation?? on: February 14, 2013, 12:03:01 pm
Another approach would be to put a large coil of wire where the car is to park. Use enough turns to get a measurable amount of inductance and use that inductance as the basis for an oscillator. When there is no metal interfering with the inductance of the coil, the oscillator will be at its design frequency. When ferrous metal interferes as when a car moves over the coil, the frequency will change.
183  Using Arduino / General Electronics / Re: Transistor Gain/Saturation question on: February 14, 2013, 11:55:03 am

Base voltage is defined as the voltage on the base of the transistor. In the case of the common emitter configuration, it would be approximately 0.7V. The 5.0V in your example is the driver voltage. Plus, one usually calculates the base resistor value based on the desired base current, not the other way around as you have done.

What I meant by Base Voltage was the driver voltage or rather, the voltage out of the Arduino pin (+5V). I used the wrong words, but I think you know what I meant now. And yes, you're right in saying that you pick a desired current and then figure out the resistor. I was just using a random resistor value to test my math smiley.

Yes, I know what you meant nowsmiley I think proper terminology is important when trying to communicate.
184  Using Arduino / General Electronics / Re: Transistor Gain/Saturation question on: February 14, 2013, 11:49:06 am
That's not how I read it.
Quote
Base Voltage - 0.7V (or whatever the base Voltage drop [Vbe] is according to datasheet) ÷ base resistor = Ib

is the same as

5.0V - 0.65V ÷ 220Ω = 19.7mA
therefore Ib = 19.5mA

Base voltage is defined as the voltage on the base of the transistor. In the case of the common emitter configuration, it would be approximately 0.7V. The 5.0V in your example is the driver voltage. Plus, one usually calculates the base resistor value based on the desired base current, not the other way around as you have done.
185  Using Arduino / General Electronics / Re: Transistor Gain/Saturation question on: February 14, 2013, 11:33:54 am
Quote
should be between 100 and 300
Saying "should be" is slightly the wrong connotation, more correctly it "will be". You
don't try to set hFE, rather you end up getting a value of hFE which is roughly related
to the ultimate Ic, but with great variability. [sound confusing, yet?].

You might try to sort this out be looking at the following thread, although there are
a lot of confusing answers there. Reply #26 pretty much straightens a lot of it out.
hFE is a characteristic of the transistor itself, and not really of the external cktry,
which wasn't quite made clear on a number of the posts. You want to choose the
external components to set Ib and Ic, and ratio of this should have relationship to
hFE as indicated in #26 [see how this plays, LOL].

http://arduino.cc/forum/index.php/topic,147852.15.html

Thank you so much for that link! Found some great stuff there and despite having 'learned' about BJTs in school this link ( http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt_theory.html ) I found in that thread cleared up things SO much better.

The logic inversion is simple. I have attached a simple schematic.

The LED dimmer is a little harder for the Arduino because, while there is A2D pins, there are no D2A pins.  Two ideas would be to use a dedicated D2A chip like the AD558.

Another simpler/cheaper option would be using a digital pot.  Look here http://arduino.cc/en/Tutorial/DigitalPotentiometer

Thank you too, I really appreciate the schematic.

If I've got this right.....

Hfe = Ic/Ib (ratio of collector and base current)
Base Voltage - 0.7V (or whatever the base Voltage drop [Vbe] is according to datasheet) ÷ base resistor = Ib
Collector voltage - Vce (collector emitter voltage drop) ÷ collector resistor = Ic

*crossing my fingers* smiley-roll-sweat

Nope, not quite.
Ib depends on voltage available from the driver, in this case the Arduino output, and the base resistor. You would usually calculate the base resistor value based on how much base current you want. Let's say you want an Ib of 1mA. If your Arduino output is 5V at that current then the voltage drop across the base resistor will be 5V - 0.7V (the BE drop) = 4.3V. Then you can calculate the base resistor value, Rb = 4.3V / 0.001A = 4K3 ohms. Use the standard resistor value nearest that value.

Calculating the correct Ic is left as an exercise. smiley
186  Using Arduino / General Electronics / Re: "accidentally" connected RS232 to TTL, how screwed am I? on: February 14, 2013, 10:59:16 am
i've connected direct output from USB-RS232 Adapter to this one here http://www.atmel.com/products/microcontrollers/arm/sam7x_xc.aspx

Now i've tried to abuse my uno as a level shifter, which i haven't thought of before, and still no signal..which probably means that the serial pins are dead :/

So, in the words of Tommy (Brad Pitt) from the movie Snatch, "Proper Screwed"
 smiley
187  Using Arduino / General Electronics / Re: daisy chaining 74HC595's on: February 13, 2013, 08:00:58 pm
or carpal tunnel syndrome smiley-cool

Yeah, that's a definite danger! smiley
188  Using Arduino / General Electronics / Re: daisy chaining 74HC595's - conflicting images, confused as heck! on: February 13, 2013, 07:51:49 pm
that's not very helpful.. i have 74HC595's from different sources.. i don't know who the actual manufactures are nor am i going to track them down... (like i don't have other things to do)... and honestly, i don't know your skill level, but i'm not going to take advice from someone new who has less posts than i do.. no punt intended.

Fortunately, 74HC devices are supposed to meet certain specifications no matter who the manufacturer is. Therefore, just about anyone's data sheet will do. That may not be true for other devices so if someone tells you to get a data sheet, then it's probably good advice. Nevertheless, it is your responsibility to either look up the data sheet or reveal the manufacturer's name when designing with a part or asking for help with that design... whether or not you're too busy doing other things.

You can take that advice or leave it as I probably don't have enough posts to be relevant. This is the Internet and anyone can have any qualifications they can type but if we're counting, I have been designing and building electronic circuits since I was ten (57 years), and have had a degree in electrical engineering (electronics not power) for 46 years. When I have 15,000 posts I will have skill as well I guess. smiley
189  Using Arduino / Audio / Re: electronic game sounds on: February 13, 2013, 03:30:14 pm
A lot of the old games used the MOS Technology 6581 SID (Sound Interface Device). You can rip one out of an old Commodore 64 or buy one on eBay.
190  Using Arduino / General Electronics / Re: Resistors, Voltage vs Current on: February 13, 2013, 11:29:41 am
I heard Ohm's Law was going to be repealed....
Ohm's Law isn't a suggestion, like speed limits.

And the suggestions are getting better and better, e.g. SH45.  smiley
191  Using Arduino / General Electronics / Re: supplier for flatted-shaft knob without pointer on: February 13, 2013, 10:39:44 am
You'll have better luck searching for 6.35mm shaft knobs. It's been my experience that they fit a flatted 6mm shaft just fine and the 1/4" (6.35mm) shaft is more of a standard, so far. Mouser has some with no markings, but they are expensive. I'd Google for 1/4" shaft knobs.
192  Using Arduino / General Electronics / Re: The ants come marching in (and release magic smoke) on: February 12, 2013, 11:29:33 am
I had a heck of a time with the fire ants getting into my stuff. They would crawl into the contactors for the AC compressors and jam in so tight the relay would quit working. You'd have to dig them out with a screwdriver. I solved that problem by getting hermetically sealed (= expensive) units. I had the same problem with some outdoor light switches.

I don't know what their fatal attraction with electricity is all about. smiley
193  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 05:43:51 pm
Quote
OP, you notice how analog design is really still an "art" [meaning 1/2 subjective and
based upon personal experience, :-)].

While there can be some "art" associated with design of analog circuits, we're talking primarily about a digital switch here and the design procedure is not that mysterious.
Unfortunately, although we're only building a digital switch, we're using an analog
component to do it, so we need to pay attention to the analog characteristics.
That's what all those resistors are for.


Well, my first job out of university was with Texas Instruments on their 7400 logic family team and I can assure you that all those ICs have transistors and resistors inside and the same design criteria that you use with discrete components apply. Technically speaking, a transistor in saturation is not an analog component.

I get your point, I just don't rely on magic when science will do the job.  smiley
194  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 05:32:46 pm
Yes, transistor a little silly for 1 LED, but the same design carries right over into 2-3-4 LEDs in series from 12V source.

Well, I'm not going to argue with you about your "Hail Mary" design approach.
195  Using Arduino / General Electronics / Re: A couple of questions on the 2N2222 transistor on: February 09, 2013, 05:27:15 pm
Quote
OP, you notice how analog design is really still an "art" [meaning 1/2 subjective and
based upon personal experience, :-)].

While there can be some "art" associated with design of analog circuits, we're talking primarily about a digital switch here and the design procedure is not that mysterious.
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