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241  Using Arduino / General Electronics / Re: Power jack for arduino prototyping on: July 09, 2014, 06:30:21 pm
5V and -only- 5V connects to both Vcc and AVcc. Do NOT connect 12V to those pins. It should go to ground and the input of the 7805, seen here:

That page is flawed - there should be several 0.1uF bypass caps across both power rails, one right at AVcc, and on ARef.
242  Using Arduino / General Electronics / Re: T4010A1 ultrasound transducer polarity on: July 09, 2014, 04:39:05 pm
This should only matter if you are driving an array of them. In that case, simply wiring all up the same is sufficient.
243  Using Arduino / General Electronics / Re: Power Issue on: July 09, 2014, 04:08:27 pm
Why is a 7809 feeding the 5V input of the Nano?
244  Using Arduino / General Electronics / Re: Using a 4 digit 7 segment display on: July 09, 2014, 03:46:07 pm
Did you look at the Datasheet for it on that page?

You'll need a PNP transistor for each digit, and seven NPN transistors. Then you use something called  multiplexing.

And you could get away with just the four PNP transistors, and use the Arduino pins directly for the seven segments. That is because each cathode pin will be driving only one LED each, so <=20mA, but the common anode has to carry a maximum of 7x that, so possibly 140mA maximum. And an Arduino pins absolute max current in or out is 40mA, better to stay around 20mA max.

This is for a PIC, but it illustrates the idea.

Then it is just a matter of lighting up each digit in turn, while illuminating the proper LEDs. I would use Blink Without Delay to accomplish the timing, using the micros() timer rather than millis(). Do NOT use delay().

Hmm... someone has written a library for seven segment LED displays:

245  Using Arduino / Project Guidance / Re: The Bat Ultrasonic Location System on: July 09, 2014, 02:39:08 pm
That website talks about a system using ultrasonic transducers on the ceiling, not 433MHz for position location. The 433MHz is purely for a data link. It merely looks for the strongest signal and places you somewhere in the vicinity of that ceiling mounted transducer.

Time of travel in a short distance is not an easy thing to do.

Rather, send out an RF signal that is sweeping across a short frequency range, compare the reflection with the transmitted signal. The further the reflections have to travel, the greater difference in frequency between the local and reflected signal. It is also called a chirp.

Otherwise, you can compare the phase of the local and reflected waveform. Not something you can accomplish with the analogRead() capabilities of an Arduino.
246  Using Arduino / Motors, Mechanics, and Power / Re: Venturing into the inner workings of brushless motors on: July 09, 2014, 02:24:51 pm
Watching with great interest!
247  Using Arduino / Sensors / Re: RFID Box Locker/Unlocker on: July 09, 2014, 12:22:22 pm
Which RFID shield are you using now?
248  Using Arduino / Sensors / Re: RFID Box Locker/Unlocker on: July 09, 2014, 12:21:41 pm
Well... if you use a Mifare RC522 chip/board such as those I linked, it is the same thing. The shields are very often just breakout boards with convenient connections to an Arduino. A schematic should be available.

Now if you change from 13.56MHz RFID to some other frequency/protocol, then it will likely require some changes to your code. But shield or not, the chips are what matter.
249  Using Arduino / Sensors / Re: RFID Box Locker/Unlocker on: July 09, 2014, 09:58:34 am
Now i just need to find decent libraries for this board and learn how to program it... Apparently the documentation for this particular board is sub par.

I'm confused. Earlier you said:

.... the code i have written for my Mifare rc5222 shield?

I must have missed something.
250  Community / Bar Sport / Re: Oscilloscope recommendation. on: July 09, 2014, 12:07:29 am
I have to wonder if the same isn't true of Siglent. They named two of their scopes the 1102 and the 1052. The 1052 is 50MHz, the 1102 is 100MHz, and they appear identical physically.

And the Siglent 1052 is still only $270 USD.

However, reading the comments, the build quality may be poor. Since Dave Jones of EEVBlog has endorsed the Rigol 1052e 50MHz, I bought one and will be applying the firmware upgrade to an 1102e 100MHz.
251  Community / Bar Sport / Re: Oscilloscope recommendation. on: July 09, 2014, 12:01:58 am
Listen to them. And stay away from those toy tiny 'scopes that are the size of a small cell phone (or smaller). They are nothing but toys. 50MHz bandwidth is barely sufficient for a 16MHz Arduino.

The Rigol DS1052e is a 50MHz 'scope that is really the Rigol DS1102E 100MHz 'scope. A firmware update can change a 1052e into a 100MHz DS1102e. FYI.
252  Using Arduino / General Electronics / Re: PNP's are they Arduino friendly? on: July 08, 2014, 11:57:07 pm
Protecting the Arduino is moot, since this circuit as drawn in replies 12 and 13 will never turn off.

With 20V on the PNP emitter, you'd want something like a 22V zener and a 1k resistor in series between the Arduino output and the PNP base. Then a 1k resistor from the PNP base to 20V.

But that's a bad idea, too, because it is so fiddly. The Zener is going to be not quite biased on when the Arduino is at 5V. It just seems simpler to add an NPN transistor.
253  Using Arduino / Sensors / Re: RFID Box Locker/Unlocker on: July 08, 2014, 11:48:47 pm
My mistake! You didn't say, and I didn't ask and I should have. I assumed 125kHz RFID tags. Forget that link.

You did mean Mifare RC522, correct? Hard to beat $5.20 each. On Amazon with free shipping, or at Electrodragon (never ordered from them):

That card uses the NXP MFRC522 chip:

The chip alone is available from Digikey for over $6 each, not dropping appreciably until you get into the thousands. Cheaper just to source the board above.
254  Using Arduino / General Electronics / Re: PNP's are they Arduino friendly? on: July 08, 2014, 11:33:58 pm
K. Anyway, I hope some of this is helping.
255  Using Arduino / Sensors / Re: RFID Box Locker/Unlocker on: July 08, 2014, 05:10:07 pm
Sure, the EM-18 or the ID-12 module, but they aren't cheap. Or there is even a circuit for an RFID reader you can build yourself using a couple of sections of a quad LM324 Op Amp and a few more parts. You'll have to do some work building it and wind a coil, but should be pretty cheap. The LM358 is a dual Op Amp version of the LM324. Let me look that up, it is here on the site:

I would not be surprised if this weren't essentially the same as above but with the 2nd IC being an interface chip:

No, it can't read two at once. You'd have to have the tags separated by at least a short distance, and kind of sweep them past the reader. Otherwise, either the closest will only be read, or neither will be read as they interfere with each other. Passive tags don't have any anti-collision in them.
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