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916  Using Arduino / Project Guidance / Re: Constant digitalWrite? on: December 02, 2012, 07:47:58 pm
In an embedded system, it is better to assume nothing, so I would vote for writing to the relay.

917  Using Arduino / General Electronics / Re: driving 11v 7 segment led displays on: December 02, 2012, 07:45:34 pm
Should I use a 7447 to drive transistors that will drive the leds?

Certainly one option: it keeps the code base small.
918  Using Arduino / Sensors / Re: BI-DIRECTIONAL PEOPLE SENSOR AND COUNTER USING ARDUINO BOARD on: December 02, 2012, 07:43:21 pm
The more I read it, the more I admire my non-blind guy idea.

919  Using Arduino / General Electronics / Re: Questions on making a minimal/custom Arduino circuit @ 3.3v/8Mhz? on: December 02, 2012, 07:42:15 pm
this can not (should not) be a chip by itself project... IMHO.. 

I mean I need it to be small.. but I want it to work and be somewhat stable!  lol

The chip-only implement will work and will be very stable.

920  Using Arduino / LEDs and Multiplexing / Re: Changing the digital ports voltage? on: December 02, 2012, 07:39:31 pm
is there a way to regulate the current that the digital ports on the Arduino Uno put out?

There is no one way to reglate the current  but multiple ways, based on your application.

1) For some applications, you can rely on the internal resistance of the mcu output pin: it is simple and can be effective for leds whose forward drop voltage is close to the supply voltage;
2) For small leds, you can use a resistor: simple but less efficient.
3) You can also use other ways, like a jfet, or a dedicated CCS driver, or a current mirror with multiple output legs.
4) There are also dedicated switching mode ccs drivers. they offer flexibility and a wide range of output current / voltage options.

Our led's can only hand up to 3.3 volts and I am unsure how much the ports put out.

You will find that your leds can take far more than 3.3v, especially those high power leds.
921  Using Arduino / Programming Questions / Re: For loops?? on: December 02, 2012, 07:33:18 pm
Fairly easy: send a set of patterns to the leds and that pattern determines how fast they blink.

For example, if '1' means on and '0' means off,

0b11 -> both leds are on
0b01 -> bit 0 is still on, but bit 1 is blinking
0b10 -> bit 0 is blinking and bit 1 is blinking
0b00 -> bit 1 is blinking and bit 0 isn't.

A sequence like that will blink one led at 1/2 of the speed of the other led.

You can expand this approach to cover multiple leds at multiple speed.
922  Using Arduino / General Electronics / Re: What voltage is given out on the digital out pins? on: December 02, 2012, 07:29:08 pm
but what voltage is the high?   

That voltage varies based on the load / power supply. Typically, it is close to 5v (the power supply), but with a heavier load, it goes down from there.

Could a digital out go to a led and then just to ground?

Yes, it can. It can also go from the 5v to the led and then to the mcu pin.

If it is small led,  you can put a resistor there to help define the current. For large leds,  you typically u se dedicated drivers.

Or does it need to go to the 5v pin?

That works as well.

923  Using Arduino / Project Guidance / Re: Larger thermostat project on: December 02, 2012, 02:56:32 pm
This sounds like a perfect project for 555: simpler and cheaper.

I would use a constant-on heater (light bulb in your case), and a 555-controller motor. The thermistor would control the 555's duty cycle so the colder the thermistor, the stronger the motor blows hot air into the housing / section.

This would be an energy-efficient / low noise solution that is cheap and simple to build.
924  Community / Bar Sport / Re: Where to get a Due or Raspberry Pi for Xmas gift on: December 02, 2012, 02:51:57 pm
it appears the Raspberry Pi population is approaching 1 million units,

Well, I tend to be skeptical of vendor claims.

But, to make an apple-to-apple comparison and given the Arduino model, you will have to compare Arduino-like devices vs. Pi after the same time since their introduction. I don't know how many Aruidno/Arduino-like devices have been sold but my sense is that its eco-system is far bigger than that of the Pi's.

But that comparison still misses the point: Roku2 vs. Pi.
925  Using Arduino / Programming Questions / Re: Multiple Random Relays on: December 02, 2012, 02:32:00 pm
I guess what I'm looking for is how to delay individual pins for random times without using the delay command and pausing everything.

Two ways:

1) use a timer to "tick": you keep track of the starting "tick" of a relay and test to see if the desired length of time has passed. The timer can be a hardware timer, or a loop counter if the loop's duration is constant; I prefer a hardware timer for consistency. This approach can track multiple delays with one timer.

2) use individual timer for each delay: you simply need to decrement the desired passage and when it reaches zero, you know that your time is up. This requires multiple timers for multiple delays.
926  Using Arduino / General Electronics / Re: Can 5V from USB burn LM7805 Voltage regulator? on: December 02, 2012, 12:28:48 pm
What did I do wrong?  Do I need put a diode between LM7805 and +5V line?

Typically a reverse-connected diode between 7805's input and output.
927  Using Arduino / Project Guidance / Re: Clock (Hours and Mins) Project on: December 02, 2012, 12:26:00 pm
So the Arduino would set the start time (by buttons) on the RTC

You can use a rtc but it is really not need for your application: the main crystal does an excellent job keeping time, if you code it so.

928  Using Arduino / Project Guidance / Re: Simple Voltage Divider Feedback on: December 02, 2012, 12:24:33 pm
Does anyone have any suggestions or tips?

You can feed the pwm output through a low-pass filter and then your divider to obtain an analog output.

BTW, unless you are very lucky, your implementation, as is, will likely be unstable. Google PID to see how it can be done.
929  Using Arduino / Project Guidance / Re: Can it be done... Arduino-based full Digital Auto Instrument Cluster on: December 02, 2012, 12:21:00 pm
Can it be done

Surely yes. But that's not the challenge.

The challenge for such kind of applications is not to make it work when it works; It is to design for failure: when it fails, it fails safe.

930  Using Arduino / General Electronics / Re: Constant current power supplies on: December 02, 2012, 09:08:56 am

That is one of those gated oscillator smps types: it turns itself on/off between 1.69v - 0.8v (on the feedback pin), making it suitable for low-power applications but largely not usable for a device needing a constant voltage.

The grandfather of those designs are the Linear LT1107.
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