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13381  Forum 2005-2010 (read only) / Troubleshooting / Re: Voltage/Current output from Arduino on: October 03, 2009, 03:30:35 pm
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ok, so I dont have to use any resistors and connect a 20mA 5V LED strait between a pin and ground?

No to wiring a LED from ground to an output pin, without also a series resistor. Yes you need to compute and install a series resistor!

Not sure about your 5V LED?, a simple LED is not rated by voltage but by it's nominal forward current and it's nominal forward voltage drop.

A typical red led may have an internal forward voltage drop specification of 1.5vdc. So you take your source voltage (5vdc) minus the LED's voltage drop (1.5vdc) and get 3.5vdc. Now you use ohm's law to calculate the resistance needed to limit current to 20ma. So 3.5vdc divided by .020 amps is equal to 175 ohms for the required series connected resistor. Other color LEDs have different forward voltage drops and different recommended continous forward current ratings.

 The important concept to understand is that the Arduion digital output pin does not control the amount of current being supplied to the load circuit, it only has a recommended maxium safe current that should be drawn from the pin before possible damage to the output pin. The digital output pin cannot protect it's self without external current limiting components, in your case with a resistor wired is series with the LED.

That make any sense?

Lefty
13382  Forum 2005-2010 (read only) / Troubleshooting / Re: Voltage/Current output from Arduino on: October 03, 2009, 10:55:51 am
Use 5vdc as the source voltage of a digital output pin for calculating a current limiting resistor for use with a LED. The pin has a maximum current rating of around 40ma, but it's best not to run at maximum ratings, better to limit at 20ma which is fine for most standard LEDs.

 Beware that there is also a processor chip package current maximum limit of around 200ma of current consumption so don't plan on driving more then 10 output pins at 20ma each.

Lefty
13383  Forum 2005-2010 (read only) / Troubleshooting / Re: Tx pin only works with USB connected on: April 11, 2010, 04:23:59 pm
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Can anyone please tell me why that happens?

On the surface that sounds like a near impossible symptom. Arduino pin 1 (AVR chip pin 3) wires to both the FTDI and to the pin 1 connector, so if the signal is sending to the FTDI then it should also be avalible on the connector pin 1.

How are you determining that it is not sending when not plugged into USB? That is what physical device are you sending to? How is it wired up? do you have a ground connection between the Arduino and the device your trying to send serial data to?

You do understand that the USB connector on the Arduino board serves two purposes, one to power the board if no external power is avalible, and two, to establish a serial connection to a PC?
 If the USB port is unplugged then you can only send serial data to some external device interfaced to pins 0 and 1 on one of the Arduino connectors.
Lefty
13384  Forum 2005-2010 (read only) / Troubleshooting / Re: Arduino MEGA resets when connecting 5V to bboard on: April 11, 2010, 01:08:03 pm
Possible something on your breadboard is drawing too much current from the Arduino's +5vdc supply? There is a limit of say 400ma or so. How are you powering your mega board, from USB or an external power source through the external power connector?

Lefty

13385  Forum 2005-2010 (read only) / Troubleshooting / Re: Common GND noise on: April 11, 2010, 12:09:01 pm
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I chose LM386 audio power amp. with gain 200db, the signal voltage is larger but not I desired.

I'm guess you mean 20db gain, cause 200db gain is not going to be happening in this universe.  smiley-wink

Lefty
13386  Forum 2005-2010 (read only) / Troubleshooting / Re: Duli connection error on: April 10, 2010, 07:49:22 pm
If there anything other software in your computer that uses serial ports?

Lefty
13387  Forum 2005-2010 (read only) / Troubleshooting / Re: Interference issues on: April 09, 2010, 12:26:00 am
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dont know how having the LED's un-resisted can cause damage since the board supplies the same amount of power either way.

The board (output pin) doesn't alone determine the power to the load (a LED for example), the load resistance (along with the supplied voltage) determines the power consumed. Once an LED is forward biased on, it acts like a direct short with no current limit. It's basic ohms law learned in DC electronics fundamentals.

Lefty
13388  Forum 2005-2010 (read only) / Troubleshooting / Re: Battery problem on: April 05, 2010, 12:02:20 pm
The small rectangular 9 volt batteries have very wimpy current capacity, so that is most likely the problem. Make up a 9vdc pack using 6 AAs and you will probably find it works fine.

Lefty
13389  Forum 2005-2010 (read only) / Troubleshooting / Re: sudenly my comptr stoped detecting comport(arduino on: April 06, 2010, 01:53:23 pm
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You might want to check the Vin pin voltage just to make sure that you are getting a 5V there (in other word the regulator is still working properly).

Vin does not suppose to measure +5vdc. It is the voltage that is being plugged into the external power connector and should be around 7.5vdc or higher. It feeds the on-board +5vdc regulator's input. To measure for proper +5vdc on should measure the +5vdc pin, that should measure +5vdc no matter if the board is being powered from USB or external power jack  smiley-wink

Lefty
13390  Forum 2005-2010 (read only) / Troubleshooting / Re: 4-20mA output options on: April 06, 2010, 11:29:23 am
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That's a little pessimistic - given you filter the output with a smoothing capacitor you can then pass this through a resistor to turn it into a current.  However you will need to know the input voltage that the controller uses for its current-sensing input - I couldn't seem to find the full specs.

Have you ever worked with loop powered 4-20ma current loops? I ask because it is industrial standard interfacing method, but is more complex then may first seem, especially for simple interfacing with a Arduino. One requirement is to deal with the 4ma 'live zero' concept. This allows a current loop measurement system to differentiate from a 0% measurement or command value and a broken or open loop connection. So one can't use a PWM output of 0% duty cycle to represent a 0% command to the controller that needs to see 4ma for a valid 0% command. Again 4-20ma current loops are a little more complex then just filtering and driving via a fixed resistor.

Then there is the issue of which side of the current loop is providing the loop current, if it's the motor controller then the Arduino interface needs to simulate a passive dynamic adjustable resistance to the loop. If the controller is expecting the Arduino to power the loop then it needs to supply the loop compliance voltage source, typically a 24vdc potential.

All in all, using the 0-10vdc interface to the controller will be a much simpler interface.

Lefty

13391  Forum 2005-2010 (read only) / Troubleshooting / Re: 4-20mA output options on: April 04, 2010, 09:37:07 am
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Do you suppose that a digital pot might work? The only reason I'd go this way is to keep the number of power supplies to a minimum.

I haven't played with digital pots yet. My impression is that they probably won't work for your application. I think most are designed for lower voltage signals, like line level adudio which is just a AC voltage of less then 0-2vac.

Lefty



13392  Forum 2005-2010 (read only) / Troubleshooting / Re: 4-20mA output options on: April 03, 2010, 08:53:22 pm
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So, is 4-20mA straight from the analog pin kosher?

No. Even if you filter a Arduino PWM analog output you will have obtained a programmable analog output voltage, not a programmable constant current output. 4-20ma current outputs require external components. You are probably better off with the 0-10vdc option for your controller. That will still require external components like an op-amp to amplify the 0-5vdc of the filtered PWM output to a 0-10vdc analog output.

Lefty
13393  Forum 2005-2010 (read only) / Troubleshooting / Re: Upload trouble on home designed board on: March 22, 2010, 03:46:20 pm
The DTR signal which passes through the series cap to the reset pin cause a momentary low going pulse, enough to cause the AVR to reset.

Lefty

13394  Forum 2005-2010 (read only) / Troubleshooting / Re: Freeduino serial v2-unable to upload program on: March 20, 2010, 12:11:31 pm
Then you will have to select the board named "Arduino NG or older w/ ATmega 8" and your mega8 chips needs to run at 8mhz either with an external 8mhz crystal or resonator, or with it's fuses set to internal 8 mhz. Does you mega 8 chip have an Arduino bootloader already burned into it? If not you will have to deal with that first before you can upload sketches to it from the Arduino IDE.

Now that I think about it, doesn't the freeduino serial v2 board use a 16mhz crystal? If so I think you can only run mega168 or 328 AVR chips in that board and then select the proper board in the IDE that associates the proper AVR chip and mhz speed. I don't think the IDE has a board selection that will program a mega8 at 16mhz.

Looked up older NG with mega8 and it does run at 16mhz, so having a bootloader in your AVR mega8 is the only question to answer.

Lefty
13395  Forum 2005-2010 (read only) / Troubleshooting / Re: Freeduino serial v2-unable to upload program on: March 20, 2010, 11:56:40 am
Have you selected the proper board type and proper comm port in the Arduino IDE?

Lefty
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