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I'm a programmer and I recently purchased an Arduino Uno and I've very scared to short circuit it. I've been playing with http://www.falstad.com/circuit/ and have been able to create a few LED sketches and a piezo buzzer circuit.

So far, I've stuck everything behind a 470Ohm resistor. I have two problems.

  • First, how far can I reduce my resistor but maintain board integrity? Bonus, what should I look for in Falstad's circuit app? Possibly current over a certain threshold?
  • Second, what are some good ways to identify scavenged supplies so I can build safer circuits? Such as unmarked piezo buzzers, LEDs, and motors.

I've been doing research these past two days and discovered resistance lowers current not voltage. In order to lower voltage I can use a voltage divider

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  • \$\begingroup\$ Reminds me of my 200 in 1 project kit when I was a kid. As the AA batteries slowly discharged I found I needed less and less resistance. Changed the batteries one day and pop! \$\endgroup\$ – geometrikal Jun 18 '13 at 9:21
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  • First, how far can I reduce my resistor but maintain board integrity? Bonus, what should I look for in Falstad's circuit app? Possibly current over a certain threshold?

You need to check the maximum (source, sink) current ratings for the pin in question and divide that into the voltage you will be using. That will give you the lowest possible fully safe resistance.

  • Second, what are some good ways to identify scavenged supplies so I can build safer circuits? Such as unmarked piezo buzzers, LEDs, and motors.

Study them. Find datasheets, catalogs, brochures... anything you can get your hands on. And don't be afraid to plug package markings into a search engine if you can find any.

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  • \$\begingroup\$ Thanks! I've looked up most (all) of the markings and gotten datasheets and such. In particular, I have several devices without markings. Resistors are moot, LEDs aren't so bad with my voltmeter, but I have a few motors and other devices with no markings that I can't figure out. Edit: Clarification \$\endgroup\$ – Nathan Goings Jun 15 '13 at 7:05
  • \$\begingroup\$ Find documentation for similar motors. That will give you a starting point as to what to expect. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 15 '13 at 7:06
  • \$\begingroup\$ I accepted this as the answer because "and divide that into the voltage" was what I wasn't grasping. I'm nearly a total newbie at circuit design. \$\endgroup\$ – Nathan Goings Jun 18 '13 at 7:32
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If you are really paranoid about destroying the Arduino, check out the ruggedized version here, which has current limiting and various protections:

http://ruggedcircuits.com/index.html

However, from a learning standpoint, sometimes it's good to not rely too much on protections built into the circuit; a circuit might only work because of the protection and you might be in for a nasty surprise when you move onto a 'normal' arduino.

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    \$\begingroup\$ Now that should be called a shield. \$\endgroup\$ – Kaz Jun 15 '13 at 18:27
  • \$\begingroup\$ Do want. And some more text since my feelings are incapable of reaching the minimum required for comment entry. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 16 '13 at 4:50
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The Uno is pretty robust but you can still kill an output pin. LEDs are mellow lil' things, even a 1K series resistor will get a glow out of most (if you need to conserve power when running on batteries, for example). Most circuits I've seen use 220 or 330 ohms for LEDs.

edited for clarity

Don't try to run motors directly off an Uno pin. 40mA is the ABSOLUTE limit per pin for this device (200mA MAX for ALL pins), these are logic drivers not heavy lifters. General recommendations are to keep it 20-30mA per pin. Use a relay to drive higher power requirements - and, don't draw from the Uno's internal 5V regulator for those power hungry critters.

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    \$\begingroup\$ The specifications for the Uno says 40mA (max). However, several tutorials I've read advised against anything over 30-35mA. \$\endgroup\$ – Nathan Goings Jun 18 '13 at 7:29
  • \$\begingroup\$ -1: Where do you get that 20 mA limit, please? Evidently not from the relevant microcontroller datasheet or the Arduino documentation. \$\endgroup\$ – Anindo Ghosh Jun 18 '13 at 9:54
  • \$\begingroup\$ While the max rating is 40mA, the TOTAL pin draw is 200mA (see forum.arduino.cc/index.php/topic,16892.0.html ) which gives you 5 pins of use. Keeping it at 20mA will give you 10 pins to play with, and keep a safe margin - you really don't want to push its current draw limits. \$\endgroup\$ – Ron J. Jun 18 '13 at 13:47
  • \$\begingroup\$ @RonJ. does your answer contain a typo? (20mA when you mean 200mA?) You are correct though, the maximum individual pin current output is 40mA, and the total collective pin output is 200mA. \$\endgroup\$ – Nathan Goings Jun 18 '13 at 15:27
  • \$\begingroup\$ I edited for clarity - 40mA is the absolute max per pin but I would not recommend testing that limit; especially when using multiple pins on the Uno. \$\endgroup\$ – Ron J. Jun 19 '13 at 12:44

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