How long... LED on time without resistors?

[mackrackit]

Read this, it gets interesting...

http://www.picbasic.co.uk/forum/show...t=11575&page=1

[rsocor01]

Read this, it gets interesting...

http://www.picbasic.co.uk/forum/show...t=11575&page=1

Hmmmm, there was a heated debate in there . However, like DT mentioned in that thread, you can drive a regular LED (using a 3V supply) from a PIC without any problems, but if you connect that same LED directly to the power rail it will burn and it can burn your fingers (I say this from experience ).

The reason it works on the PIC is the internal resistance of the pin in the PIC. For example, the following graph is taken from the 16F84A datasheet from the AC/DC Characteristic Graphs section.



By calculating the slope in the dashed line it can be determined that the internal resistance for VOH = 3.0V is equal to ROH = 130 Ohms. Now, in the case the output is low, VOL = 0 then the VOL-IOL graph is



Again, by calculating the slope of the curve it can be determined that the internal resistance ROL = 36 Ohms.

So, in the case that you are using a regulator of 3.0V you will have an equivalent PIC resistance of 130 Ohms; therefore, there is no need for an external resistor.

More detailed information about port characteristics can be found in the next book in chapter 3.

http://www.amazon.com/Designing-Embe...0073607&sr=1-1

[Heckler]

Thanks to everyone who replied...

The project is a very NON-Critical Boy Scout electronics merit badge project. I know I need to teach proper LED current limiting to the Scouts (and I will), as well as OHM's law, capacitors, inductors, transistors, resistors, etc. (oh yea, and Software concepts)

In this case due to time limitations and dollar limits on project costs... I am trying to keep the project as simple as possible. They need to be exposed to soldering and basic electronics. I want to "wet" their appetite for programmable microcontrollers. The kit will be an 8x8 LED matrix with a PIC 2-AA batteries and that's it. It will end up being a neckerchief slide that scrolls a scouting message. I may include two pushbuttons to include a simple game.

Similar to these excellent projects...

http://www.instructables.com/id/Mini...-with-built-i/

http://tinkerlog.com/howto/64pixels/

[ardhuru]

Along with the other parameters you listed that would matter, its also important *how much time the LEDs stay off* between their 1.5 ms on state; the time they get to catch their breath, so to say. As long as this duty cycle is small enough the design should work fine even at 5 volts. I say this on the basis of a 4 digit 7 segment display I have integrated in a product; no resistors, running 22X7 the past 8 odd years, and no failures...yet.

Regards,

Anand

[ardhuru]

running 22X7 the past 8 odd years, and no failures...yet.

Sorry... "24X7".

Anand