RE: Getting to Know Arduino : Part 2 : Hello, Again!

jc2048 — Sat, 18 Jun 2016 18:33:24 GMT

Good blog. Explaining how to connect a circuit is a nice step on from watching an LED on a board blink.

I hope it's all right if I comment on one or two things you wrote - don't think of it as criticism, think of it as me adding to and refining what you've done:-

"typically a 1Kohm resistor will do"

The photograph has a 220 ohm resistor.

Doesn't really matter. Either would do, they just result in different currents, but it might be a bit confusing for a beginner who isn't very sure about reading resistor colour codes.

"...the forward current (or just current) in mA (milliamperes)..."

If using Ohm's Law, calculate with the current in amps, not milliamps. So if you want 10mA, use 0.01A in the equation.

For example, if the forward voltage of your LED were 2V, then for 10mA the resistor would be

R = V / I

R = (5V - 2V) / 0.01 = 300 ohms.

The voltage in the equation is 3V, and not 2V, because it's the voltage across the resistor we are interested in, not the voltage across the LED (and the voltage across the resistor is what's left after the LED has had 2V from the 5V that's on offer).

"It has been known that cheap LEDs you can acquire have been manufactured incorrectly and that the casing on the LED has been rotated. So sometimes it is a lucky dip as to whether or not it will turn into a FED depending on the build quality."

That's a bit unfair to the manufacturers. There never was a correct way. In the early days you used to see both; it's just that, later on, manufacturers started choosing the one that got a lead in the market and it's ended up with mostly one orientation with just an odd few the other way.

If you have the resistor in place, it's unlikely you'll damage the LED by putting it in the wrong way round. If the LED has an anti-static protection diode, the resistor will limit the current enough not to blow it up [don't run it for too long like that, though], and if there isn't a protection diode, the LED will normally survive a reverse voltage of 5V [datasheets often used to give 5V as the minimum absolute max reverse breakdown voltage, but in practice it would be higher than that because they give themselves a safe margin]. Just remember not to connect it without the resistor.

"... and the source voltage (you can find out from the specifications on the Arduino, but I can easily tell you it is 5volts)..."

Look up the Atmel datasheet for the ATMega328 and refer to the graph on page 356 (Figure 30-71). There you'll see that with a supply voltage of 5V, the output pin will sit down to 4.45V if you sink 20mA from it (at 25C).