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June 04, 2009
by HarmVeenstra
HarmVeenstra's Avatar

The yellow and the green wire from the programming header go to pin 2 (Yellow) and pin 3 (Green ) that is OK and that's what it says on top of page 33 ( step 7b ). This is also as shown in the picture on that same page.

BUT in the diagram at the bottom of page 34 the colors are switched !!

June 04, 2009
by mrobbins
(NerdKits Staff)

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Hi there,

The diagram is correct as shown, but taking a look at it now, I see how it could be a bit confusing. Where it says "DB-9 pin 3", it refers to pin 3 out of the industry-standard 9-pin serial port connector. But you're right, that signal goes through the level shift and then goes to MCU pin 2. It's just a coincidence that these pins 2 and 3 happen to serve a similar (but switched) purpose to pins 2 and 3 of the microcontroller. Hope that helps!


June 13, 2009
by BobaMosfet
BobaMosfet's Avatar


I was looking at the back page of the NerdKit (for USB), because I want to turn on the backlight. Page 72 (Appendix C: The LCD Backlight). Your math is wrong in the current calculation, and I'm still not certain how you derived to use a 33 Ohm resistor?

I = E/R

I = (5.0 - 4.2) / 33 Ohms
I = 0.8 / 33
I = 0.0242424A  (24.2mA not 27.3mA)

Even if I use the Amperage you suggest as the result, I don't get 33 Ohms.

R = E/I

R = (5.0 - 4.2) / 27.3mA
R = 0.8 / 0.0273
R = 29.3042 Ohms (not 33 Ohms).

Which leaves me with several questions:

  1. Was 33 chosen simply because it is the closet match on the resistor scale to 29.3 Ohms?
  2. You gave us the maximum current as 180mA, but no minimum- how did you know that 27.3mA wasn't too low?

In order for me to get a 33 Ohm number, I have to have a backlight supply current of 5.0VDC with a minimum current of 150mA. Then I get 33.33 Ohms.

June 13, 2009
by hevans
(NerdKits Staff)

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Hi BobaMosfet,

Thanks for spotting that, you are right (5.0-4.2)/33 is 24.2 milliamps not 27.3 like we state in the guide. However those 3 milliamps don't make much a difference in terms of brightness.

When dealing with LEDs, there is not really a minimum current, it just gets less bright the smaller the current until at some low enough current you can't really see it go on anymore. It really is just a game of how much current you are willing to burn to have a nice bright LED backlight (as long as you don't exceed the max you will not damage the LED). For our calculations, we sort of picked somewhere around 30mA as a reasonable amount of current to burn (you do build up some intuition about that as you work more and more with electronics) and just tried the 33 ohm resistor to see if we liked the amount of brightness we got.


June 13, 2009
by BobaMosfet
BobaMosfet's Avatar


Thank you. I appreciate the quick and knowledgeable response. I went ahead and hooked it up and it works great. I also put a DPST switch in between pin 16 on the LCD and the 33 Ohm resistor so I could turn the backlight on & off. I left one side of the switch open. I couldn't connect it to anything else. Is this what you would have done?

An important observation: Since you guys chose the 30mA current range for the backlight, the 9V battery easily powers the whole thing AND the backlight. That may change once I get the USB Header and so forth installed.


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