8 replies to this topic

[jimox]

Ok, this one has me baffled. I created an LCD backpack using the wiring from http://geekswithblog...id_crystal.aspx (actually the upside down wiring) and everything is working with my project when using usb to power it. However when I unplug the usb and switch to wall power the LCD becomes significantly brighter and the characters do not show up correctly. The rest of my application works fine (it is network connected so I can check the sensor remotely). The power to the backpack is coming from the 5v on my plus. Anyone have an explanation for this one?

[Chris Walker]

Hi jimox, The USB power on your computer is "5V", but it could be anywhere from say 4.5V-5.5V. When you use the DC power brick, the voltage is downconverted to almost exactly 5V. It's quite possible that the display is not really rated for 5V--and is depending on "not quite 5V" power from your PC. Do you have a multimeter? Can you test the voltage on the 5V pin when plugged into your computer...and then when plugged into the DC power brick? Also, can you measure the voltage on VIN when the power brick is plugged in (to verify that it's a reasonably steady and accurate voltage)? Chris

[Nevyn]

However when I unplug the usb and switch to wall power the LCD becomes significantly brighter and the characters do not show up correctly.

Probably a silly question but have you tried adjusting the pot as this should control the contrast.

Regards,
Mark

[jimox]

Nevyn, Thanks but yea that was the first thing I checked. Chris, Here is what I found. When plugged into USB the 5v pin is putting out about 4.08V but when plugged into wall power I am getting about 5.08V. As for the VIN when plugged into wall it is a fairly steady 9.18V - 9.20V (it's a 9V power brick). Assuming you are right and the wall power is too much for the LCD what do I need to do to drop that voltage down closer to the 4.08? Thanks for the help, --Jim

[Stefan]

jimox, What kind of potmeter (which range) did you use? Can you try add an additional 10M resistor in serie with it, and adjust it a bit again?

[Chris Walker]

jimox--it sounds like your USB port is putting out very low power! 4V, wow. That's technically low enough that the 3.3V MCU is on "the edge" after a ~1V voltage drop. You can use a resistor "voltage divider" or a power regulator to cut down your power usage. But assume the LCD is designed for 5V--perhaps there's another setting on it? Chris

[jimox]

Stefan, I am using a 10k pot on the contrast. However I am not convinced that it is a problem with contrast. Chris, So I was using a powered usb hub and that seems to account for the low power. I moved the cable to a plug right off the motherboard and now it is outputting 4.74V from USB and everything seems to still work with USB on that plug. I added a 220ohm resistor to the backlight to limit the current and it did bring down the backlight a bit but I still have the issues on wall power. I checked the datasheet on my LCD http://www.adafruit....TC1602A-01T.pdf and it looks like the 5.08V coming from the wall should not be a problem. I also tried another LCD that I have and put it on the backpack and I am getting the same issues from wall power but it works fine on USB. To describe the issue a little more in depth when it tries to write something to the LCD I get a bunch of '?' characters being written quickly across both lines of the screen with what appears to be a blinking cursor. This lasts for a second or two then it stops writing and sits with that on the screen. It appears to me to actually somehow be impacting the logic. Do you still think that I should cut down the power to the backpack? Thanks, --Jim

[Mark H]

It sounds like you could have a dodgey power pack, or you're overloading the regulator on the netduino. Is the voltage regulator getting really hot? The 9v in with your current draw may be putting it into thermal shutdown. What happens if you supply 5v to the 5v pin on the netduino from an external source (such as a lab supply)?

[kellermd]

I'm brand new to NetDuino, and I have a similar issue with an LCD used with batteries vs USB. I use the 10K potentiometer and adjust it with USB power to see the display correctly. The batteries are plugged into the barrel power connector, and are showing about 10 volts. The display shows a little life, but no characters show and I see a little flicker. These are alkaline batteries, and I have read that alkaline cannot deliver the power that NiMH does. I'm charging a set of NiMH now to test out my theory about the power. It's interesting property that alkaline is rated for a nominal 1.5V each, but can drop voltage quickly down to 1.2V, whereas NiMH will maintain the voltage for a longer time - again, from what I've read, not done the actual measurements myself.