## LCD Backlight Problem

### LCD Backlight Problem

Hi,

First time question from a beginner.

I hooked up an LCD display to an Arduino UNO. I'm using a TMP36 to get the temperature and display it on the LCD. When I turn off the backlight, the temperature displayed is correct, when I turn the backlight on, the temperature jumps up a few degrees.

Has anyone else experienced this problem? Does anyone know how to solve it?

Thanks,

Belgian
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

Could be a number of things. How are you powering your project? Does the voltage on the Arduino 5v pin drop at all when you turn on the backlight? That would affect your analog reference and your temperature reading.

Posts: 15898
Joined: Sat Feb 07, 2009 9:11 am

### Re: LCD Backlight Problem

The Arduino Uno is power by 12V batteries. I'm getting 5.02V on the Arduino when the project is running. When I lit the LCD, the voltage is still 5.02V. Yet the temp goes up (from 75F to 84F).

The voltage on the TMP36 is 5.02V without the LCD lit. When lit, it drops to between 4.70 and 4.74V. That is my problem, and I don't know how to solve it.

Thanks,

Belgian
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

When lit, it drops to between 4.70 and 4.74V.

I agree, that is likely the problem. The question is how does that happen if your Arduino +5 regulated voltage stays constant?

Can you post a photo or schematic of how you have things connected?

Posts: 15898
Joined: Sat Feb 07, 2009 9:11 am

### Re: LCD Backlight Problem

Do you have a current-limiting resistor with your back light. You will need one. I assume your back light is drawing too much current.

liudr

Posts: 31
Joined: Sat Jan 15, 2011 10:08 pm
Location: USA

### Re: LCD Backlight Problem

how it's all connected
Thermometer 5v LCD.bb.png (58.21 KiB) Viewed 2473 times

back light connected
IMAG0477.jpg (899.96 KiB) Viewed 2473 times

back light NOT connected
IMAG0476.jpg (969.91 KiB) Viewed 2473 times

Thanks for the suggestion liudr. I don't have resistor for the back light. I tried with a 47 Ohm resistor, the temp went up by a little.

Attached are some pictures with "evidence" and how everything is connected - as requested by adafruit_support.

Thanks,

Belgian
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

The Arduino Uno is power by 12V batteries. I'm getting 5.02V on the Arduino when the project is running. When I lit the LCD, the voltage is still 5.02V. ...

The voltage on the TMP36 is 5.02V without the LCD lit. When lit, it drops to between 4.70 and 4.74V. That is my problem, and I don't know how to solve it.

The question is how does that happen if your Arduino +5 regulated voltage stays constant?

It happens because the wires and the connections between the wires and the breadboard are not ideal, they have resistance. If the backlight does not have a current limiting resistor, and if it should have one, then the excessive current is causing a voltage drop in the non ideal wiring between the Arduino and the breadboard.

Post a link to the LCD datasheet and perhaps we can tell if a resistor is required and what size it should be.

Don
floresta

Posts: 214
Joined: Thu Jul 31, 2008 9:27 am
Location: Western New York, USA

### Re: LCD Backlight Problem

I downloaded the datasheet a while ago and I don't remember from where. I can't attache PDFs here. So here is the part number I have: MDL-20464B
I found another copy of the PDF here: http://www.pdfsea.com/down/down.php?downid=12200&id=0

On DigiKey, I found this: http://media.digikey.com/pdf/Data%20She ... PDFs/MDL(S)-20464.pdf
It just doesn't have the "B" in the part number.

I got the part from an alarm keypad they sold at Halted (a local electronics spare parts store). It sold for about \$8, had the LCD, a keypad and some other electronics. So I thought it was a good deal. I have never tried one of the slightly more expensive ones you can buy here at adafruit or other places.

Thanks,

Bart
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

The first link does not point to a usable datasheet. It gives the physical dimensions and the pinout but not much more.

Don
floresta

Posts: 214
Joined: Thu Jul 31, 2008 9:27 am
Location: Western New York, USA

### Re: LCD Backlight Problem

I'm going to ask a silly question, what is your power supply?

I agree with everyone that if your V+ voltage drops from 5V to 4.xxV when you turn on the backlight then your power supply doesn't have the current capacity to drive your circuit. In your picture you just have USB connected. That won't be enough in this configuration.

--Chuck
ChuckM

Posts: 139
Joined: Thu Dec 24, 2009 2:31 am

### Re: LCD Backlight Problem

I agree with everyone that if your V+ voltage drops from 5V to 4.xxV

But he said: "I'm getting 5.02V on the Arduino when the project is running. When I lit the LCD, the voltage is still 5.02V."

Don
floresta

Posts: 214
Joined: Thu Jul 31, 2008 9:27 am
Location: Western New York, USA

### Re: LCD Backlight Problem

My power supply doesn't matter. I tried with USB or with a 12V battery. The results are the same.

I don't have a better datasheet. The two links above is all I found.

Maybe I'll buy an LCD from adafruit and see if I get the same results. The mystery remains.
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

In any case, Floresta is probably correct that the voltage drop across the power jumpers is due to the current draw of the LCD. You can isolate your TMP36 from that by disconnecting it from the power & ground bus on the breadboard and running separate power and ground jumpers direct from the Arduino.

Posts: 15898
Joined: Sat Feb 07, 2009 9:11 am

### Re: LCD Backlight Problem

adafruit_support wrote:In any case, Floresta is probably correct that the voltage drop across the power jumpers is due to the current draw of the LCD. You can isolate your TMP36 from that by disconnecting it from the power & ground bus on the breadboard and running separate power and ground jumpers direct from the Arduino.

That did the trick!!! Awesome. So simple, why didn't I think of that myself All I did was connect the ground from the TMP36 to it's own ground pin directly from the Arduino rather than from the bus on the breadboard.

Thank you so much!!!
Belgian

Posts: 6
Joined: Sun Sep 04, 2011 9:30 pm

### Re: LCD Backlight Problem

what was the trick dude, I didn't get you !

breast actives
bredhedden1

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Joined: Wed Jan 18, 2012 9:56 am