LCD Backlight Problem

[Belgian]

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

[adafruit_support_bill]

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.

[Belgian]

Thanks for your reply.

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

[adafruit_support_bill]

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?

[liudr]

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.

[Belgian]

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

[floresta]

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

[Belgian]

Thanks for the reply Don.

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

[floresta]

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

The second link is bad.

Don

[ChuckM]

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

[floresta]

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

[Belgian]

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.

[adafruit_support_bill]

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.

[Belgian]

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!!!

[bredhedden1]

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

breast actives