I'm trying to do a project for a friend using a wiring diagram online. My problem is that the backlight for my LCD display is not turning on. Does anyone know which pin specifically on the LCD controls the backlight?
You have not specified enough information. There are many different types of LCDs, text LCDs, graphic LCDs, and then there are different interfaces, manufacturers, and chipsets.
How the backlight is controlled varies depending on which LCD you have.
--- bill
I'm using an Arduino Uno Revision 3 with a 4x20 text lcd made by RioRand. The Amazon listing doesn't specify what chipset it has.
But if you have a suggestion for a different display that will work better, let me know since I just killed that display.
scottriehm:
But if you have a suggestion for a different display that will work better, let me know since I just killed that display.
You need to be far more organised than that! 
What makes you think you "killed" the display?
If you want information, you have to provide the necessary information. Why should we need to guess which Amazon listing you mean? 
scottriehm:
... I just killed that display.
That certainly is possible, if you were guessing how to wire up the device.
If you are trying to re-create a project you found online and it had a wiring diagram, it should have shown what type of LCD device was needed and should have shown how to wire it up.
You still have not provided enough information to get meaningful advice.
--- bill
After borrowing a display from someone on campus, I took my time to completely tear the circuit apart and rebuild it. My problem with the backlight was entirely me not putting one of the jumper wires in the right spot on the breadboard.
Paul__B:
What makes you think you "killed" the display?If you want information, you have to provide the necessary information. Why should we need to guess which Amazon listing you mean?
My apologies, I was completely unorganized yesterday and for some reason, I was not able to log into my Amazon account on my computer. The link will be down below. I know I killed the display because when I was removing the header pin solder to resolder them (it was my first ever attempt at soldering so it wasn't as good as I liked for this project) and I popped off the pad for the 5 volt pin and when put it back on my breadboard it would not respond in any way.
So you have a hd44780 LCD.
If you look at the left side of page you linked to, you can see the pinout information for the LCD module.
In the second picture down, it indicates where pin 1 is.
In the third picture down, you can see what each pin on the module is used for.
The backlight is powered by pins 15 and 16.
It is possible that the first LCD module you were using can be repaired by just soldering an additional thin wire that runs from the header pin where the PCB pad is broken to the PCB trace that ran to that pad.
If you want an easier solution for using that type of LCD, I'd recommend using an i2c backpack.
They are only about $1 USD from places like Ebay.
You solder it to the LCD then you only have 4 wires that connect to the Arduino/ 2 for power and 2 pins for I2C.
Couple that with the hd44780 library and you get a plug and play device and can even control the backlight from the sketch.
--- bill
So I ordered a new display (same display) and it boots up fine but does not write to the display. The circuit works with minor character problems with one of the displays from the Arduino starter pack (inconsistent and I'm also getting connection failures so I think it may have something to do with API connection issues). I tried downloading and adding the hd44780 library (1.0.1 from the link below) and it's still not working. Any suggestions?
I would recommend installing the hd44780 library from the IDE using the library manager and not from a zip file.
It is simpler as the IDE will go out and grab the library an install it for you.
It ensures that it is installed in the proper location and that it can be easily updated using the library manager.
See the installation instructions on the hd44780 github page: GitHub - duinoWitchery/hd44780: Extensible hd44780 LCD library
Not sure what you mean by "API connection issues".
Misbehaving LCD devices is typically due to either bad/poor/incorrect wiring, or library misconfiguration.
What specific LCD device are you using? does it have i2c backpack or is it just the hd44780 lcd module with 16 pins?
--- bill
If you look on that Amazon listing and go to reviews... there's a full listing of the PIN# and function: THIS IS FROM THE POSTING:
15 and 16 are the backlight pins. Do not power through Arduino - it needs too much power. You can control it with a digitally controlled switch like a transistor.
Pin # Function
1 VSS (Gnd)
2 VDD (+5V)
3 Contrast Adjustment - Connect the center tap of a 10k pot connected between Gnd and 5V
4 RS Register Select Input
5 R/W Read/Write Signal, normally at Gnd
6 E Enable
7 DB0
8 DB1
9 DB2
10 DB3
11 DB4
12 DB5
13 DB6
14 DB7
15 LED (+) (+5V through a current limiting resistor - I used 220ohm)
16 LED (-) Gnd
wolframore:
3 Contrast Adjustment - Connect the center tap of a 10k pot connected between Gnd and 5V
Actually, wrong!
Do not connect the potentiometer to 5 V, and a 1k or 2k will make contrast setting easier. Please remember this when posting advice.
If the display has a "101" or 100 Ohm resistor as R8, you can control the backlight directly with an Arduino pin.
ok 
I guess the manufacturer is wrong too. Which is entirely possible.
My 1602 needs 160mA for backlight. You want to run that through a pin? The controls for it is in the library. Paul are you thinking of LCD? Having said that the 220 ohms is prob too dim and so is 100 ohm. I’ll have to see what I have on mine.
wolframore:
I guess the manufacturer is wrong too. Which is entirely possible.
That is indeed the case. As I explained to you in a reply to your PM, this is simply a blunder made by someone way back in the day and mindlessly copied by others simply because it seemed to work and they did not actually think about the reasons for it. That in itself is a fascinating insight into human enterprise in general, it is by no means the first time in electronics or engineering; it happens all the time. Don't even mention computer engineering in this regard! 
wolframore:
My 1602 needs 160mA for backlight. You want to run that through a pin? The controls for it is in the library. Paul are you thinking of LCD? Having said that the 220 ohms is prob too dim and so is 100 ohm. I’ll have to see what I have on mine.
That is an unusual backlight. Most of the common "1602" modules as sold on eBay have a single white LED rated at 20 mA and a "101" or 100 Ohm resistor as R8 limiting it to about 25 mA.
This is the dfrobot one: 1602 Datasheet
This one is rated 130mA most range 120-160
I assure you it is quite common as the backlight on most of these are an array. Not one led off to a side or top/bottom.
I have boxes of them with 5 or 6 LEDs you can see from an angle.
So I want to clear the air on the contrast setting. Paul is correct. I’ve checked the Datasheet and the contrast trim pot does not need a 5v connection. There is already power going through a series of resistors. So the pot just needs to be grounded. Also 10k is high. 2.5k is about right giving you finer tuning and more useful range. A permanent resistor could be used but contrast can be affected by various factors like voltage and temperature and a trim pot may be useful.
Check your Datasheet on the backlight. Most new ones have a number of leds which make them nice and bright but use a lot of power.
We are hoping OLED gets bigger and cheaper. They don’t need backlight and offer a nice high contrast image using waaaay less power
wolframore:
So I want to clear the air on the contrast setting. Paul is correct. I’ve checked the Datasheet and the contrast trim pot does not need a 5v connection.
Ah, I have a "convert"!
wolframore:
We are hoping OLED gets bigger and cheaper. They don’t need backlight and offer a nice high contrast image using waaaay less power
We sure are.
wolframore:
This is the dfrobot one: 1602 DatasheetThis one is rated 130mA most range 120-160
I assure you it is quite common as the backlight on most of these are an array. Not one led off to a side or top/bottom.
I have boxes of them with 5 or 6 LEDs you can see from an angle.
Have you measured the current?
Or just quoting from the datasheet.
You wrote in #10:
15 LED (+) (+5V through a current limiting resistor - I used 220ohm)
16 LED (-) Gnd
According to the datasheet the backlight LED typical Vf is 4.2V
Your 220R would limit the current to 3.6mA which is much less than 130mA
Some modules mount a 0R on the module pcb. This lets you use PWM or external resistor.
Most modules mount a 100R on the module pcb. This gives a typical 20mA.
Just to put things in perspective (for normal Ebay 16x2)
Typical controller current = 1.2mA
Typical 10k potentiometer = 0.5mA
Typical Backlight = 20mA
Yes, a 1k variable resistor instead of 10k pot would reduce typical module current from 1.7mA to 1.2mA
But the significant current is down to the Backlight.
Personally, I find the Yellow reflective displays easier to read than Blue transmissive.
They do not need a backlight (except at night)
David.
Ok I should have used quote brackets for what I copied and pasted out of that amazon listing.
Now you’ve made me curious. I plan on checking the current on the backlight... I have been using the datasheets to calculate current. I can’t imagine every single Datasheet being wrong. So yeah I’ve seen it over 5 times in different sources and they all say 120-180mA. Paul says his only has one LED. the ones I have more than one most likely in parallel. Ok maybe the issue is the word “typical”
The yellow ones are easy to read but they’re also slightly more finicky to adjust.
I just saw a commercial DCC controller that had a 2” OLED it’s really beautiful and blows the LCD’s away.
David I just checked the current on mine one. It lights up dimly with as low as 10 mA which would be fine for dark situation. It gets a little better with 60 mA. I have mine set to 100mA on a project and you can see it in bright sunshine.
If you tilt the display you can clearly see 8 separate LEDs for backlighting.
I have a sainsmart lcd but this one has a built in resistor so all you need is 5v at it draws 25mA. It is a single led from the side.
Another Chinese one part# MDLS16166D has only one LED and something limiting the current to 35 mA. Looks great.
Seems like there are a lot of variety and the designs have changed. Looks like you can’t trust the datasheets.
I have some more but it does seem like there are a lot of variety with the Chinese LCD designs.