Hi folks
I've built a regulated 5V power supply, by means of simple 7805 circuitry, to power devices on a breadboard, for testing.
When I supply power to the LCD from the Arduino's 5V , all goes well and LCD does its job.
However when I supply it 5V from the regulated power supply it wont initialize or show anything else for that matter 
...
Sequence being devices (i.e. the LCD) powered 1st then the arduino.
Any pointers appreciated 
All grounds connected?
groundfungus:
All grounds connected?
Yes, LCD connected to ground 
When I say "doesn't initialize" I mean it just turns on the backlight when powered by external power source and shows alternate rows of block characters.
I thought that by turning on the arduino later would then properly intialize the display.
The only thing that I can think of is perhaps the rise time of the external supply is greater than 10mS so the internal reset is not working right. Hopefully one of the LCD experts will have better ideas. Which library are you using? I understand that some don't do the initialize properly.
Just for clarity this is the one I have
There's a jumper connecting those pins on the left (marked Vcc and Gnd)
I'm powering the LCD with 5V from the right Vcc, Gnd pins ... perhaps that's the problem?
Should I power it (from external power) using the pins on the left?
TIA
7805 : (TO-220 package right ?
With or without heatsink?
What's the output voltage ?
What is the input voltage ?
Do you have a DMM to measure the current ?
How hot is it getting (the 7805) ?
Can you squeze your thumb and forefinger on the 7805 and hold it for 60 seconds ?
It has internal temp sensing . If you don't provide enough heat sinking it goes into thermal shutdown.
What is the rated load current for you 4x20 display ?
Without a heatsink it is probably only good to 500mA. With a good heatsink, 1.5A.
YES. YOU SHOULD USE THE EXTERNAL POWER INPUT CONNECTOR (GRN) TO CONNECT YOU EXT PS.
raschemmel:
7805 : (TO-220 package right ?
With or without heatsink?
What's the output voltage ?
What is the input voltage ?
Do you have a DMM to measure the current ?
How hot is it getting (the 7805) ?
Can you squeze your thumb and forefinger on the 7805 and hold it for 60 seconds ?
It has internal temp sensing . If you don't provide enough heat sinking it goes into thermal shutdown.
What is the rated load current for you 4x20 display ?
Without a heatsink it is probably only good to 500mA. With a good heatsink, 1.5A.
YES. YOU SHOULD USE THE EXTERNAL POWER INPUT CONNECTOR (GRN) TO CONNECT YOU EXT PS.
Thank you raschemmel
Yes, it's a TO-220 package to which I've attached a good heatsink
Input voltage 9-15V from a wallwart (15V on the 12V switch position when I'm clumsy and forget to set it back to 9V 
-- hence the heatskink )
And the 7805 never gets hot
Yes I do have a DMM but haven't measured the current drawn by the LCD yet (expect it to be ~200mA ?) 
So I'll simply connect my ext. ps to the pins on the left... 
Again thank you very much for your input
Your welcome.
You still didn't tell us the output voltage.
raschemmel:
Your welcome.
You still didn't tell us the output voltage.
Oops sorry
5.05V
pdoriam:
There's a jumper connecting those pins on the left (marked Vcc and Gnd)
Why?
When you gave us the output voltage you DID mean WITH the display connected , right ?
Also, regarding the connector on the left, it is ENTIRELY possible that it is similar to the Arduino Vcc in
meaning it can accomodate an input voltage in the range of 9 to 12Vdc.
Without any documentation, the only thing you can do is remove the jumper (which is a tiny plastic case with
metal pins inside) the type used for configuration jumpers on computer boards and connect your 5V there.
If indeed that input is meant for 9 to 12V then the voltage on the regular Vcc & GRD on the right will drop
below 5V. IF that is the case then if your wallbrick has a 6V setting then connect it there and see if
the output voltge (on the regular power pins on the right) comes up to close to 5V. It doesn't make any
sense to have two 5V inputs. We know the one on the right is a 5V input so that means the one on the
left (with the jumper) is probably meant for more than 5V. It's too bad you don't have access to a variable
supply but you can make your own with AA batteries, starting with 4 of them to get 6V and adding one at
a time to get to 9V, monitoring the voltage on the other pins. IF on the other had BOTH are
inputs then your safest course is to use the LCM1602 ic to measure the voltage. Pin 2 is Vdd, pin -1 is Vss
(from a chinese datasheet). I found one datasheet that showed an "LCD driving voltage " on the datasheet
with a rating up to 12V (13 max) according to the datasheet. If so it will not
provide any power to the logic because it only supplies the LCD . The only thing I am fairly certain of
in all of the above is that the IC on the PCB is a PCF8574T for I2C BUS made by NXP (formerly PHILLIPS) , the inventor
of the I2C bus. Pin 14 is SCL and pin 15 is SDA. Pin 16 is +Vdd and Pin 8 is Vss (GND). If you do a continuity
check with an ohm meter you can verify that 14 & 15 go to your SCL & SDA on your interface connector.
Incidently, you are using 4.7k pullup resistors on A4 & A5 right ? So check the voltage on pins 16 & 8.
Opps. I forgot to delete that part about pin 1 and 2 being power and ground .
That was bogus. Disregard that stuff along with the part about the chinese data sheet.
The part about the PCF8574T (pins 16 & 8) you can take that to the bank. You get the datasheet online. (NXP).
and check it youself.
pdoriam:
There's a jumper connecting those pins on the left (marked Vcc and Gnd)
I'm powering the LCD with 5V from the right Vcc, Gnd pins ... perhaps that's the problem?
Should I power it (from external power) using the pins on the left?
The pins on the left with the green jumper are not Vcc and ground. These annotations refer to the unconnected four pin header underneath the jumper which is as you will note, an exact parallel of the connector on the right hand side.
The jumper itself is there to enable the backlight of the LCD which is also controlled by one of the pins of the PCF8574T. It is possible to damage the circuit by attempting to put 5V into one of the jumper pins. There is no need or reason to remove or otherwise interfere with the jumper.
We do need to clarify that if you connect an external power supply, you must have "GND" on the backpack module connected to "GND" on the Arduino as you did when you were powering it from the Arduino, as well as the "SDA" and "SCL" lines just as when you were powering it from the Arduino. If you are going to provide your external power to this module, you connect your extra 5V to the "VCC" on this module instead of the Arduino Vcc, and you connect the negative of your 5V to the "GND" of this module as well as the "GND" from the Arduino. It matters relatively little where you actually connect the "GND" terminals together as long as they are all connected together.
How does it looks from the front ?
Are the lines 1 and 3 filled with "bargraphs" after connecting to power ?
That is the only hardware feedback of the lcd that shows, that power and init was OK
As i see that you are using an i2c backpack - is the pinout well known ?
There's a jumper connecting those pins on the left (marked Vcc and Gnd)
The pins on the left with the green jumper are not Vcc and ground.
Well, that's confusing.
Most of these boards have no additional connection on the left, therefor confusing.
And as like most items from asia: no manual, no description, no pinout...only the power and i2c connection (and sometime the adressing "jumpers") are printed on the pcd - but whats with the most urgent part, the data pins ?
If they connect them like standard (GND,Vcc,GND,Rs,EN, DB0-DB7) then no threads would appear with problems with those boards, but as there are at least 6 different boards that wont work with the same settings as the other five.... 
Point taken. Well I guess there's no debate about what it means if there is continuity (with an ohmeter) from
the pin labeled Vcc on the LEFT to pin-16 of the PCF8574T. It means that my supposition about that connector
being an optional input for greater than 5V was incorrect. As a matter of fact, you CAN see the trace going
from pin-16 toward the connector with the jumper on it but it is partially obscured. If you take an ohmeter
an check from the power pins of the PCF8574T to both connectors you might find out how the circuit could
be working with Vcc & GND Jumpered together. That's a bit of a mystery. I have to agree with you, putting
more that 5V is probably not a good thing, but putting 6V as I suggested on pin 16 is still 1V BELOW the
maximum rated input voltage (7) of the chip. If Vcc of that connector goes to pin16 of the PCF8574T then
it is clearly not an option to apply more than 5V there. I would want to know if Vcc(1) and Vcc(2) are connected.
While you're at it, it wouldn't hurt to know if Gnd(1) and Grd(2) are connected. Heck, why stop there, why
not ohm out the whole board. How long could that take ? It' only 16 pins. You can never have too much
information.
PCF8574.pdf (132 KB)
I'm so glad I didn't test connecting power to the left pins yesterday! 
Was dead tired and called it a day
I really appreciate everyone's input for it cleared out that confusion.
Special thanks to Paul__B! That was some timely advice!
The whole point on powering the LCD from an external power supply is that I have some other things to power besides the LCD itself. (like DS1307, shif registers for hardware keyboard (tact btns), temp probes, etc). Didn't want to stress arduino's regulator.
Again thank you very much everyone!
My apologies. Paul was right. I shouldn't have told you to connect power there. There's probably some components
that run on 3.3V .
The whole point on powering the LCD from an external power supply is that I have some other things to power besides the LCD itself. (like DS1307, shif registers for hardware keyboard (tact btns), temp probes, etc). Didn't want to stress arduino's regulator.
The display part of the LCD module requires only a few milliamps, it is the backlight that consumes most of the current. You might want to consider not using the backlight if that is possible.
Some surgery on the connection between pin 16 of the display and the I2C board would enable you to power the just the backlight from your extra supply.
Did you put the recommended capacitors on the input and output of the 7805?
from reply #3:
The only thing that I can think of is perhaps the rise time of the external supply is greater than 10mS so the internal reset is not working right. Hopefully one of the LCD experts will have better ideas. Which library are you using? I understand that some don't do the initialize properly.
These are valid points. You didn't answer the question.*
Don
EDIT: "You didn't answer the question."* <--- This might not have been clear. I meant that the original poster did not tell us which library he was using.
