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Aug 28, 2013, 04:06 pm Last Edit: Aug 06, 2014, 02:42 pm by palavro Reason: 1


So - how do you know at what value to set the voltage divider if it is fixed?

The "contrast" control only makes the display visible over a quite small part of the range.  Whilst many modules sold use a multi-turn potentiometer, it is a "set and forget" function and you do not adjust it in normal use.  A simple miniature trimpot (as as on many other modules) is a bit touchy to set perfectly, but you only ever do it once.

If the "contrast" control is not in the correct range, you will either see nothing, or black blocks.

If the supply voltage is fluctuating as you suggest it is, it may be briefly passing a range where the LCD becomes visible and you can see whatever characters it may be trying to display, possibly random due to failed start-up.

Is that a genuine Iphone charger?  "Cheapie" chargers are known for errant regulation.  If it drops to 4.5V, who knows what it is doing on transients ...


Posting the wiring schematic you used will help.
If you see a solid row of black blocks on the 1st (and 3rd) line then contrast is probably fine. You can run Vo (contrast pin of the LCD) to GND for testing, this will give you maximum contrast and let you know if something is displayed or not.
A row of blocks indicates that the LCD isn't being initialized. Post your code and the wiring schematic, and we'll try to help you.


I have saved it for future reference.

It is not the usual sort of display module - certainly not the one I have in front of me; I have others at home which I will look at later.

I trust that J7 is not bridged on the module, which would connect the LEDs (apparently multiple) directly across the power rails and be a rather bad idea.

I for one am still not clear as to what is connected to what, from where you are getting your various power rails - are you using the 3.3V from the (genuine) FTDI board for the Pro Mini or what?  You suggest you were supplying it from the iPhone supply - presumably to the "raw" input.  Have you measured all the voltages, including "Vcc" on the Pro Mini?


Do you have a more standard, 5v arduino board to try this with?

Reading the datasheet of a HD44780 controller, an input signal is considered high when the voltage is over 0.7, so I don't understand why it doesn't work for you.
While the LED backlight requires 5v, the controller will be happy at 3.3v.
Try running everything off the 3.3v rail, only give 5V to the LED anode.

Recheck your wiring, make sure that everything makes proper contact.



You still have not illustrated the FTDI board connections, but I gather it is one with a 3.3V output and presume you do not connect its 3.3V supply connection - for programming - at the same time as the 5V supply.

To check the Pro Mini, just re-load the "blink" sketch and see that it works.  It is relatively unlikely you have damaged it.

And do measure each of the voltages - including the Vcc - and see whether they make sense.


With the LCD vcc at "0.2v" it'll never work. It should be used at 2.5v or more.


Aug 29, 2013, 09:50 am Last Edit: Aug 29, 2013, 09:52 am by TheCoolest Reason: 1
Glad I could help :) That happens sometimes, bad wiring problem can be hard to track down and fix.

I'm not sure I follow, though, how can VCC be 0.15? Are you talking about Vo (pin3) or VDD (pin2)?
0.15v on the contrast pin (Vo) makes sense to give a good result, 0.15v on VDD does not. When you say that it isn't working at 3.3v, what do you mean? The backlight or the actual display?


Both the display and the backlight are not showing anything at all when I power the Arduino through the FTDI with 3.3V due to low voltage.
The 3.3V from the FTDI drop to 2.3V. That's not enough to power the display.

Well, no-one said it would, did they?  If the display (LCD) is designed to work at 5V, it works at 5V.

The backlight - according to that spec sheet - draws 150 mA - I think you should measure it (separately, via its own connection) to see if it is appropriate - in fact, it would be a good idea to measure all the various currents in the system.

The Vcc on the Arduino is at 3.3V but its not connected.

But this is the terminal to which the FTDI connects.

And by the way, the backlight is the reason for the voltage drop from 5 to 4.6V.
This is not happening when the backlight is off.

Yeah, should measure the current - perhaps assess the voltage/ current regulation of the Apple power supply using some sturdy resistors (or a piece of electric jug element) if you can.


And by the way, the backlight is the reason for the voltage drop from 5 to 4.6V.
This is not happening when the backlight is off.

I don't see any reference to you using a current limiting resistor for your backlight.  The backlight should have a typical forward voltage of 4.2 volts when the current is 150 mA.  It is your responsibility to limit the current which you do not seem to be doing.  This is what is causing your power supply voltage to 'sag'.

I don't understand how you expect a 5 volt LCD module to operate properly at 3.3volts.  When operating with a typical 5 volt power supply the contrast voltage should typically be 4.5 volts.  This 4.5 volts is measured between VO and VDD which means it should be about 0.5 volts above GND.  We don't know what VO should be when VDD is 3.3 v but it very well could be below 0v (negative) with respect to GND.



Aug 29, 2013, 06:06 pm Last Edit: Aug 29, 2013, 06:08 pm by TheCoolest Reason: 1
@ floresta
Take a look at the HD47780 datasheet. It is specced to work at 2.5v or higher.
Regarding contrast, you have a point, but it has to be tested.
As far as I can understand from the datasheet of that module (which is actually a proper datasheet), is that there is a resistor built into the PCB, but J7 (I think it was) should be open. I think he closed it, so you are right, the backlight is probably chewing up more current than it should and causes the voltage to sag.

I suggested palavro to run the arduino and the LCD VDD from the 3.3v line of the TFDI chip while driving the LED backlight from the iphone charger, I don't think he tried to do that yet.


Aug 29, 2013, 08:16 pm Last Edit: Aug 29, 2013, 08:18 pm by TheCoolest Reason: 1
In my experience with this specific LCD: http://www.ebay.com/itm/400448319287
The backlight alone consumes about 33mA running off USB power, it has a 33ohm backlight resistor built in.
Simply run the backlight as a "separate" circuit, charger+ to the Anode and charger- to the cathode.


Take a look at the HD47780 datasheet. It is specced to work at 2.5v or higher.

That's the LCD controller chip.  We are talking about his LCD board which is a different story.  It's minimum operating voltage appears to be 4.7 volts.



I don't really need the LCD to work at 3.3V, it just would have been handy so I don't have to switch the power supply after programming.

Well, it clearly won't, but during development you can leave the Pro Mini connected to and powered by the FTDI only, and the LCD (both parts) powered by the 5V supply.  It would be nice to know the current details and the behaviour of the 5V supply.

The Ampermeter part of my Multimeter is broken so I can't measure the current at the moment.
I did not bridge J7, the resistance between the J7 leads is 12 ohms.

Checked that out.  Good.

Must be a newbie though - only one multimeter!  Cost about $5, need at least two (if only for simultaneous voltage & current readings) and clipleads.  Note: for currents in excess of a milliamp, the voltmeter should be connected on the component side of the ammeter as the ammeter has a significant voltage drop; for currents less than 1 mA, the reverse is likely to be the case.

I'll try powering the arduino from 3.3V FTDI and the display from 5V, will post the result in a bit.
Isn't that a short to frame though?

Not sure what you mean.  Don't make any sort of power connection between the Pro Mini and the display, but there must always be a common ground connection between all parts.

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