Hey!
I bought a LCD backpack for my LCD display to get rid of some cables, but it just wont work.
This is how I mounted it on the display, and I wonder if anybody knows if this is wrong or right?
If you wonder why the pins looks messed up, its because I've been trying to unsold it - without succeeding...
Thanks in advance
// Andreas
Well your display has 18 pins so I guess it is one of those that has a multi-color backlight. There was a discussion about this topic not long ago, I'll see if I can find it. Meanwhile, in order to figure out why your setup is 'not working' we need more information.
Don
Well I found the thread (Arduino Forum) and I see that it was yours as well. Hopefully this thread will not get sidetracked like the last one did.
Were you ever able to disconnect the display from the backpack to see if it is still functioning?
Don
I just took a closer look at the schematics for your display and your backpack.
It looks like the display has three LEDs connected in a common cathode configuration with the cathode on pin 15 and the anodes on pins 16, 17, and 18.
The backpack however is designed to provide a switched positive voltage on JP2 pin 2 which I believe corresponds to pin 15 on your display. If my analysis is correct this would make the backlight circuitry on your backpack incompatible with your display.
Don
floresta:
I just took a closer look at the schematics for your display and your backpack.It looks like the display has three LEDs connected in a common cathode configuration with the cathode on pin 15 and the anodes on pins 16, 17, and 18.
The backpack however is designed to provide a switched positive voltage on JP2 pin 2 which I believe corresponds to pin 15 on your display. If my analysis is correct this would make the backlight circuitry on your backpack incompatible with your display.
Don
Hey!
Yes, I'm aware of that its not a RGB compatible backpack, but the thing is that the display wont work at all with the backpack - its completely blank.
I tried to unsold it earlier today, but its very hard to release it from the display, but I just tried connecting normal wires to the lower pins of the display -( the opposite side of where the backpack is fitted), and it works with no problems at all.
The weird thing is that its flowing current trough the pins of the backpack,even tough the display is blank, so I'm afraid that I've mounted the backpack upside down, which is the reason to why i posted the picture so you could see if I've fitted it wrong on the display, or if the backpack is just broken
... so I'm afraid that I've mounted the backpack upside down, which is the reason to why i posted the picture so you could see if I've fitted it wrong on the display ...
The backpack looks as though it is mounted correctly as far as it's orientation is concerned. I say this because the traces from the potentiometer seem to head up and to the right which is the correct location for it's connections to pins 1, 2, and 3 of the LCD module. Speaking of the potentiometer ... you did try to adjust it, didn't you? It's that small square device just to the left of the 'RX' connection.
The backlight connections at the other end are another story. It's not the fact that two of the backlight connections are open that makes the backpack incompatible with your display. The devices are incompatible as supplied because the two backlight pins that are connected are exactly backwards as I tried to point out in my previous post.
This backwards connection applies a reverse voltage to the backlight LED which is not conducive to it's long and happy life but it shouldn't keep the LCD from functioning.
Don
floresta:
... so I'm afraid that I've mounted the backpack upside down, which is the reason to why i posted the picture so you could see if I've fitted it wrong on the display ...
The backpack looks as though it is mounted correctly as far as it's orientation is concerned. I say this because the traces from the potentiometer seem to head up and to the right which is the correct location for it's connections to pins 1, 2, and 3 of the LCD module. Speaking of the potentiometer ... you did try to adjust it, didn't you? It's that small square device just to the left of the 'RX' connection.
The backlight connections at the other end are another story. It's not the fact that two of the backlight connections are open that makes the backpack incompatible with your display. The devices are incompatible as supplied because the two backlight pins that are connected are exactly backwards as I tried to point out in my previous post.
This backwards connection applies a reverse voltage to the backlight LED which is not conducive to it's long and happy life but it shouldn't keep the LCD from functioning.
Don
Hey!
I've done lots of troubleshooting now, and it seems like the problem lies in pin number 3 - the contrast pin that is.
The thing is that the voltage doesn't change on the contrast pin when I adjust the potentiometer, which makes it stay at about 5 v.
I connected a separate potentiometer on the other side and measured what voltage on the contrast made the display visible, which was about 1 V or less, while 5 v was completely blanked out.
I did also measure voltage on all other pins while adjusting the potentiometer, but nothing happen how much I adjusted, so I cant think of any other than that the potentiometer is simply broken.
What do you think?
When it comes to the incompatibility of the backpack, I see what you mean, but find it very strange since a staff from Sparkfun as well as the shop I bought it from refereed to this backpack for my display.
// Andreas
The thing is that the voltage doesn't change on the contrast pin when I adjust the potentiometer, which makes it stay at about 5 v.
Well you definitely have to get the contrast problem fixed before you can get the display to be usable. As you have discovered you need a low voltage, typically around 0.5 volts, on pin 3 to get a usable display. This is about the simplest problem you could have since it does not involve the microprocessor but you may not be able to fix it without removing the backpack from the display.
I connected a separate potentiometer on the other side and measured what voltage on the contrast made the display visible...
Other than being visible did the device appear to work? Did you get the splash screen?
I did also measure voltage on all other pins while adjusting the potentiometer, but nothing happen how much I adjusted...
Voltage measurements on any of data or control pins are meaningless and will not be affected by the potentiometer even if it is working.
... so I cant think of any other than that the potentiometer is simply broken.
It could also just have a bad connection to the PC board. It's time to use your voltmeter to check out the potentiometer. It looks like the lower left connection is GND, the lower right is +5V, and the top center is the wiper. Do you know what to do next or do you need some hints on how to proceed?
Don
Other than being visible did the device appear to work? Did you get the splash screen?
I cant get out anything from the display with the backpack since I cant connect a separate potentiometer while the contrast input on the display is reserved by the defective contrast pin from the backpack.
However, if I use normal wires on the other side of the display, with a separate potentiometer etc, I can type messages without problems.
Do you know what to do next or do you need some hints on how to proceed?
No, I'm not really sure what to do, I was thinking about removing the contrast pin from the backpack and replace it with a separate potentiometer instead, but other than that I don't really know, so a little hint would be great 
// Andreas
I cant get out anything from the display with the backpack since I cant connect a separate potentiometer while the contrast input on the display is reserved by the defective contrast pin from the backpack.
However, if I use normal wires on the other side of the display, with a separate potentiometer etc, I can type messages without problems.
It seems to me that even though you are using normal wires on the other side of the display the two sides are connected together by the traces on the pc board. I don't see any evidence of an eighteen pole double throw switch anywhere that would isolate the two sets of connections.
I was thinking about removing the contrast pin from the backpack and replace it with a separate potentiometer instead
You may wind up doing that, but lets check out the pot on the board first.
One possibility is that there is a problem at the GND end of the potentiometer. With the negative voltmeter lead on the PC board GND (pin 16 or pin 1) you should read +5 volts at the lower right connection on the potentiometer and 0 volts on the lower left connection no matter how you vary the setting. You should, however be able to read a voltage anywhere from 0 to +5 at the top center connection as you vary the setting.
If that is OK then check the voltage between GND and the backpack end of pin 3 of the header and then between GND and the the LCD end of pin 3 of the header.
Don
One possibility is that there is a problem at the GND end of the potentiometer. With the negative voltmeter lead on the PC board GND (pin 16 or pin 1) you should read +5 volts at the lower right connection on the potentiometer and 0 volts on the lower left connection no matter how you vary the setting. You should, however be able to read a voltage anywhere from 0 to +5 at the top center connection as you vary the setting.
Hey!
I did as you said and ended up with these results:
Lower right connection : +4,5 volts
Lower left connection : 0 volts
Top center connection : +4,5 volts no matter how much I rotate the potentiometer
I did also notice that the potentiometer is very loose - I can just touch at it to make it rotate 180 degrees.
I'm pretty sure that I needed a screwdriver to make it rotate previously..
// Andreas
I did also notice that the potentiometer is very loose - I can just touch at it to make it rotate 180 degrees.
I'm pretty sure that I needed a screwdriver to make it rotate previously..
I would say that there is a mechanical problem with your potentiometer. Most likely the metal part that you are turning is not actually engaging the actual wiper that is supposed to move within the potentiometer.
You can probably pry that metal part with the '+' shaped hole away from the rest of the potentiometer and then you should be able to see the wiper below. I think that the metal piece that you can see inside the '+' shaped hole is part of the wiper so try not to damage that when you do the surgery.
At this point you may be able to move the wiper and see if the voltage at pin 3 changes. If the voltage changes then you should be able to set the pot where you need it and perhaps use some hot glue to keep it from moving again.
Otherwise it's time to chip away at the rest of the potentiometer to get it completely off the board. I would attack it with diagonal cutters a bit at a time. At the bare minimum you have to disconnect at least two of the three connections to the PC board so that it's remains won't interfere electrically with the replacement.
Don
floresta:
I did also notice that the potentiometer is very loose - I can just touch at it to make it rotate 180 degrees.
I'm pretty sure that I needed a screwdriver to make it rotate previously..I would say that there is a mechanical problem with your potentiometer. Most likely the metal part that you are turning is not actually engaging the actual wiper that is supposed to move within the potentiometer.
You can probably pry that metal part with the '+' shaped hole away from the rest of the potentiometer and then you should be able to see the wiper below. I think that the metal piece that you can see inside the '+' shaped hole is part of the wiper so try not to damage that when you do the surgery.
At this point you may be able to move the wiper and see if the voltage at pin 3 changes. If the voltage changes then you should be able to set the pot where you need it and perhaps use some hot glue to keep it from moving again.
Otherwise it's time to chip away at the rest of the potentiometer to get it completely off the board. I would attack it with diagonal cutters a bit at a time. At the bare minimum you have to disconnect at least two of the three connections to the PC board so that it's remains won't interfere electrically with the replacement.
Don
The potentiometer cracked into two parts when I tried to ply the metal part, so I guess I don't have any other alternatives than to remove it?
That would be the next step.
Don
floresta:
That would be the next step.Don
Is it possible to replace it when I've removed it, or should I just remove the contrast pin and replace it with a cable from a separate potentiometer?
You should be able to tack a new one on where you removed the old one. You could dangle it from short leads if the footprint isn't exactly the same and you could connect those leads at the header itself instead of where the pot used to be.
You could also just try connecting pin 3 to GND as you may get an acceptable display with 0 Volts at pin 3.
You could also try fixed resistors as a voltage divider. Start with 1K between pins 1 and 3 and 10K between pins 2 and 3.
Don