Warning to users of some vendors LCD keypad shields

CarlosGz: I have been done the mod adding a Diode (IN5819) how you can see in the schematic (I have marked green the new diode in the graphic) . Now I have a new issue with the brightness wich have been decreased...

That is because the diode does not go in series with the pullup resistor to the base of the transistor but in series with the D10 to connection of the base of the transistor.

--- bill

bperrybap:

CarlosGz:
I have been done the mod adding a Diode (IN5819) how you can see in the schematic (I have marked green the new diode in the graphic) . Now I have a new issue with the brightness wich have been decreased…

That is because the diode does not go in series with the pullup resistor to the base of the
transistor but in series with the D10 to connection of the base of the transistor.

— bill

maybe now i have a bigger problem lol, so, in this case I have to connect the Anode to BT (D10) and the Catode to the the base of the transistor?

To be clear, something like this?

Almost, The diode is flipped/reversed. (I think you would have quickly figured it out) The diode is used to isolate D10 from the circuit when D10 is high. The pullup resistor is used to turn on the transistor. The backlight will be on if D10 is input or HIGH. Setting D10 to low turns off the backlight. The diode allows D10 to yank the pullup down to gnd to turn off the transistor when D10 is low.

It works but really isn't a good design, I wouldn't design a circuit like this.

I did it this way because I couldn't easily desolder the LCD on the shield I had to replace the transistor with an FET. (The transistor was underneath the soldered LCD module)

--- bill

bperrybap:
Almost, The diode is flipped/reversed. (I think you would have quickly figured it out)
The diode is used to isolate D10 from the circuit when D10 is high.
The pullup resistor is used to turn on the transistor.
The backlight will be on if D10 is input or HIGH.
Setting D10 to low turns off the backlight.
The diode allows D10 to yank the pullup down to gnd to turn off the transistor when D10 is low.

It works but really isn’t a good design, I wouldn’t design a circuit like this.

I did it this way because I couldn’t easily desolder the LCD on the shield I had to replace
the transistor with an FET. (The transistor was underneath the soldered LCD module)

— bill

Hey, thanks for your help, now I understand the question.
I finally won’t be able to fix my lcd because of the pcb design and the components placement, so I decided to buy a new one at http://www.electrofunltd.com/ and I have been testing the FMalpartida’s library with nice results…
By the way, I would like to reupload the schematic with the diode in a correct placement in case it can help someone :slight_smile:

Hello,

Not quite sure how to pick the most appropriate diode here for the D10 fix, could somebody take a look at the following

http://www.maplin.co.uk/search?criteria=diode&page=1

and advise which would be a good choice?

Many thanks,

Amadeus

To answer my own question, and to help other users, I found that the "1A Schottky Barrier Rectifiers" on that page, in particular a 1N5817 worked well. Trick was to cut the track after bugging the diode on.

Worked well until I got over-confident with the "brief" part below:

Briefly (and I mean just a brief touch) connect the wire from D10 to Vcc.

Cue smell of burning and a slighty dimmer backlight, no on/off control. I imagine the transistor went.

Oh well. You win some you lose some.......

SolarPanels: Thank you dweston and bperrybap

I just noticed that the product page says

"Do not use Pin 10 while this shield is connected"

http://www.robotshop.com/productinfo.aspx?pc=RB-Dfr-07&lang=en-US

I do not remember that statement when I ordered it

impressive if they have responded to this info so quickly

It looks like robotshop is not dfrobot. No warning here, and the schematic has not changed... http://www.dfrobot.com/wiki/index.php?title=Arduino_LCD_KeyPad_Shield_%28SKU:_DFR0009%29

Bummer...

[quote author=bwims link=topic=96747.msg912548#msg912548 date=134667261 It looks like robotshop is not dfrobot. No warning here, and the schematic has not changed... http://www.dfrobot.com/wiki/index.php?title=Arduino_LCD_KeyPad_Shield_%28SKU:_DFR0009%29

Bummer... [/quote] Yes, robotshop is a distributor for various parts manufactured by others (DFRobot in this case makes the lcd shield). I've used them in the past rather than ordering straight from DFRobot because Robotshop is USA based and shipping times are much less. As I mentioned upthread, evidently robotshop hasn't changed the warning, but when I ordered the lcd shield in May, it was the fixed version where button 10 does control the contrast.

IIRC, you can't use servos and the lcd shield together on UNOs because the servo library disables PWM on pins 9 and 10.

bwims: It looks like robotshop is not dfrobot. No warning here, and the schematic has not changed...

Bummer...

Yes, robotshop is a distributor for various parts manufactured by others (DFRobot in this case makes the lcd shield). I've used them in the past rather than ordering straight from DFRobot because Robotshop is USA based and shipping times are much less. As I mentioned upthread, evidently robotshop hasn't changed the warning, but when I ordered the lcd shield in May, it was the fixed version where digital pin 10 does control the contrast.

IIRC, you can't use servos and the lcd shield together on UNOs because the servo library disables PWM on pins 9 and 10.

FYI, I am working on my phi-3 shield design right now, a revision of my popular LCD keypad shield with lots of add-ons. It will have what these generic shields don’t. You may choose which pin drives the LCD back light and only 3 resistors for 6 buttons on one analog input, of course compatible with phi-1 and phi-2 software and phi_prompt user interface library. I will be releasing the 20X4 version shield first. It will also feature a small proto area for and IC up to 16 pins, for example, shift registers etc can be used to sense/control multiple things via the RJ-45 ports. You can also use DAC or other stuff on the proto area.

Here is an idea:

phi-3 shield.png

MichaelMeissner:

bwims: It looks like robotshop is not dfrobot. No warning here, and the schematic has not changed...

Bummer...

Yes, robotshop is a distributor for various parts manufactured by others (DFRobot in this case makes the lcd shield). I've used them in the past rather than ordering straight from DFRobot because Robotshop is USA based and shipping times are much less. As I mentioned upthread, evidently robotshop hasn't changed the warning, but when I ordered the lcd shield in May, it was the fixed version where digital pin 10 does control the contrast.

IIRC, you can't use servos and the lcd shield together on UNOs because the servo library disables PWM on pins 9 and 10.

Hi, I'm curious about this.

My understanding is that pin 10 would control the brightness on boards with this bug - just that it puts a high current on the pin with the danger of frying it. Have you done the test to see if the problem exists on your board, as described by the original poster?

Otherwise, I cannot see why DFRobot would not have updated their schematic. Could it be that you are just lucky?

Also, you say pin 10 controls the contrast - do you mean brightness? I thought a VR controlled the contrast?

B.

I've just received an LCD shield that came in a DFRobot branded box from this vendor on ebay:

http://www.ebay.co.uk/itm/LCD-Shield-Arduino-/320919256375?pt=LH_DefaultDomain_0&hash=item4ab8473d37

The circuit diagram, for the version 1.0 I have, has a 10K resistor between D10 and the base of the transistor. I haven't tried it yet but I would say that the board I have has been fixed.

Martyn.

bwims: Hi, I'm curious about this.

My understanding is that pin 10 would control the brightness on boards with this bug - just that it puts a high current on the pin with the danger of frying it. Have you done the test to see if the problem exists on your board, as described by the original poster?

No, I haven't done a detailed test.

bwims: Otherwise, I cannot see why DFRobot would not have updated their schematic. Could it be that you are just lucky?

There are 2 schematics at DFRobot, one labeled V1.0 that has the 10k resistor on pin D10 and the other V0.01 that does not have the resistor. So to me it looks like they have updated the schematic. Robotshop.com also has both schematics, but it has a third that is essentially the same as V0.01, and it still has the warning.

bwims: Also, you say pin 10 controls the contrast - do you mean brightness? I thought a VR controlled the contrast?

An onboard potentiometer on the LCD shield controls the contrast. Pin 10 controls the brightness. The LCD backlight is visible on the side, and when I set pin 10 to low numbers, I can see it visibly becoming less bright, in addition to the screen.

Hi

Im new here and have only owned my Arduino Uno and SainSmart LCD keypad for 3 days, but in reading how to use it I came across this thread. It has been very enlightening.

Is it right to assume that if I can only see 1 resistor beside the transistor under the LCD that my board has the issue?

Will it be ok as long as I dont use D10 in my code for brightness?

I have only made a few sketches so far like the sample scrolling message and I modified the key grabber that goes with this board.

Cheers

GannonW: Hi Is it right to assume that if I can only see 1 resistor beside the transistor under the LCD that my board has the issue?

It might or might not, it depends on the circuit.

Will it be ok as long as I dont use D10 in my code for brightness?

It should be ok. The cirsuits with the problem are ok when D10 is an input. So as long as D10 remains an input (which it is by default after are reset) the backlight should be on and should not create any issues.

--- bill

Thankyou for the quick response.

Is it possible to remove the D10 pin from the LCD board and solder a pot on there to make the backlight adjustable like the contrast is?

This is really a non-issue.

It would have been nice if the resistor is on the transistor's base.

But even if it is not, you can still control the backlight via the pin (D10): first, clear D10. After that, if you wish to turn on the back light, turn D10 into an input - the pull-up resistor will now turn on the switch / led; if you wish to turn off the back light, turn D10 into an output - now, D10 will be cleared thus turning off the switch / led.

If you do that in a timer, you can do software PWM and control the backlight brightness continuously.

dhenry: This is really a non-issue.

I disagree. If the shield is unmodified and you don't handle the output of D10 correctly it is possible to damage the AVR.

It would have been nice if the resistor is on the transistor's base.

But there is a resistor on the transistors base. It pulls up the base to VCC to provide a default "on" state. The problem is that D10 is directly connected to the base. My guess is that the circuit was designed for an FET and in that case D10 would connect directly to the gate and the circuit would work perfectly "as is". Then somebody cost reduced the design a few pennies and changed it to a NPN transistor, which doesn't work the same as an FET and creates a short when D10 goes high. - But that is just my guess.

But even if it is not, you can still control the backlight via the pin (D10): first, clear D10. After that, if you wish to turn on the back light, turn D10 into an input - the pull-up resistor will now turn on the switch / led; if you wish to turn off the back light, turn D10 into an output - now, D10 will be cleared thus turning off the switch / led.

If you do that in a timer, you can do software PWM and control the backlight brightness continuously.

From the original post:

There is also a software alternative which is to set the pin to input mode to turn the backlight on and to output and low to turn the LED off.

And also in reply #3.

Do not set D10 to HIGH or use PWM on this shield unless you modify it. You can still control the backlight . First set D10 to LOW. (only need to that once)

Then set the D10 to OUTPUT for off and INPUT for on.

PWM above meaning hardware PWM or analogWrite().

--- bill

AVR's output, even in the push-pull mode, isn't a strong voltage source in that it has substantial output resistance. So even if you short the output pin (in a logic 1), you don't get much current (40-50ma maybe) out of it. The design here isn't a great design but it is not a deadly design either.

One of the schematics linked here had the led between Vcc and the transistor, without a resistor. If that's really the design, the led can draw substantial current under a 5v rail. Leds are pretty good at taking abuses but it is wise to put a resistor in serial with the led.

But that is just my guess.

It could have been designed for an open - collector type of output pin.