VMA203 LCD Keypad Shield


#1

Since there is no entry in Arduino™ Shields, here my questions about the user manual vma203_a4v01.pdf:
1.Pin lay-out: Digital 9 => RW: not correct: RW is permanent connected to ground (only write possible). Digital 9 goes to E (Enable)
2.Pin lay-out: Digital 10 => Backlight: ? What does this mean? In the sketch, there is no control of pin 10. Please clarify. Schematic could explain something.
Is it possible to use digital 10 for other purposes? I only need backlight constant on.
3.Pin lay-out of the duplicated I/O-pins on the VMA203 board:
On the lower side: it is clear: all pins are through-connected with the respective Arduino-pins: except the A0 pin, used as input for the 5 keys
On the upper side: it is not document to what I/O the pins are through-connected: most probably from right to left:
D0/Rx, D1/Tx, D2, D3, D11, D12, D13.
4. Is there no schematic available, which should explain everything
Thanks


#2

Dear MRSjones
Digital 9 must be E indeed, however this makes no difference concerning functionality. Thanks for notifying this.
With digital 10 , the LCD backlight can be switched on and off (0 = off)
A schematic of the VMA203 will be available later today from our website. (velleman.eu/products/view/?id=435510 under downloads)
Thanks for Your support !


#3

OK, this will do, but:

Still waiting for the schematic for the pin-out.
Thanks


#4

Dear MRSJones,
please see : velleman.eu/downloads/29/inf … scheme.pdf
Best regards


#5

Schematic: 2 questions on the back-light issue

  1. Is this the latest schematic, since I saw schematics with a 10k serial resistor connected the base of Q1. Looks much better since it doesn’t short D10 when high.
  2. Is there no need for a resistor in the collector of Q1 to limit the LED current (say 220 ohm), or is this resistor in the LCD-module?
    Best regards

#6

Dear MRSJones

  1. We will check this with the manufacturer of the board
  2. The LCD-module accepts 5V for the back-light, so there are no additional resistors needed
    Best regards

#7

1.: no need for verification: Unfortunately my board doesn’t have that series resistor, I only see 1 resistor and a transistor. Apparently this module was initially designed for an “open collector interface”. This is a problem for the Arduino, since it doesn’t have open collector outputs. This is a known problem in the Arduino forum : http://forum.arduino.cc/index.php?topic=96747.0, counting already more the 11 pages!
However: for those who do not want to make board modifications, as suggested in the forum, one could use the module in the following (unusable) manner: play with the pinMode command:
Backlight on: set pinMode(D10,INPUT)
Backlight off: set pinMode(D10,OUTPUT); digitalWrite(D10,LOW), again, never do a digitalWrite high!
This method works also on those boards that have a series resistor between output D10 and the base of Q1.

2.: [quote]The LCD-module accepts 5V for the back-light[/quote]: doubtful:
If you look at the LCD backlight specification https://www.arduino.cc/documents/datasheets/LCDscreen.PDF:
Page 16 indeed specifies it as 5 V compatible. Page 9 shows an R7 in series with the LED. Page 7 defines R7 = 110 ohm (I measure 100 ohm), but also in table 4.2 LED absolute maximum ratings: absolute maximum forward current: 25 mA at 25 °C: With a power supply of 5 V, 0.2V over the transistor, say 1,8 V over the LED you have a nominal current of (5V - .2V -1.8V)/110 ohm = 27.27 mA!
Also strange is that they specify the typical current only 5 mA lower than the absolute maximum current.
In the Arduino projects book one uses a 220 ohm series resistor. This is a reasonable value.

Conclusion: I will use this module, but since I only need backlight always on: use it with Backlight off (transistor not conducting) and with an additional resistor of 220ohm between PIN16 (cathode) and ground.