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.