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Topics - ngatgi

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The contrast of the LCD module referenced in the subject can be adjusted by adding a variable resistor between the VEE (negative supply), GND and Vo (input for contrast adjustment).  When tracing where the interface header pins are connected, there appears to be resistor pads that will allow either a variable or fixed contrast setting directly on the module.  As this display is replacing a different type of display, I don't want to require external components for this purpose.

LCD Module Datasheet

The component locations in question are R6, R7, VR1, R81, R82 and Q2.  The stock configuration has 0Ω jumpers at R6 and R7 and I believe Q2 is a PNP 2SB1198K based on the device markings.  VR1, R81 and R82 are not populated.  I have included pictures of the actual PCB and a hand-drawn schematic as I see it.  Note that there is one node in the schematic that I couldn't determine it's connection, but it would appear to be a supply connection with voltage greater than VEE.



I am sure there must be a configuration that would only require a change in population (no cuts or jumps) otherwise why would they bother to include all these component locations ... ESPECIALLY when you see the pads for the variable resistor (VR1)!

The transistor is what is throwing me.  I would like to know what it's purpose is in the default configuration.  If R6, R7 and Q2 were removed, a fixed voltage divider can be created using R6 and R81 and a 0Ω jumper at R82, just as recommended for the external voltage divider.  Obviously, this could be adjustable by populating a pot for VR1 and not populating R6.

If any of you have been down this road before, I would appreciate you sharing your experience!

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