AXE133 Backlight Control

nick12ab

Senior Member
#1
The extra pads on the AXE133 which are provided for a backlight don't get any explanation in the AXE133 PDF, therefore a blog entry is necessary.

The AXE133 PCB has extra pads that allow a transistor to be installed in order to control the backlight on a backlit LCD if one is attached. Rev-Ed do not sell the AXE133 PCB with any backlit LCD module; only non-backlit LCDs and OLEDs are available. However it's nice that they put the pads there.

The AXE133 PCB is designed to take a PNP transistor.



[TABLE="width: 500"]
[TR]
[TD]Pad[/TD]
[TD]Description[/TD]
[TD]Connected to[/TD]
[/TR]
[TR]
[TD]E[/TD]
[TD]Emitter of PNP transistor[/TD]
[TD]Vdd[/TD]
[/TR]
[TR]
[TD]B[/TD]
[TD]Base of PNP transistor[/TD]
[TD]3[/TD]
[/TR]
[TR]
[TD]C[/TD]
[TD]Collector of PNP transistor[/TD]
[TD]1[/TD]
[/TR]
[TR]
[TD]1, 2[/TD]
[TD]LCD backlight resistor pads[/TD]
[TD]C, A[/TD]
[/TR]
[TR]
[TD]3, 4[/TD]
[TD]Transistor base resistor[/TD]
[TD]B, PICAXE pin C.2[/TD]
[/TR]
[TR]
[TD]A[/TD]
[TD]LCD backlight anode[/TD]
[TD]2[/TD]
[/TR]
[TR]
[TD]K[/TD]
[TD]LCD backlight cathode[/TD]
[TD]Vss[/TD]
[/TR]
[/TABLE]
Notes
  • For the LCD backlight resistor pads, a resistor or jumper wire should be soldered there depending on whether the LCD to be used will not have a built in resistor or will have one.
  • If through hole resistors are to be used, the board design means that the resistors need to be standing on their ends rather than flat to the board unless creative positioning and lead bending is used.
  • If the resistors are too big in value, the PNP transistor is the wrong way round, or the power supply voltage is wrong then the LCD backlight may work but will be dim.
  • The backlight is turned on by setting pin C.2 low and the backlight is turned off by setting pin C.2 high because a PNP transistor is used.
  • The backlight pins on the AXE133 and on the 16x2 LCDs/OLEDs Rev-Ed sell are at the opposite ends to the signal pins than on cheap eBay LCDs and 20x4 LCDs.
  • If (and only if) modifying an LCD for 3V operation, solder jumpers on the LCD module should be configured so that the anode of the backlight is connected to A, the cathode of the backlight is connected straight to Vss, and the negative voltage output is connected to K. The K pad is used to connect other vital 0V connections to the 0V connector on the 3-pin header so this pin cannot be connected to the LCD module and instead a wire must be ran from the pin on the LCD module to the potentiometer.
 

nick12ab

Senior Member
#2
This picture shows the AXE133 PCB equipped with a BC556B transistor for backlight control.



The 1k resistor is for the base of the transistor and lies flat against the PCB. Pads 1 and 2 (see above) are shorted together on the other side of the PCB because my LCDs have the resistors built-in.
 
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