How can I switch the STU off when not in use?

As a general note, there is no real need to switch off the backlight on an STU in order to prolong its life.
STUs are designed to keep working for a long time.

If there are other reasons for switching off the backlight then the information below is applicable.

The brightness of the STU-520, 530, 540 and 541's backlight can be set using the following API method:

setBacklightBrightness ([in] UINT16 backlightBrightness)

This is mentioned in the API index which is installed with the STU SDK and can be accessed via a browser here:

file:///C:/Program%20Files%20(x86)/Wacom%20STU%20SDK/COM/doc/interfacewgss_s_t_u_1_1_i_protocol.html#a7c7f39ff7e5419bc332be3e56e55d7b6

The above API entry is not entirely accurate when it warns against setting the backlight brightness frequently because it is stored in non-volatile tablet memory.
This is true for the STU-520 but is configurable for the 530, 540 and 541.

For the 520 the values below can be used, but not frequently because the EEPROM has a limitation on the number of times it can be written to.

• 0x0000 - 25% brightness
• 0x0001 - 50% brightness
• 0x0002 - 75% brightness
• 0x0003 - Brightest (this is the default setting)

For the 530, 540 and 541 the brightness can be set temporarily as follows, without any concerns about frequency:

• 0x0000 - 25% brightness
• 0x0001 - 50% brightness
• 0x0002 - 75% brightness
• 0x0003 - Brightest (this is the default setting)
• 0x0004 - Backlight off

Alternatively, the backlight can be written to the flash memory as follows, but this should be done sparingly:

• 0x8000 - 25% brightness
• 0x8001 - 50% brightness
• 0x8002 - 75% brightness
• 0x8003 - Brightest (this is the default setting)
• 0x8004 - Backlight off

N.B.

1. Using 0x0004 with the 540 may only make the display dim, depending on the individual unit - this is currently awaiting a fix.

2. Switching the backlight off does not actually disable the pad - it still remains active and can generate pen data and accept uploaded images from the PC, even though nothing is visible on the pad itself.