The touch function only works with batteries: See instructions for use (following copy of the manual).
8 Display lighting 8.1 Operation with adapter Caution: The permanent screen illumination only works with the adapter. You can change the brightness of the screen on 3 levels (L1=high, L2=medium, L3=low) using the DOWN key. If automatic brightness adjustment is activated (A-on), the low background lighting (lighting level L3) will automatically switch off between 7pm and 6:59am. 8.2 Battery operation (energy-saving mode) - To activate the lighting, press the alarm clock or make a loud noise (such as clapping your hands). - The background lighting is activated for 15 seconds.