Adjusting Search Illumination/Lighting

208 as of Aug 99 & 210 as of July 99

The brightness of the search illumination of the instrument cluster (A1) can be set with the buttons for instrument and search illumination (bright/dark) (A1s4, A1s5). The control takes place via the on/off ratio of terminal 58d.







Signal flow:
- If the side light is switched on, the light module (N7-1) transmits the discrete signal terminal 58K to the instrument cluster (A1).
- The instrument cluster (A1) receives this signal and transmits "On/off ratio terminal 58d" as a message to the CAN/engine compartment, to the CAN (A1-A2, A40/3) and as a discrete signal terminal 58d.
- The lamps can now be actuated by the output terminal 58d of the instrument cluster or of other control modules.







On/off ratio terminal 58d
The lamps for search and switch illumination are actuated with a square-wave voltage U(5Bd) (see figure above):
- The lamps are energized in the time (t1), followed by a pause, energized again after the pause and so on. This change happens so quickly that the eyes do not notice any flickering.
- The ratio of the times t1 to T is called the on/off ratio. T is the constant period and t1 is the time in which current flows within a period.
- The higher the on/off ratio (thus the energizing period t1 the higher the average power and therefore the brightness of the lamps.

The voltage curve U(58d) as it is applied to the outputs terminal 58d of the various control modules is sketched in the figure. The upper sketch shows an on/off ratio of 80% and the lower an on/off ratio of 40%.

An on/off ratio of 100% corresponds to a normal direct voltage. No voltage is applied with an on/off ratio of 0%.

Advantages of this brightness control
- Much higher efficiency,
- The brightness of the lamps is independent of the number of lamps,
- Every control module which is connected to the CAN/engine compartment can actuate lamps with the brightness specified by the instrument cluster (A1).
COMMAND receives the information via a separate CAN bus between the instrument cluster (A1) and the COMMAND operating, display and control module (A40/3).


NOTE:
When the side light is OFF the instrument cluster (A1) sends the message "on/off ratio terminal 58d = 0%".
When the side light is ON the on/off ratio can be set between 12 and 100% using the buttons for instrument and search illumination (A1s4, A1s5).