The display and keyboard are illuminated by LED's. The light is normally switched on when a keypad is pressed. The rules for light switching are defined in the SW UI specifications. The display and keyboard light is controlled by the MCU. The LED's are connected two in series to reduce the power consumption. Due to the amount of LED's required for the keyboard and display light they are divided into three groups. Each group has it's own control transistor. The LED switch transistor is connected as a constant current source, which means that the current limiting resistor is put in the emitter circuitry. This arrangement will reduce LED flickering depending upon battery voltage and momentary power consumption of the phone. The LED's are connected straight to the battery voltage. This connection allows two LED's to connected in series. The battery voltage varies a lot depending upon if the battery is charged, full or empty. The switching transistor circuitry is designed to improve this as mentioned earlier.
The LED transistor control lines are coming from PSCLD. The MCU controls these lines by writing to PSCLD using the serial control bus. There are two LED control lines provided by the PSCLD. The display light control is connected to a separate control line. The keyboard light control is common to the two transistors. This means that the keyboard and display light can be controlled separately. The advantage of this is that the power dissipation and heating of the phone can be reduced by only having the required lights switched on.
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You can now recondition your old batteries at home and bring them back to 100 percent of their working condition. This guide will enable you to revive All NiCd batteries regardless of brand and battery volt. It will give you the required information on how to re-energize and revive your NiCd batteries through the RVD process, charging method and charging guidelines.