3
Each key is read in turn when the relevant channel is selected via pins p3.5 and p3.6.
1.8 Buzzer Operation
TR2 and TR3 are configured as an astable multivibrator with a nominal frequency of
2.7kHz. TR7 and 8 are used to switch the buzzer with TR7 acting as an inverter to
turn the buzzer off during a microprocessor reset.
1.9 Illumination Drive
TR10, 12 and 16 provide a constant current drive for lamps LP1 and LP2. The
current is set by resistors R11 and R12. TR6 is an inverting buffer to turn off the lights
during a microprocessor reset. Illumination level's are given by pulse width
modulation of drive line P3.3 (pin 15).
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