Footswitch Board
5-14
LigaSure Vessel Sealing Generator Service Manual
Audio Circuit
The audio system consists of an audio oscillator, tone control signals, a volume
control potentiometer, an audio amplifier, and a speaker.
The audio circuit sounds a tone when RF output is active and a different tone
when alarm conditions occur. A potentiometer (R19) allows you to adjust – but
not turn off entirely – the volume of RF output activation tones. You cannot adjust
the volume of the alarm tone.
When pulled low, the RF_TONE\ signal turns on diode D1 and actively shuts off
MOSFET Q1 to enable the audio oscillator, U4.
The feedback microcontroller supplies an analog level control signal, V_FREQ,
that determines the output frequency at U4, a voltage controlled oscillator.
V_FREQ varies in amplitude to provide the appropriate audio tone.
Normal activation tones are mode dependent and have the following typical
operating frequencies:
Audio IC U5 amplifies the activation tone signal from U4. Potentiometer R19
allows audio volume control.
The ALARM signal interrupts the activation tone and resets the volume to a fixed
level. U7A and U7B are configured in an exclusive OR arrangement that is
interactive with volume potentiometer R19.
Under normal operating conditions, the ALARM signal is low, the U7A output
floats, and the U7B output transistor remains on. The output of U7B creates a
voltage divider through R5, R19 (the volume control potentiometer), and R6 to
attenuate the audio signal to levels acceptable for input to the audio amplifier. R6
determines the maximum audio volume and R5 determines the minimum audio
volume. R35 determines the audio alert volume level. R7 provides an alternate
audio signal path in the event of an open volume control potentiometer.
When the ALARM signal is high, the U7A output transistor is on and the output
of U7B floats. The U7A output transistor, when on, pulls R35 to ground and
creates a fixed voltage divider with R6 to produce the alarm volume level at the
input to audio amplifier U5. Meanwhile, the output of U7A floats, thus removing
the variable resistor divider from the circuit. In this case, the volume control
potentiometer becomes a small resistance in series with the high impedance input
from the audio amplifier, negating the effect of the volume setting.
Audio amplifier U5 and speaker SP1 constitute the final stage in the audio
system. C13 AC couples the audio signal to the amplifier. When its gain select
pins float, the voltage gain of U5 is about 20. Because the U5 output signal is
internally biased to Vcc/2, it is necessary to AC couple the speaker through C16
to prevent the amplifier from DC biasing the speaker.
Mode
Frequency (Hz)
Seal
440
Macrobipolar
520
Bipolar
660
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