Panasonic KX-FPC135 Service Manual page 211

9.6.5. Sending Signal
The audio signal from the microphone is amplified by Mic Amp,
compander, and 3 kHz LPF built into IC201. It is then mixed
with the TX DATA signal from the CPU, the modulation is
adjusted by VR201, and input to the modulator in the RF UNIT.
9.6.6. Reset/Power Down/Battery Low/ID
When the battery is installed in the Handset, the reset circuit
consisting of R217 and inside IC201 functions, inputting a reset
signal to the CPU. This ensures that the unit will operate
normally without the user´s needing to switch the power off and
on. When the voltage from the batteries drops to 3.5 V, 3.5 V
voltage detector inside IC201 operates and output a battery low
signal to the CPU. This causes the RECHARGE LED to turn
on. If voltage continues to drop and reaches 3.2 V, 3.2 V
voltage detector inside IC201 operates and outputs a power
down signal to the CPU. This causes power to be cut off
automatically and prevents the battery from over discharging.
Q203 is a charge detector that informs the CPU whether or not
the handset is currently being charged. During charging, ID
data is sent from the base unit. Q204 receives this ID data and
sends it to the CPU.
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KX-FPC135 / KX-FPC141