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9

This circuit is designed to automatically provide an LED indication when either one telephone or another telephone which is con-

nected in parallel across the line is engaged. 

Q351 turns ON. Then the DC is converted at T351 (9 pin 

 +, 8 pin  -). Q352 base becomes (High) and Q352 turns ON. 

When Q352 is ON, Q351 base becomes (Low) and Q351 turns OFF. And no electric current flows in the collector of Q351. 

Thus Q351 starts oscillating. 

In this circumstance voltage is generated at T351 (1, 2) as well as T351 (3, 4). 

This output voltage is then rectified by D351 and D352, and smoothed by C351, C352 thus supplied to the IN USE circuits of 

Line 1 and Line 2 respectively. 

When the circuit voltage is less than 18 V (unit and telephone are on-hook). 
Telephone Line 

 D110 [D111]  D506 [D556] OFF  R520 [R570]  Q506 [Q506] base a low level.  

Q506 [Q556] OFF 

 Q507 [Q557] base a high level.  

Q507 [Q557] ON 

 Q508 [Q560] ON  D507 [D557]  R821 [R823]  LED D803-R [LED D805-R] ON.  

When the circuit voltage is more than 18 V (unit and telephone are on-hook). 
Telephone Line 

 D110 [D111]  D506 [D556] ON  R520 [R570]  Q506 [Q506] base a high level.  

Q506 [Q556] ON 

 Q507 [Q557] base a low level.  

Q507 [Q557] OFF 

 Q508 [Q560] OFF  LED D803-R [LED D805-R] OFF.                                                                                            

[ ] : in case of Line2 

Battery     T351 (6)     T351 (5)     collector of Q351

R351    base of Q351

10

The HOLD circuit holds the line from the talk condition. 
The green LED light at the time of holding. 

When line 1 [line 2] is selected with HOLD switch and the handset is off-hook, Q508 [Q560] goes ON and IN USE LED D803-R
[D805-R] lights. At this time, C507 [C557] is charged. When the Hold key is depressed, Q501 [Q551] is triggered and the hold cir-
cuit comes on. 
R118 

 D103  C507 [C557] is charged. When the Hold key is depressed, current flows as follows  C507 [C557]  SW110

[SW120] 

 SW510 [SW510]  D509 [D559]  R505 [R555]  Q502 [Q552] ON. 

  

At this time, the LED D803-R [D805-R] for the IN USE indicator goes out and green LED D803-G [D805-G] for Hold 

indicator lights. 

Q501 [Q551] collector 

 D803-G [D805-G] ON. 

When the line select switch is pressed ON and the voltage drops because of the handset going off-hook or another telephone
connected in parallel has been raised (off-hook), Q504 [Q554] goes OFF 

 Q503 [Q553] goes OFF  Q502 [Q552] goes OFF,

and hold is cancelled. 
[ ] : in case of Line2  
  

Telephone Line      Q501 [Q551] ON      Q505[Q555] ON      D502[D552]                  Q503[Q553] ON      D508[D558]

D503[D553] ON      Q504[Q554] ON   

R504[R554]      Q502[Q552] ON   

11

The circuit controls the automatic switching of the transmitted and received signals, to and from the telephone line, when the unit is 

used in the hands -free mode. 

The speakerphone can only provide a one-way communication path. 

In other words, it can either transmit an outgoing signal or receive an incoming signal at a given time, but cannot do both 

simultaneously. Therefore, a switching circuit is necessary to control the flow of the outgoing and incoming signals. 

This switching circuit is contained in IC601 and consists of a Voice Detector, TX Attenuator, RX Attenuator, Comparator and Attenu-

ator Control. The circuit analyzes whether the TX(transmit) or the RX(receive) signal is louder, and then it processed the 

signals such that the louder signal is given precedence. 

The Voice Detector provides a DC input to the Attenuator Control corresponding to the TX signal. 

The Comparator receives a TX and a RX signal, and supplies a DC input to the Attenuator Control corresponding to the RX 

signal. 

The Attenuator Control provides a control signal to the TX and the RX attenuator to switch the appropriate signals on and off.       

The Attenuator Control also detects the level of the volume control to automatically adjust for changing ambient conditions. 

1. Transmission signal path:

The input signal from the microphone is sent through the circuit via the following path:                                                                 
MIC 

 IC601 (9  10  3  4)  R601  C602  Q108  Tel line. 

2. Reception signal path:

Signals receive from the telephone line are outputted at the speaker via the following path:                                                          
Tel line 

 Q108  Q109  C112  R600  C603  IC601 (27  26  19  15)  Speaker. 

3. Transmission/Reception switching

The comparison result between TX and RX outputs as a DC level of IC601 (25), In short the voltage difference between IC601
(20) and IC601 (25) switches TX/RX. 

4. Voice detector

The output of the Mic Amp (IC601 (10)) is supplied to IC601 (13) as a control signal for the voice detector. 

5. Attenuator control

The attenuator control detects the setting of the volume control through IC601 (24) to automatically adjust for changing ambi-
ent conditions. 

6 mV

150 mV

“TX” mode

“RX” mode

Idle mode

12

This circuit is used for to reset the microcomputer when inserting batteries.

When the batteries is inserted into the unit, then the voltage is shifted by D304 and power is supplied to the CPUas shown
below.

IC303

IC304

VCC

(CPU)

Battery

GND

Reset

(CPU)

Power

Down

(CPU)

R

3

12

C

3

0

3

R

3

11

C

3

05

C

3

0

8

NC

Q302

R310 C304

VOUT

VCC

GND

NC

R313

3

2

1

OUT

VSS

IN

Vcc

CPU (7)

3.7 V

3.7 V

3.7 V

CPU (9)

CPU (8)

Reset

CPU Reset

Power down