7/30/2019 Automatic Telephone Conversation Recorder

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Ritika Dua

ere is a circuit which enables auto-Hmatic switching ON of the taperecorder when the handset is lifted. Thetape recorder gets switched OFF when

the handset is replaced. The signal issuitably adjusted to a level at which theycan be recorded using the MIC-INsocket of the tape recorder.Connections X1 and X2 in the circuit areconnected to the telephone lines.Resistors R1, R2 and VR1 act as a volt-age divider. The voltage appearingacross R2 and VR1 is fed to the MIC-IN socket of the tape recorder. Thevalues of R1 and R2 may be changeddepending on the input impedance ofthe tape recorder's MIC-IN terminals.Capacitor C2 is used for blocking the

flow of DC.The second part of the circuit controlsrelay RL1, which is used to switchON/OFF the tape recorder. A voltageof 48 volts appears across the tele-phone lines in on-hook condition. This

voltage drops to about 9 volts when thehandset is lifted. Diodes D3 through D6constitute a bridge rectifier. This ensuresthat transistor T1 gets voltage of proper

polarity, irrespective of the polarity ofthe telephone lines.During on-hook condition, the outputfrom the bridge (approx. 48V DC)passes through 12V zener D7 and isapplied to the base of transistor T1 viathe voltage divider comprising resistorsR3 and R4. This switches on transistorT1 and its collector is pulled low. This, inturn, causes transistor T2 to cut off andrelay RL1 is not energised.

When the telephone handset is lifted,the voltage across points X1 and X2 fallsbelow 12 volts and so zener diode D7does not conduct. As a result, base of

transistor T1 is pulled to ground poten-tial via resistor R4 and thus is cut off.Thus, base of transistor T2 gets forwardbiased via resistor R5, which results inthe energisation of relay RL1. The taperecorder is switched ON and recordingbegins.The tape recorder should be keptloaded with a cassette and the recordbutton of the tape recorder shouldremain pressed to enable it to record

the conversation assoon as the handset islifted. Capacitor C3

ensures that the relay isnot switched on-and-off repeatedly when anumbe r i s be ingdialled in pulse dialingmode.

EMN. SHARMA

TESTED

Fig. 1: Circuit diagram of the Automatic Telephone Conversation Recorder.

AUTOMATIC TELEPHONE

CONVERSATIONRECORDER

X1

230V AC PRIMARY TO

6V-0-6V, 100mASECONDARY

TRANSFORMER

D1

D2 +

230VAC

MAINS

D3 D4

D5D6 T1

T2

Teleph

one

Line

R2

R1

VR1

C2 D7

R3

R4 C3

D8

230 V AC

MainSupply

fo

rTapeRecorder

C1

X1

X2

Y1

Y2

R5

RL1

MainTape

Recorder

RL1

Y1

Y2X1

X2

TelephoneLine

Mains

MIC-IN

AutomaticTelephone

ConversationRecorder

Connection Details

7/30/2019 Automatic Telephone Conversation Recorder

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CSEMICONDUCTORS

T1 BC548

T2 SL100D1-D6, D8 1N4001, Rectifier DiodeD7 12V Zener

RESISTORS

R1 18kWR2 1.8kWR3 470kWR4 100kWR5 4.7kWVR1 1kW Preset

CAPACITORS

C1 1000F/25V

C2 0.033F/100V Polyester C3 100F/25V

MISCELLANEOUS

X1 230V AC Primary to 6V-0-6V 100mASecondary Transformer

RL1 6V, 100WFig

.2:

Ac

tua

l-

size,

so

lder-s

ide

PCBlayou

t.

Fig

.3:

Componentl

ayou

tfor

the

PCB

.

D3

D4 D5

D6

C2

R1

R2

D7

R3

R4

VR1

C3

C1

D1

D2

T1

R5

D8

T2

R

L1

TOEXTMICTERMINALOF

TAPERECORDER

6V-0-6V

TOMAINPLUGOF

TAPERECORDER

C B E

C

B

E