e02ch09.pdf

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009

Turn That Down, Please

A. Baur

Many people, both at home and at work, like to have the radio

on for some background music or news. Your preferred sound

volume, however, may well be annoying when a phone call

has to be made or answered. This little circuit keeps music

lovers and phone addicts happy at the same time by auto-

matically reducing the music volume when the phone is taken

off the hook. No need to walk over to the radio!

The circuit is simple and built from off the shelf components.

As long as the receiver is on the hook, the phone line volt-

age will be high, causing the optocoupler to conduct and the

relay to remain off. Via the relay contacts, the radio is then

connected to the loudspeakers in the normal fashion.

Once the receiver is lifted, the situation reverses with R2 pro-

viding enough base current for T1 to start conducting. With

the relay energized, the contacts connect 22- Ω resistors in

series with the loudspeakers lines, causing much reduced vol-

ume and allowing you to complete the phone call without

yelling.

Switch S1 when closed defeats the action of the optocoupler

and so acts as a ‘bypass’ control.

The relay may be 12-volt type with two change-over (c/o) con-

tacts or just two make contacts. Preset P1 is adjusted until the

relay comes on reliably when the receiver is lifted.

Finally, of the two alternatives shown in the circuit diagram,

only the CNY17-2 optocoupler will provide an isolation volt-

age specification compliant with Class 2 equipment. The 4N35

will still satisfy Class 1, however.

The circuit is conveniently and safely powered from a battery

eliminator (a.k.a. mains adaptor) with a (loaded) output volt-

age of about 12 VDC.

R7

22

Ω

5W

RE1a

ANTENNA

LS R

R

R7

22

Ω

5W

RADIO

RE1b

LS L

L

+12V

R2

R5

D2

D1

RE1

D6

D 7

R1

1N4007

D4

D3

T1

4x

1N4007

R3

10k

P1

50k

BC517

1

6

5

S1

R4

D5

C11

4

µ

7

50V

1N4007

2

4

IC1

4N35

CNY18-2

BYPASS

024021 - 11

(024021-1)

12/2002

Elektor Electronics

27

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