Add a headset to
the Speakerphone
It's easy to add a headset with inbuilt
microphone to the Speokerphone
described in the September issue. Now
you con enjoy hands-free operation
combined with total privacy.
Although the Speakerphone is
great for in-home use, it's sometimes not practical for use in the office. Other people can be distracted
by the two-way conversation or the
inbuilt electret microphone can
68
SILICON CHIP
pick up too much unwanted noise.
Sure, you could plug in a pair of
headphones but that doesn't solve
the noise problem and it isolates
you from others in the office.
These problems can be solved by
adding a lightweight telephone
headset. Only a couple of minor
modifications are required to the
original circuit and all switching
takes place automatically when you
plug the headset into the microphone socket. The on/off, mute and
volume controls on the front panel
of the Speakerphone all operate exactly as before.
The headset pictured here is
available from Jaycar (Cat. AA2018} and is especially designed for
telephone use. It features a single
earpiece and a boom microphone
that's fully adjustable for position.
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SPEAKERPHONE
SC12-1·0988-1
Fig.1: this section of the circuit carries all the necessary modifications. These involve swapping the locations of the
electret microphone and its 22k0 bias resistor, and rewiring the headphone socket and mute switch.
It's just the shot for use with the
Speakerphone but note that other
headsets may not be suitable
without modification.
Because the microphone is now
positioned closer to the user's
mouth, there is less extraneous
noise pickup than before. And the
single earpiece means that you can
still hold conversations with other
workers.
Of course, the big advantage is
that you can now use the phone
without disturbing others while
keeping both hands free for note
taking or operating a computer
terminal.
Circuit modifications
In order to connect the headset to
the Speakerphone we have to do
two things. First, we have to switch
the output signal from the loudspeaker to the headset earpiece.
That's taken care of by the existing
headphone socket circuit. Second,
we have to switch out the existing
electret microphone and switch in
The lightweight headset is available from Jaycar (Cat. AA-2018) and features a
single earpiece and boom mounted microphone. It plugs into the Speakerphone
via a 3.5mm to 6.35mm stereo plug adaptor.
NOVEMBER 1988
69
Silicon Chip Back Issues
Issue Highlights
November 1987: Car Stereo in Your
Home ; 1GHz Frequency Meter;
Capacitance Adapter for DMMs.
December 1987: 1 00W Power
Amplifier Module ; Passive lnfrared
Sensor for Burglar Alarms; Universal
Speed Control and Lamp Dimmer;
24V to 12V DC Converter.
January 1988· Bowtie UHF
Antenn ·
I g aO
ower Supply; Custo,,...~Prrr, oa--r:-iinger; Subcarrier
Adapter r FM Tuners.
February 1988: 200 Watt Stereo
Power Amplifier; Deluxe Car Burglar
Alarm ; End of File Indicator for
Modems; Simple Door Minder; Low
Ohms Adapter for Multimeters.
March 1988: Remote Switch for Car
Alarms ; Telephone Line Grabber;
Low Cost Function Generator;
Endless Loop Tape Player.
April 1988: Walkaround Throttle for
Model Railroads; pH Meter for Swimming Pools; Slave Flash Trigger;
Headphone Amplifier for CD Players.
May 1988: Optical Tachometer for
Aeromodellers; High Energy Ignition
for Cars; Ultrasonic Car Burglar
Alarm.
June 1988: Stereo Control Preamplifier; Breakerless Ignition For
Cars; Automatic Light Controller;
Mega-Fast Nicad Battery Charger.
July 1988: Fitting a Fuel Cut-Off
Solenoid ; Booster for TV & FM
Signals; The Discolight Light Show.
August 1988: Plasma Display;
Remote Chime/Doorbell; High Performance AC Millivoltmeter; Getting
the Most Out of Nicad Batteries.
September 1988: Hands-Free
Speakerphone; Electronic Fish Bite
Detector; Switchmode Charger for
12V Gel Batteries; Vader Voice.
October 1988: Stereo FM Transmitter; High Performance FM Antenna ;
Led-Light House Number; Matchbox
Crystal Set.
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70
SILICON CHIP
You will have to temporarily remove the PCB from the case to modify the wiring. We used PC stakes to terminate the
leads from the 2.2k0 resistor and to make connections to the headphone socket easier.
2.2k0 resistor from circuit and
disconnect the appropriate leads to
the microphone and to the mute
switch. The 2.2k0 resistor can then
be installed in its new location and
the leads from the microphone and
switch connected to their new
positions.
The headphone socket modification is quite straightforward and involves connecting two extra leads
- one to the microphone and the
other to one side of the 2.2k0
resistor. The shorting link on the
back of the socket must also be
relocated.
Fig.2: here's how to modify the wiring to accept the headset. Note the
different linking arrangement on the back of the microphone socket.
the headset microphone instead.
To avoid complication, the circuit
is arranged to switch over automatically when the headset is plugged in. But before we can do this, it
is first necessary to modify the bias
arrangement for the electret
microphone so that one side of the
microphone goes to ground. In practice, this involves swapping the
locations of the microphone and the
2.2k0 resistor on the circuit.
After that, it's simply a matter of
rewiring the headphone socket and
the mute switch (which must remain in parallel with the microphone). Fig.1 shows all the circuit
details.
Construction
Fig.2 shows how the wiring to the
headphone socket, microphone and
mute switch is modified.
You will first have to remove the
Testing it
You will need a 3.5mm to 6.35mm
stereo plug adaptor to use the
headset with the Speakerphone.
These are available from Dick
Smith Electronics (Cat. P-6500) or
from Arista outlets (Cat. PA90).
Once the wiring has been •completed, the Speakerphone should be
tested to ensure that it still functions correctly. The unit can then
be checked out with the headset
plugged in. We found that it's best
to keep the microphone well away
from the user's mouth, to prevent
overload and stop breathing noises.
NOVEMBE R 1988
71
