Silicon ChipAdd a Headset to the Speakerphone - November 1988 SILICON CHIP
  1. Outer Front Cover
  2. Contents
  3. Publisher's Letter: Servicing and serviceability
  4. Feature: Screws & Screwdrivers by Leo Simpson
  5. Feature: Quieten the Fan in Your Computer by Leo Simpson
  6. Project: High Power PA Amplifier Module by Leo Simpson & Bob Flynn
  7. Feature: The Way I See It by Neville Williams
  8. Vintage Radio: What to do about the loudspeaker by John Hill
  9. Project: Poor Man's Plasma Display by Leo Simpson
  10. Serviceman's Log: My friend (the enemy) Flicker by The Original TV Serviceman
  11. Project: Build a Car Safety Light by John Clarke
  12. Project: Add a Headset to the Speakerphone by John Clarke & Greg Swain
  13. Back Issues
  14. Subscriptions
  15. Feature: Amateur Radio by Garry Cratt, VK2YBX
  16. Feature: The Evolution of Electric Railways by Bryan Maher
  17. Market Centre
  18. Advertising Index
  19. Outer Back Cover

This is only a preview of the November 1988 issue of Silicon Chip.

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Items relevant to "High Power PA Amplifier Module":
  • Silicon Chip article scan, November 1988 - 120W PA Amplifier (AUD $10.00)
Articles in this series:
  • The Way I See It (November 1987)
  • The Way I See It (December 1987)
  • The Way I See It (January 1988)
  • The Way I See It (February 1988)
  • The Way I See It (March 1988)
  • The Way I See It (April 1988)
  • The Way I See It (May 1988)
  • The Way I See It (June 1988)
  • The Way I See it (July 1988)
  • The Way I See It (August 1988)
  • The Way I See It (September 1988)
  • The Way I See It (October 1988)
  • The Way I See It (November 1988)
  • The Way I See It (December 1988)
  • The Way I See It (January 1989)
  • The Way I See It (February 1989)
  • The Way I See It (March 1989)
  • The Way I See It (April 1989)
  • The Way I See It (May 1989)
  • The Way I See It (June 1989)
  • The Way I See It (July 1989)
  • The Way I See It (August 1989)
  • The Way I See It (September 1989)
  • The Way I See It (October 1989)
  • The Way I See It (November 1989)
  • The Way I See It (December 1989)
Articles in this series:
  • Amateur Radio (November 1987)
  • Amateur Radio (December 1987)
  • Amateur Radio (February 1988)
  • Amateur Radio (March 1988)
  • Amateur Radio (April 1988)
  • Amateur Radio (May 1988)
  • Amateur Radio (June 1988)
  • Amateur Radio (July 1988)
  • Amateur Radio (August 1988)
  • Amateur Radio (September 1988)
  • Amateur Radio (October 1988)
  • Amateur Radio (November 1988)
  • Amateur Radio (December 1988)
  • Amateur Radio (January 1989)
  • Amateur Radio (April 1989)
  • Amateur Radio (May 1989)
  • Amateur Radio (June 1989)
  • Amateur Radio (July 1989)
  • Amateur Radio (August 1989)
  • Amateur Radio (September 1989)
  • Amateur Radio (October 1989)
  • Amateur Radio (November 1989)
  • Amateur Radio (December 1989)
  • Amateur Radio (February 1990)
  • Amateur Radio (March 1990)
  • Amateur Radio (April 1990)
  • Amateur Radio (May 1990)
  • Amateur Radio (June 1990)
  • Amateur Radio (July 1990)
  • The "Tube" vs. The Microchip (August 1990)
  • Amateur Radio (September 1990)
  • Amateur Radio (October 1990)
  • Amateur Radio (November 1990)
  • Amateur Radio (December 1990)
  • Amateur Radio (January 1991)
  • Amateur Radio (February 1991)
  • Amateur Radio (March 1991)
  • Amateur Radio (April 1991)
  • Amateur Radio (May 1991)
  • Amateur Radio (June 1991)
  • Amateur Radio (July 1991)
  • Amateur Radio (August 1991)
  • Amateur Radio (September 1991)
  • Amateur Radio (October 1991)
  • Amateur Radio (November 1991)
  • Amateur Radio (January 1992)
  • Amateur Radio (February 1992)
  • Amateur Radio (March 1992)
  • Amateur Radio (July 1992)
  • Amateur Radio (August 1992)
  • Amateur Radio (September 1992)
  • Amateur Radio (October 1992)
  • Amateur Radio (November 1992)
  • Amateur Radio (January 1993)
  • Amateur Radio (March 1993)
  • Amateur Radio (May 1993)
  • Amateur Radio (June 1993)
  • Amateur Radio (July 1993)
  • Amateur Radio (August 1993)
  • Amateur Radio (September 1993)
  • Amateur Radio (October 1993)
  • Amateur Radio (December 1993)
  • Amateur Radio (February 1994)
  • Amateur Radio (March 1994)
  • Amateur Radio (May 1994)
  • Amateur Radio (June 1994)
  • Amateur Radio (September 1994)
  • Amateur Radio (December 1994)
  • Amateur Radio (January 1995)
  • CB Radio Can Now Transmit Data (March 2001)
  • What's On Offer In "Walkie Talkies" (March 2001)
  • Stressless Wireless (October 2004)
  • “Degen” Synthesised HF Communications Receiver (January 2007)
  • WiNRADiO: Marrying A Radio Receiver To A PC (January 2007)
  • PICAXE-08M 433MHz Data Transceiver (October 2008)
  • Half-Duplex With HopeRF’s HM-TR UHF Transceivers (April 2009)
  • Dorji 433MHz Wireless Data Modules (January 2012)
Articles in this series:
  • The Evolution of Electric Railways (November 1987)
  • The Evolution of Electric Railways (December 1987)
  • The Evolution of Electric Railways (January 1988)
  • The Evolution of Electric Railways (February 1988)
  • The Evolution of Electric Railways (March 1988)
  • The Evolution of Electric Railways (April 1988)
  • The Evolution of Electric Railways (May 1988)
  • The Evolution of Electric Railways (June 1988)
  • The Evolution of Electric Railways (July 1988)
  • The Evolution of Electric Railways (August 1988)
  • The Evolution of Electric Railways (September 1988)
  • The Evolution of Electric Railways (October 1988)
  • The Evolution of Electric Railways (November 1988)
  • The Evolution of Electric Railways (December 1988)
  • The Evolution of Electric Railways (January 1989)
  • The Evolution Of Electric Railways (February 1989)
  • The Evolution of Electric Railways (March 1989)
  • The Evolution of Electric Railways (April 1989)
  • The Evolution of Electric Railways (May 1989)
  • The Evolution of Electric Railways (June 1989)
  • The Evolution of Electric Railways (July 1989)
  • The Evolution of Electric Railways (August 1989)
  • The Evolution of Electric Railways (September 1989)
  • The Evolution of Electric Railways (October 1989)
  • The Evolution of Electric Railways (November 1989)
  • The Evolution Of Electric Railways (December 1989)
  • The Evolution of Electric Railways (January 1990)
  • The Evolution of Electric Railways (February 1990)
  • The Evolution of Electric Railways (March 1990)
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. 4.7k ,,.L 0.1 4.7k .068 3 TXI .068 .o ~ 10 MCO 0.1 27 RXI 7 RU X01 v+ 16 TXO +2.9V HEADPHONES ~ I 21 VB 1 16VW 47 + 16VW! - +HAGC 24 .,. VDLUME~---'-1 VLC VR1 20k I IC1 MC34018 I I (1 / / 4 2.2k .047 -------,---t1■·--t9 MCI 8 1 vcc 20 ACF CP1 11 RTX RR 25 4.7 7 100k + 16V + 1 +5.4V_......,._ _ __ 91k RRX 28 + Kl 19 CP2 12 XDC 23 LO GNO RLO 22 I I .,. I I 4.3k :vw 47 16VW RXD 26 30k 4.7 18k 16VW .01 + - 200k 4.7 + 16VW _ 2.2M 1 + 16VW _ 2.2M /SPEAKER RS { SKG CS 18 S2 MUTE 220 I .,. 1 + 16VW _ I I II I I I L _ ______________ . - - - - - - - - - - - - - - - - - - - ~ *R1+RS=250 B ("7'\ E~ VIE::ra:oM 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. BACK ISSUES ORDER FORM Please send me a back issue for : D March 1988 D August 1 988 D April 1988 D Sept.1988 D November 1987 D December 1987 D February 1988 D May 1 988 D June 1 988 D July 1 988 □ October 1 988 Enclosed is my cheque/money order for $ _ _ _ _ __ or please debit my D Bankcard Card No I~~~~~~~~~~~~~~~~ Signature _ _ _ _ __ _ _ _ _ Card expiry date_/_/_ _ Name_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ (PLEASE PRINT) Stree.__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ Suburb/town, _ _ _ _ _ _ _ _ _ _ _ _ _ Postcode____ □ Visa Card Price: $A5 .00 each (includes postage). Overseas orders add $A 1 .00 per issue for postage. NZ & PNG orders are sent by air mail. Detach and mail to : SILICON CHIP PUBLICATIONS PO BOX 139 COLLAROY BEACH NSW 2097 ~--- ----------------------~-----------~ 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