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TR4 and surrounding components form a standard RC phase shift oscillator. The output of this is fed into the power amplifier via R16 and C9. TR2 has been added to short out the signal at one of the phase shift points, disabling the oscillator. This transistor is normally held on by R3, however when one of the talk buttons is pressed it is turned off for about half a second. This time is set by the values of R3 and C4. Thus the oscillator produces a brief tone when a talk button is pressed.

The circuit is mains powered using a small six volt transformer. Since the current requirement when the unit is silent is minimal, a 100mA transformer is adequate. The supply to TR3 and TR4 is additionally decoupled by R4 and C13.

Construction
The circuit is assembled on a single sided PCB. Assembly is straightforward and should present no difficulties. None of the component values are critical, there should be no problems using the value above or below if the correct part is not available. If you are using different transistors, ensure that TR7 and TR8 have a maximum collector current rating of at least 100mA.

Ceramic disk capacitors can be microphonic and are not recommended for C2, C3 and C5. The oscillator phase shift capacitors, C6, C7 and C10, should have a good Q rating or the circuit may fail to oscillate. Ceramic disk and dipped polyester types are fine, but the small resistor sized tubular devices are not suitable.

The PCB should be mounted in a suitable enclosure, together with a loudspeaker. A similar box will be needed for the remote unit. The prototype system was built into a pair of small stereo loudspeaker cases, containing 16R oval speakers. These are readily available at car boot sales for a few pounds, complete with a matching defunct stereo record player!

Suitable holes should be made on the top surfaces of both case, for the push button switches. The PCB and transformer can be mounted in any convenient position, in one unit. A suitable connector should be fitted on the rear of each unit for the interconnection cable. If the installation will be reasonably permanent, electrical choc-block connectors will suffice.

The units should then be wired up as shown in figure **. If the interconnection cable does not have polarity markings there is a 50% chance that C17 will be the wrong way round! In this case fit two 220uF capacitors in series but opposite ways round as shown.

Testing
Initially set VR1 to the centre position and VR2 fully anticlockwise. Connect the two units together and connect the master unit to the mains. Connect a voltmeter across R18 or R19 and adjust VR2 to obtain a reading of 20mV. This gives a quiescent current of 2mA, which is sufficient to eliminate crossover distortion with 16R speakers. With 8R speakers it may be necessary to increase this to 3 or 4mA (30 or 40mV), this should only be done if distortion is noticeable.

Both speakers should now be silent. Press the talk button on the remote unit. A brief tone should be heard from the local unit, and you should then be able to hear yourself speaking through the system. Check this the other way too. VR1 should be set so that speech is loud enough to be clear without being deafening. This setting can only be finalised once the units are installed, since feedback will be a problem with both units in the same room.

If the volume of the bleep is too loud (unlikely), it can be reduced by increasing the value of R16

In Use
What can I say - push the button to talk and release it to listen. If both buttons are pressed simultaneously, the master unit will dominate.

The units will pop and click as the buttons are pressed and released, this is due to the varying bias on TR3. In practice it is useful to be able to hear when the other party has released the button.

You can now keep in touch with your loved one, and enjoy your hobby. However, there is one problem with such a useful communication system installed - it is much easier for your partner to nag you about how much time you spend in the workshop! Make sure you have the master unit, then you can always "accidentally" unplug it!