This is a very sensitive, sound or voice activated switch (VOX) which plugs into the REMOTE plug of a tape recorder and turns it on when it detects a preselected level of sound.  If the sound continues then the switch will self-trigger and remain on.  Once the sound goes away then a delay in the circuit will turn off the tape recorder after about 6 seconds.

It is ideal for hands-free operation of a tape recorder, eg, for taking notes.   The kit can be calibrated to turn on at the slightest sound or at a very high sound level.

****Note that this kit does not improve the sensitivity of the tape recorder itself.   It is just a switch which turns the recorder on.  So that the sound which turns ON the tape recorder may not be picked up by the tape recorder if the microphone in the tape recorder is not sensitive enough.

The kit is constructed on a single-sided printed circuit board (PCB).  Protel Autotrax and Schematic were used to design the board.

CIRCUIT CALIBRATION
The kit can be adjusted to switch on at the slightest sound.  However, at these levels of sensitivity there are several points to remember.  Firstly, the switching level is sensitive to the supply voltage.  So that a battery is not a good power source for this method of operation over a long period of time.  A plug in pack or a mains operated power supply would be better.  If you do use a battery pack for extended periods then disconnect the LED as it will just consume power for no purpose.

Note that the electret microphone of th sound switch may be more sensitive than the microphone in the tape recorder itself!  This could result in the recorder switching on but nothing being recorded.

The PCB mounted switch is to allow for the polarity of the tape recorder.  In one position the tape recorder will be ON and the kit will have no effect.  In the other position the kit should work and the 5K pot will control the sound level at which the tape recorder turns ON.

If the kit is too sensitive: we have chosen a good quality 5K pot to give fine adjustment at low levels of sound.  If this is too low for your purpose then replace it by a good quality 50K or 100K pot.  Also you can change the 47K load resistor R1 on the microphone to alter its sensitivity.

To change the delay: Lower R9 to say 100K to get an approximate 2 second delay.   Increasing R9 will increase the delay up to point.  For long 1 minute delays or more it would probably be best to incorporated a delay circuit on the base of T3.   The 470R could be increased to 10K say with an electrolytic of around 47uF from it (on the other side to the connection to the base) and ground.