Kit 13. VOICE-ACTIVATED TAPE RECORDER SWITCH
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 much 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.
ASSEMBLY INSTRUCTIONS
Assembly is straight forward and components may be
added to the PCB in any order. Note that the electret
microphone should be inserted with the pin connected to
the metal case connected to the negative rail (that is, to
the ground or zero voltage side of the circuit). This is
marked by the minus sign on the overlay of the PCB. It is
generally best to add the lowest height components first
for ease of soldering. Be sure to put the electrolytic
capacitors and the diodes in the correct way as indicated.
The two PCB-mounted switches may go in either way.
Note the two links required next to the DPDT switch (the
larger of the two switches.) Use some wire cut off from the
leads from the other components. Note that T3 is different
to T1 and T2. T3 can handle the higher current
which flows through the REMOTE switch when the it is
turned ON. The 10" of two strand wire goes into the 'TO
TAPE REMOTE' pads. Attach the 2.5mm plug to the
other end. Be sure the two wires within the plug do not
short-circuit when you screw on the cover.
Use the two 1% metal film 1M resistors (the two resistors
with the extra band- they are a different colour to the 5%
carbon film resistors) for the voltage divided input to pin 1
of the IC. The purpose of the pad marked 'A' will be
explained in the next section.
CIRCUIT DESCRIPTION
At the start of the circuit is an electret microphone. An
electret is a permanently charged dielectric. It is made by
heating a ceramic material, placing it in a magnetic field
then allowing it to cool while still in the magnetic field. It
is the electrostatic equivalent of a permanent magnet. In
the electret microphone a slice of this material is used as
part of the dielectric of a capacitor in which the diaphram
of the microphone forms one plate. Sound pressure moves
one of its plates. The movement of the plate changes the
capacitance. The electret capacitor is connected to an FET
amplifier. These microphones are small, have excellent
sensitivity, a wide frequency response and a very low
cost.
The electret microphone is followed by two standard
audio amplification stages. A good quality 5K pot
provides sensitivity control for the leval at which the
following digital part of the circuit will be activated. The
22nF capacitor isolates the microphone from the base
voltage of the transistor and only allows alternating
current (AC) signals to pass. The 100nF acts similarly after
the second and third stages.
The input to the digital section is biased at mid-way
between the supply voltage to the Kit by the two 1M (1%
metal film) resistors. Before we look at the digital section
let us review the operation of the 74HC14, the Inverting
Schmidt Trigger IC. At the maximum operating voltage
(6V) there is a 1V hysterisis gap between 2.2V and 3.2V.
This means that as the input voltage increases from zero
to six volts the Schmidt circuit will not trigger until it gets
to 3.2V. But as the voltage falls from six volts to zero the
circuit must fall to 2.2V before it changes state.
Assume that pin 1 of the IC is taken HIGH by sound input
to the microphone. This means that pin 2 is LOW. C5, the
22uF electrolytic capacitor will charge very quickly via the
diode D2. The low on pin 2 will make pin 4 HIGH. This will
turn ON transistor T3 and the tape recorder will turn ON.
Now the LOW on pin 3 will also go to pin 11 and to pin 8.
This LOW will pull pin 1 LOW in the absence of any
sound input signal from the microphone. This in turn
makes pin 2 HIGH. The diode D2 now blocks discharge via
the path it was charged up. C5 will start to discharge
via the 470K resistor R9 with a time constant of R9.C5 or
about 10.3 seconds. The turn off time is determined by the
relative switch on voltage which is about 50% on the rail
voltage, or about 6 seconds. Pin 3 will then move through
3.2V and pin 4 will go LOW thus turning OFF the tape
recorder.
It can be seen that continuous noise input to pin 1 will
keep pin 4 HIGH but that as soon as there is an absence of
noise input the time-out process automatically begins.
The LED is connected so that it lights up when there is
power to the Kit. However, it may easily be reconnected to
light up when the tape recorder is switched ON. That is
the purpose of the pad marked A next to the IC. Pad A is
connected to the unused sixth Schmidt trigger circuit. Pin
4 is connected to pin 5 so pin 6 goes LOW when the tape
recorder is ON. Pin 6 is connected to the pad marked A.
Cut the connection from the 1K resistor in series with the
LED between the resistor and the ground track. Solder in
a jumper wire between the 1K and pad A/pin 6 of the IC.
Now the LED will light up when the tape recorder is turned
ON.
