Kit 13. VOICE-ACTIVATED TAPE RECORDER SWITCH
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 the 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.
WHAT TO DO IF IT DOES NOT WORK
Poor soldering is the most likely reason that the circuit
does not work. Check all solder joints carefully under a
good light. Next check that all components are in their
correct position on the PCB. Thirdly, follow the track with
a voltmeter to check the potential differences at various
parts of the circuit particularly across the base, collector
and emitter of the transistors and the integrated circuit.
A check list of other items:
•
are the transistors and IC in the correct way.
•
are the electrolytic capacitors and diodes in the
correct way.
•
are the 2 wires in the REMOTE socket shorting out
inside the plug.
For the circuit before the IC check that the collector-
ground voltage for T1 is about 2V. For T2 it should be 4V.
There should be about 1V across the electret microphone.
At the IC pin 3 taken to ground by a jumper wire should
turn ON the recorder. So should pin 1 taken HIGH to the
6V track. If this does not happen the the IC or its
attachment to the PCB is at fault.
After the IC short out the collector and emitter of T3. The
recorder should turn ON. If not then the fault lies in the
plug or switch.
It is possible that due to variations in tolerance the 47K
load resistor of the microphone may have to be increased
or decreased to get the best response. Reducing the value
will increase the sensitivity but if you decrease it too much
you will get an oscillation.
If the tape recorder does not turn OFF completely but very
slowly turns then there is a dry solder joint or hair-line
crack in a track somewhere. This type of fault can be hard
to find. Resolder everything and inspect all trackwork.
WHAT TO LEARN FROM THIS KIT
Many VOX use an op-amp at the front end instead of
cmos logic as we have done. An op-amp provides the
amplification and comparator operations all in one.
However, the total component count is almost the same.
(We have in fact built up two other commercial Kits using
op-amps and we found them to be surprisingly insensitive
at lower noise levels. This lower tech. cmos circuit we
have developed is the most sensitive circuit of all three.) It
is quite easy to modify the output of the PCB to attach a
relay to T3 instead of the tape recorder.
COMPONENTS
Resistors(1/4W, 5% carbon):
470R yellow violet brown
1
1K brown black red
1
3K9 orange white red
1
10K brown black orange
1
47K yellow violet orange
1
470K yellow violet yellow
2
1M brown black green
1
2M2 red red green
1
Resistors(1/4W, 1% metal):
1M brown black black yellow
2
Capacitors:
22nF mylar
2
100nF monoblock
2
22uF electrolytic
1
47uF electrolytic
1
5K potentiometer
1
1N4148 diode
2
74HC14 IC
1
14 pin IC socket
1
BC338 transistor
1
BC547 transistor
2
5mm LED
1
Electret Microphone
1
Two strand wire
10"
2.5mm plug
1
6V battery snap
1
Kit 13 PCB
1
PCB mounted switches SPDT
1
DPDT
1
