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H ere
is a circuit of a remote control unit which makes use of the radio
frequency signals to control various electrical appliances. This
remote control unit has 4 channels which can be easily extended to
12. This circuit differs from similar circuits in view of its
simplicity and a totally different concept of generating the
control signals. Usually remote control circuits make use of
infrared light to transmit control signals. Their use is thus
limited to a very confined area and line-of-sight. However, this
circuit makes use of radio frequency to transmit the control
signals and hence it can be used for control from almost anywhere
in the house. Here we make use of DTMF (dual-tone multi frequency)
signals (used in telephones to dial the digits) as the control
codes. The DTMF tones are used for frequency modulation of the
carrier. At the receiver unit, these frequency modulated signals
are intercepted to obtain DTMF tones at the speaker terminals.
This DTMF signal is connected to a DTMF-to-BCD converter whose BCD
output is used to switch-on and switch-off various electrical
applicances (4 in this case). The remote control transmitter
consists of DTMF generator and an FM transmitter circuit. For
generating the DTMF frequencies, a dedicated IC UM91214B (which is
used as a dialler IC in telephone instruments) is used here. This
IC requires 3 volts for its operation. This is provided by a
simple zener diode voltage regulator which converts 9 volts into 3
volts for use by this IC. For its time base, it requires a quartz
crystal of 3.58 MHz which is easily available from electronic
component shops. Pins 1 and 2 are used as chip select and DTMF
mode select pins respectively. When the row and column pins (12
and 15) are shorted to each other, DTMF tones corresponding to
digit 1 are output from its pin 7. Similarly, pins 13, 16 and 17
are additionally required to dial digits 2, 4 and 8. Rest of the
pins of this IC may be left as they are. The output of IC1 is
given to the input of this transmitter circuit which effectively
frequency modulates the carrier and transmits it in the air. The
carrier frequency is determined by coil L1 and trimmer capacitor
VC1 (which may be adjusted for around 100MHz operation). An
antenna of 10 to 15 cms (4 to 6 inches) length will be sufficient
to provide adequate range. The antenna is also necessary because
the transmitter unit has to be housed in a metallic cabinet to
protect the frequency drift caused due to stray EM fields. Four
key switches (DPST push-to-on spring loaded) are required to
transmit the desired DTMF tones. The switches when pressed
generate the specific tone pairs as well as provide power to the
transmitter circuit simultaneously. This way when the transmitter
unit is not in use it consumes no power at all and the battery
lasts much longer. The receiver unit consists of an FM receiver
(these days simple and inexpensive FM kits are readily available
in the market which work exceptionally well), a DTMF-to-BCD
converter and a flip-flop toggling latch section. The frequency
modulated DTMF signals are received by the FM receiver and the
output (DTMF tones) are fed to the dedicated IC KT3170 which is a
DTMF-to-BCD converter. This IC when fed with the DTMF tones gives
corresponding BCD output; for example, when digit 1 is pressed,
the output is 0001 and when digit 4 is pressed the output is 0100.
This IC also requires a 3.58MHz crystal for its operation. The
tone input is connected to its pin 2 and the BCD outputs are taken
from pins 11 to 14 respectively. These outputs are fed to 4
individual ‘D’ flip-flop latches which have been converted into
toggle flip-flops built around two CD4013B ICs. Whenever a digit
is pressed, the receiver decodes it and gives a clock pulse which
is used to toggle the corresponding flip-flop to the alternate
state. The flip-flop output is used to drive a relay which in turn
can latch or unlatch any electrical appliance. We can upgrade the
circuit to control as many as 12 channels since IC UM91214B can
generates 12 DTMF tones. For this purpose some modification has to
be done in receiver unit and also in between IC2 and toggle
flip-flop section in the receiver. A 4-to-16 lines demultiplexer
(IC 74154) has to be used and the number of toggle flip-flops have
also to be increased to 12 from the existing 4
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