Here 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.
