Electronic circuits > Radio > Radio Remote Control using DTMF
Radio Remote Control using DTMFH 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
Published on: 2004-08-21 (35273 reads)
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