It is often required to switch electrical appliances from a distance
without being a direct line of sight between the transmitter and
receiver. As you may well know, an RF based wireless remote control
system (RF Transmitter & RF Receiver) can be used to control an
output load from a remote place. RF transmitter , as the name
suggests, uses radio frequency to send the signals at a particular
frequency and a baud rate.
The RF receiver can receive these signals only if it is configured
for the pre-defined signal/data pattern. An ideal solution for this
application is provided by compact transmitter and receiver modules,
which operate at a frequency of 434 MHz and are available ready-made.
Here, the radio frequency (RF) transmission system employs Amplitude
Shift Keying (ASK) with transmitter (and receiver) operating at 434
MHz. The use of the ready-made RF module simplifies the construction of
a wireless remote control system and also makes it more reliable.
RF Transmitter
This
simple RF transmitter, consisting of a 434MHz license-exempt
Transmitter module and an encoder IC , was designed to remotely switch
simple appliances on and off. The RF part consists of a standard 434MHz
transmitter module, which works at a frequency of 433.92 MHz and has a
range of about 400m according to the manufacture. The transmitter module
has four pins. Apart from “Data” and the “Vcc” pin, there is a common
ground (GND) for data and supply. Last is the RF output (ANT) pin.
Pin Assignment of the 434MHz Transmitter module
Note that, for the transmission of a unique signal, an encoder is
crucial. For this, I have used the renowned encoder IC HT12E from
Holtek. HT12E is capable of encoding information which consists of N
address bits and 12N data bits. Each address/ data input can be set to
one of the two logic states. The programmed addresses/data are
transmitted together with the header bits via an RF transmission medium
upon receipt of a trigger signal. Solder bridges TJ1 and TJ2 are used
to set the address and data bits.
The current consumption with a supply voltage of near 5.4V is about
10 mA. Since the current consumption is very little,the power can also
be provided by standard button cells. Recommended antenna length is 17
cm for 433.92 MHz, and a stiff wire can be used as the antenna. Remember
to mount the antenna (aerial) as close as possible to pin 4 (ANT) of
the transmitter module.
RF Receiver
This circuit complements the RF transmitter built aorund the small
434MHz transmitter module. The receiver picks up the transmitted signals
using the 434Mhz receiver module. This integrated RF receiver module
has been tuned to a frequency of 433.92MHz,exactly same as for the RF
transmitter.
434MHz receiver module
The miniature 434MHz RF receiver module receives On-Off Keyed (OOK)
modulation signal and demodulates it to digital signal for the next
decoder stage. Local oscillator is made of Phase Locked Loop (PLL)
structure. Technically, this is an Amplitude Shift Keying (ASK)
receiver module based on a single-conversion, super-heterodyne receiver
architecture and incorporates an entire Phase-Locked Loop (PLL) for
precise local oscillator (LO) generation. It can use in OOK / HCS / PWM
modulation signal and demodulate to digital signal.
The receiver module has eight (4+4) pins. Apart from three “ground
(GND) ” and two “Vcc” pins, there are two pins (one for Digital Data
& other for Linear Data) for data output. Last is the RF input (ANT)
pin.
Pin Assignment of the 434MHz Receiver module
Pin Connections
- 1 Antenna
- 2 Ground
- 3 Ground
- 4 Vcc
- 5 Vcc
- 6 Linear Data (Normally NOT used)
- 7 Digital Data (Normally Used)
- 8 Ground
The “coded” signal transmitted by the transmitter is processed at the
receiver side by the decoder IC HT12F from Holtek. VR1 and R1 are used
to tweak the oscillator frequency of the decoder to that of the
transmitter. Any possible variations due to component tolerences and/or a
different supply voltage can be compensated by this arrangement. HT12F
is capable of decoding informations that consist of N bits of address
and 12N bits of data. HT12F decoder IC receives serial addresses and
data from the HT12E encoder that are transmitted by the RF transmitter
module. HT12D compare the serial input data three times continuously
with the local addresses.
If no error or unmatched codes are found, the input data codes are
decoded and then transferred to the output pins. The “Valid
Transmission” (VT) pin also goes high to indicate a valid transmission.
For proper operation, a pair of HT12E/HT12F ICs with the same number
of addresses and data format should be chosen. The data bits are set up
using solder bridges RJ1 and RJ2. Output of the decoder is brought out
on a pinheader K1 , making the logical signal available to circuits that
need it. This output is also fed to the relay driver transitor T1. The
RF Receiver circuit can be powered from a standard 5VDC supply. Just as
for the RF Transmiitter, the aerial (17 cm for 433.92 MHz) has to be
mounted as close as possible to the RF IN (ANT) pin of the 434MHz RF
receiver module.
RF Receiver – Schematic Diagram
Notes
- RF transmitter circuit can be safely powered from DC 4.5V to 6V
power supply. Here, diode D1 is added to introduce a 0.65V drop, but
this is not very crucial
- The 434 MHz RF module (Tx &Rx) is available from many sources.
Connection terminals are usually labelled on the PCB. In case of any
doubt, refer datasheets of the RF modules
- In practice, the transmitter is usually powered by batteries. But
you can power the receiver from an onboard /external dc supply too. In
this case, make an optional “noise filter” arrangement , as shown here,
to “clean” the 5V power rails of the 434MHz RF receiver module
optional “noise filter” circuit
