June 2017 9
OPEN COMMUNICATION ; BY LOU FRENZEL W5LEF
The Radio Modules
The modules I used are made
in the UK by a company called
Radiometrix. They are available in
the US from their distributor, Lemos
International. The transmitter module
TX3A operates in the ISM band at
914.5 MHz. It uses FSK modulation
and can take data at a rate up to 64
kbps. The nominal output power is
0 dBm or 1 m W. That’s not much
power, but good enough for a range
of 25 to 100+ meters depending on
the environment. It can operate from
a DC voltage in the 2. 2 to 16 volt
range.
The matching receiver RX3A is a
standard superhet with a sensitivity
of up to -107 dBm. It operates from
DC in the 2. 7 to 16 volt range. The
modules have pins that conveniently
plug into a standard breadboarding
socket.
Figure 1 shows the transmitter
circuit. I used a 555 timer to
generate a 480 Hz rectangular wave
to modulate the transmitter. The
voltage divider at the output of the
555 is used to reduce the input to
the transmitter to less than 2.5V
as required. The antenna is a three
inch vertical wire. The whole thing is
powered by a 9V battery.
The 15K resistor is a pull-up
that provides an enable signal to
the module. A standard slide switch
turns the 555 on and off as desired.
When the contacts are closed,
the oscillations are stopped as the
timing capacitor is shorted. At this
time, the remote device will be off.
Opening the switch contacts starts
the oscillation, and the remote circuit
will turn on.
The receiver circuit is shown in
Figure 2. I used a nine volt battery
to power the receiver. The antenna
is also a three inch vertical wire. The
15K resistor is a pull-up that provides
an enable signal to the module. The
receiver simply recovers the data sent
by the transmitter. The data appears
at pin 9 of the RX3A and drives a
logic circuit using CD4093 quad
NAND Schmitt trigger gates with an
LM393 comparator.
This circuit keeps the relay from
turning on if there is no data. Even
with no data, the output signal on pin
9 is noise that is of a sufficient level to
keep the relay on. The received signal
strength indicator (RSSI) signal on pin
5 is used to alert the availability of
real data.
The clean data signal at pin
4 of the CD4093 is used to drive
a 2N3904 transistor switch that
operates a relay. The 47 ohm resistor
ealing with RF circuits is daunting for most experimenters. It is tricky and a bit
frustrating if you build your own RF circuits. Now you don’t have to — thanks to the
many available wireless modules. These units are designed for short-range projects,
and most feature FSK (frequency-shift keying) data transmission in the industrial-scientific-medical (ISM) bands. Recently, I tried out some modules to implement a remote control off-on circuit. It was easy to put together and an immediate success.
D
Cheap and Dirty Wireless
Remote Control
Wireless modules make it easy.
FIGURE 1. Transmitter diagram with 555 timer IC and Radiometrix TX3A
transmitter module.