10 June 2017
was selected to drop the unnecessary
voltage to the five volt relay coil. You may
need a different value for the relay you
select. The 1N4148 diode protects the
transistor from a voltage spike when the
relay coil turns off. The data pulses will turn
the relay on, closing its contacts to actuate
something else. The LED is used to check
the contact closure.
Figure 3 shows the breadboarded
transmitter and receiver. They are messy,
but they work, and I can rebuild them in a
neater more compact way now that I know
they work.
To test the system, separate the
transmitter and receiver by several feet
or more (like across the room from one
another). Apply power to both. The off-on
switch at the transmitter should be closed
to prevent data from being generated.
The relay should be off at this time as
determined by the LED operated by the
contacts. If the relay is on, that means
some noise is getting through. Adjust the 50K
potentiometer until the relay and LED turn off.
Turn on the data by opening the switch at
the transmitter. The LED should turn on. Turn off
the data by closing the switch. The LED should go
off. If not, adjust the 50K pot until it does. Repeat
this process until you get a setting for the pot that
works best.
Turn off the power to avoid running down
the batteries. The receiver is fairly power hungry
and eats nine volt batteries pretty fast. So, unplug
the battery if you are not using the receiver. One
possible solution is to use four AA cells to give six
volts. This supply will last longer. You may need to
trade the 47 ohm relay resistor for one of 22 ohms
to ensure relay pull-in. Plus, the 50K pot will need
readjustment.
How It Works
You would think that a steady 480 Hz
recovered square wave from the 555 would simply
turn the relay off and on at that rate. However,
that’s not the case. The relay is a mechanical
device and cannot switch off and on that fast.
What happens is that capacitor Cx averages the
pulses into a DC voltage that turns on the 2N3904
transistor. That holds the relay on. When the signal
pulses stop, the average DC on Cx drops to zero,
turning off the transistor and relay.
A key point here is that you should select the
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Transmitter
TX3A transmitter module from Radiometrix
555 timer IC
10K ohm 1/4 watt resistors ( 3)
15K ohm 1/4 watt resistor
27K ohm 1/4 watt resistor
0.01 µF disc capacitor
0.1 µF disc capacitor
SPST slide switch
Receiver
RX3A receiver module from Radiometrix
2N3904 NPN transistor or equivalent (e.g., 2N2222, etc.)
1N4148 silicon diode
LED (light emitting diode); common type
LM393 dual comparator
CD4093 CMOS quad two-input NAND Schmitt trigger gates
Relay 5V coil with contacts to match desired load
47 ohm 1/4 watt resistor
470 ohm 1/4 watt resistor
1K ohm 1/4 watt resistor
10K ohm 1/4 watt resistor
15K ohm 1/4 watt resistor
100K ohm 1/4 watt resistor ( 2)
50K ohm potentiometer
0.05 µF disc capacitor
0.1 µF disc capacitor
10 µF electrolytic capacitor
FIGURE 2. Receiver diagram with Radiometrix RX3A receiver, control
logic, and control relay.
The Radiometrix modules
are available from Lemos
International at www.
lemosint.com. Go to the
Radiometrix website at
www.radiometrix.com and
print out the datasheet and
app notes on the TX3A and
RX3A.
Parts List
