Nuts and Volts - June 2017

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.