Easy Black Box RF Design

Today’s IoT way of thinking assumes that everyone is a “programmer”. There’s Arduino and MicroPython, which are relatively simple programming languages that emulate the more complex programming platforms like C, Python and C++. When all of the programming smoke clears, Arduino can generate most of the same logic patterns one would scare up with C or C++.  The same holds true for MicroPython and its big brother Python.

Sometimes “programming” is overkill.  For instance, what if I show you how to remotely control a pump or valve with a simple switch? The “switch” can be mechanical or electronic depending on how much you want to program or how much you want to solder.  If you’re interested, read on as I describe how to assemble a long-range MURS radio link that is capable of controlling large electrical loads that are associated with electric pumps and valves.

No Previous RF Design Experience Required..

As I alluded to earlier, the main radio link will consist of a MURS (Multi-Use Radio Service) transmitter and a companion receiver. The Multi-Use Radio Service needs no license and uses channels in the 151 – 154 MHz spectrum range. Our MURS design will consist of a 1-watt MCT1-154.600 transmitter and a matching MCR1-154.600 receiver.

We really don’t care what’s inside the MURS transmitter and receiver or how it works.  We only need to know how to feed the transmitter and how to use the resultant transmission’s information on the receiving end. In other words, the MURS radios are simply “black boxes” that we will incorporate into our overall design.

I’ve popped off the cover of the MURS transmitter in Photo 1. The receiver is shown in Photo 2.  All you need to know about the units is how to power them, how to set the DIP switches, how to trigger the transmitter and how to connect to the receiver’s relay contacts.

Powering the receiver and transmitter is simple.  Just connect a suitable +12VDC power source to the transmitter and receiver power input terminal blocks. Since we’re using a 1-watt transmitter, you’ll need to make sure your transmitter power supply can source enough current to support the transmit current requirements of the MURS transmitter.  You can find the transmitter’s current requirement in the MCT1-154.600 user manual. The receiver will use less current, but you should be sure your receiver power supply can handle the receiver’s maximum current requirement.

Setting the DIP switches is easy. All you need to do is make the transmitter and receiver 12-position DIP switch positions match.  This “matching set” of positions is the encryption scheme that will be used to establish a logical link between the transmitter and receiver.  In other words, if the DIP switches don’t match, the receiver and transmitter will not talk to each other.  The same “make it the same” idea goes for the 3-position DIP switches and the single-position DIP switches.  The smaller DIP switches determine the frequency that the transmitter and receiver operate on.  Again, if these switches don’t match, the transmitter and receiver will never communicate with each other.

The remaining 4-position DIP switch on the receiver determines if the relays will operate in continuous mode or on-off mode.
Continuous mode operation is defined as the relay being energized as long as the transmitter signal is received (think of this like a push-button switch: the bulb is only on if your finger is on the switch).
On-off mode is akin to switching a lightbulb on and off.  A signal from the transmitter trips the “lightbulb” (the relay in our case) on and the bulb stays on until a second signal is received. The second signal turns the light off and the process is repeated with the reception of the next transmitted signal (just like an ordinary domestic light switch, that you switch on, or switch off, with a flick of the finger)
Each of the 4 DIP switches corresponds to relay on the receiver.  So, we can select which relays will operate in continuous mode or on-off mode.  The mode we choose to operate in depends upon how we want to control the electrical pump or valve.

Triggering the transmitter is as simple as grounding the selected input pin of the transmitter’s terminal blocks. There are a total of 4 inputs, with each terminal block supporting 2 inputs and a ground pin. Each transmitter input corresponds to one of the receiver’s relays. Each relay is configured as SPDT. If the pump or valve we intend to remotely control draws less than 8A, we can control the pump or valve directly from the receiver’s on-board relay. In reality, we really don’t want to tie our receiver directly to a load. An overloaded relay could damage our receiver. So, we’ll use the receiver relays to drive devices that can handle higher voltage and current loads.

The final requirement for our RF design is to mount the proper antenna on the transmitter and receiver. We’ll need antennae that are tuned to the frequency band used by our MURS transmitter and receiver. At this point, we can power up our transmitter and receiver.  If we’ve done everything right so far, absolutely nothing should happen.

Triggering the Transmitter..

All we need to do is ground the input terminal associated with the relay we want to energize on the receiver. These inputs have a (4k7) pull-up resistor to about +5v so will work with any “switch”.
There are many ways to provide that ground signal.  We can use a simple SPST mechanical switch, an optoisolator, or an open drain/open collector MOSFET or bipolar transistor output.
A microcontroller (5v logic level) I/O pin will also work. This all depends on where your “control” input comes from.
Simplest is the mechanical switch, which only requires some simple “doorbell” wiring.
The microcontroller I/O pin method requires some soldering and some programming.
An optoisolator is good where your “control” signal is an external voltage, but where you can’t guarantee that voltage will always be 5v, that it will be free of spikes or overloads, or when it’s not referenced to ground

If your microcontroller I/O pin produces a logic level that isn’t compatible with the +5v logic used in the transmitter then either feed your I?0 pin signal into the optoisolator (see above) or use a MOSFET or transistor as a buffer. (The output of a typical optoisolator is an LED-triggered MOSFET or bipolar transistor, which are integral to the optoisolator. If a microcontroller I/O pin is used, it simply replaces the optoisolator’s internal LED. )

If building up a microcontroller subsystem is beyond your capabilities, you can use easily obtainable Arduino or MicroPython hardware and software to drive the input via any of the methods detailed above .

There are multitudes of examples on how to perform this operation using Arduino or MicroPython on the internet.

Effectively Utilizing the Receiver’s Relays..

The MURS receiver’s relay contacts can switch AC or DC.  However, I recommend not using the receiver relays for heavy or inductive loads. I suggest getting familiar with the high-power switching devices offered by machine automation companies such as Automation Direct.  You will find that contactors and solid-state relays (SSRs) may be the buffers you need to drive that pump or valve.  If motor control is your target, you’ll need to bone up on servo controllers, which are also listed at Automation Direct.

Basically, you can find a MURS receiver relay interface to most any pump, valve or motor at Automation Direct.  Why Automation Direct?  I have in-depth experience with their product line.  When possible, it’s always best to go with things that have worked for you in the past. When I was working at Kennedy Space Center, we called that go-with-what-you-know hardware “flight tested hardware”.

Flight Testing Your MURS Design..

The best way to design with the MURS radio set we’ve been discussing is to get the actual hardware and twiddle with it. Toggle the DIP switch settings and observe the results. Power the remote transmitter and/or receiver with your power source of choice and observe the results. I use inexpensive +12VDC power supplies that can be obtained from Amazon to initially setup and test my MURS designs. If your MURS design requires remote battery power, take a look at the solar systems used to power automated gates.

If you are not a programmer and need to control the transmitter via a microcontroller, get some Arduino or MicroPython hardware and learn the basics.  The Arduino and MicroPython hardware is cheap and the software is free.

The most important thing is not to be shy of trying things out. These units are robust (if you are sensible: don’t connect AC mains to the inputs, leave them out in the rain or use them as improvised hammers) and should allow any amount of tinkering.

Get out there and test your ideas. If the resulting performance isn’t what you want, try something else. Enjoy yourself.

For more information on the Lemos line of MURS modules please log on to

www.lemosint.com  or email us at dlemos@lemosint.com

Best regards,  Dan Lemos