First published in Electronics World magazine, February 2009
Previously I’ve written articles concentrating on the radio module hardware, and how it can best be used. This discussion is aimed more at answering the question “what is an ISM wireless module actually for ?”
Take a look at any typical module datasheet. After the inevitable (and meaningless) claims of “high quality” and universal usefulness, and the necessary (but often all too sparse) technical data, there is usually a sad little list of generic “application examples” (frequently the same list for every module a particular maker has in their whole catalogue) which could leave you believing that radio telemetry is only used for bar-code readers and dockyard crane control.
It can, of course be used here, but there is a whole other constellation of potential applications just waiting for a radio module that just requires a little thought:
Consider what the radio does. It is a device that transfers information from one point to another. That information can be a simple baseband waveform (carrying speech, or tones) or a coded digital data-burst containing as little as a single bit (“controlled device on/off”) right up to multi-megabyte image files.
The distance over which the radio link operates is similarly variable, depending on transmitter power, operating frequency band and receiver sensitivity. These figures can combine to give ranges that start in meters, and end in thousands of miles.
By concentrating on practical, available, wireless modules, we can close in on more practical figures: with the exception of WiFi LAN systems, most SRD radios offer peak data rates of between 5kbit/sec (narrowband, long range, low power types) and 250kbit/sec (wideband, short range designs) with practical (as opposed to “theoretical” or “claimed”) ranges between 10m (low end Bluetooth) and 1km (VHF).
That’s still a fairly broad church, so let us consider a few more parameters. Few modules are smaller than two cubic centimetres, or bigger than twenty. (4 x 3 x 1 cm is about average).
Current consumption will be (in most cases) between five and fifty milliamps (transmit current may be more for certain, high power units, but these are unusual) at five volts or less.
Prices range from (very roughly) ten pounds, up to over a hundred. Increased range is the main factor gained with increased cost. (To a much lesser extent, improved functionality and easier interfaces are also gained)
So, what is our imaginary, ‘average’ wireless module ? It may be either a one way (transmitter / receiver) or two way (transceiver) link. It has a (fairly) slow datastream, with a range roughly in proportion with a typical building, from a device that will fit in the palm of a hand, and can be run from a battery.
Now we can consider applications for this class of devices, by thinking of them:
1. From the point of view of the data channel.
Sending constant (or long) streams of information (such as human voice) is one of the most obvious uses for a communication link. In the literal sense, several allocated ISM bands (433MHz in the UK, parts of 869MHz, and the 2.4GHz band) permit the transmission of voice, audio or video signals, while the ability to stream large blocks of data suggests data download or even low-speed LAN type tasks.
For example: Wireless audio devices (headphones, radio microphones), wireless hearing-aids, remote audio and video surveillance, contact-less download from data logging devices (such as vehicle telematics or environmental monitors), industrial process monitoring, remote computer peripherals, remote reading of GPS datastreams (in asset tracking devices or differential GPS systems)
2. As a distant actuator:
Think of the device as a wireless replacement for a control cable, or a remote mounted switch, or remote monitor of an indicator or ‘state’. Very little information (very few bits) is sent. This category contains most simple ‘industrial switching’ tasks, but also (when one thinks about it) the entire class of ‘alarm’ applications, where the remote identification of a discrete state-change (a smoke detector, a door-contact, a ‘help’ button) is required.
For example: Remote lighting control, irrigation pumps, fluid level monitors, wireless door entry controls (including the traditional ‘garage door openers’), security (‘burglar’ or ‘unauthorised entry’) alarms, fire, gas or chemical leak alarms, personal ‘nurse-call’ alarms, man-down alarms, simple remote vehicle control, heavy machinery (crane) control, remote key devices (the automotive ‘key fob), wireless ‘machine stop’ safety systems.
3. As a mobile data terminal:
Low power and compact dimensions are the key issues, usually combined with higher data volumes than ‘cable replacement’ applications.
Examples include: Portable instruments linked to remote data gathering networks (temperature or humidity monitors, and of course bar code readers and EPOS terminals), data communication to vehicles (bus or lorry telemetry, marine data buoys, weather balloons), personal alarms (pager-type devices)
Or as something completely new.
There are thousands of short range wireless applications in the world’s marketplaces but there are many, many more yet to be imagined. The hardware already exists. The challenge, and the opportunity, is to imagine something new to do with it.
by Myk Dormer, Senior RF Design Engineer, Radiometrix Ltd