Radio Days

Some Thoughts about Low Cost Wireless Networks

This is the first of a few posts looking at short range wireless devices. I've experimented with these for a few years and believe that there is a requirement for a really low cost transceiver which allows smart sensors to communicate with each other. In the first of these posts I will look at simple super-regenerative receivers which I have encountered already by pulling apart devices such as wireless doorbells. These can be quickly re-purposed by adding your own microcontroller, which is what I did with the Lidl remote control socket.

Compared to WiFi and other 2.4GHz systems the 433MHz link is cheap and tacky, but it serves its purpose adequately. It offers a very low cost means for devices to get small amounts of data across a wireless link. It is used often in wireless doorbells, remote control systems and burglar alarm systems.

The Lidl remote socket used an example of a super-regenerative receiver and my transmitter pcb uses a SAW stabilised single transistor amplitude modulated (On/Off keyed) transmitter from Telecontrolli. The link runs at a pedestrian 1200 baud - it's best to always pick a standard baudrate to make it easy to debug with hyperterminal. 1200 baud is perfectly adequate for sending a few relay commands and temperature readings around the house.

Super-regenerative receivers are designed to be very cheap - but they can lack in sensitivity. They use a technique discovered in the 1920s by Edwin Armstrong, when manufacturers wanted a cheap wireless that only used one valve. Here's some Wikipedia info for those interested in early super-regenerative sets

http://en.wikipedia.org/wiki/Regenerative_circuit

Today this has translated into one active RF transistor but the principle is the same. A super-regenerative receiver is small and cheap to make often using less than 30 components.

The transistor is configured to be a 433MHz RF oscillator and is biassed up so that it is just on the point of oscillation. It is swept through this oscillation point using a lower frequency of a few tens of kHz. Any incoming RF signal will add to the RF transistors desire to oscillate, and this appears as a change in collector current. An op amp or a chain of 3 cascaded transistors is usually used to detect this change in current, and turn it into a logic level pulse which follows the detected signal exactly. The result is a signal that you can feed into a micros input pin and begin to decode the serial bits.

The super-regenerative receiver has at its heart an RF oscillator. In the 1920s super regen receivers could re-radiate and produce a lot of interference picked up by neighbouring sets. This might be seen as a disadvantage - but its not a million miles from what you need to automatically transmit a packet of serial data. In fact some cheap super-regens emit so much RF carrier that they can cause other signals to be blocked. When not receiving, this RF oscillator could be re-purposed to be a low power transmitter. In theory you could have a whole 433MHz transceiver costing about $1 to implement. This would be a really scungy transciever with only about 10m of range - but if you are using it in a network with other sensors which can retransmit messages, then in theory you could bounce your packet across a series of nodes and thus get to the gateway as a series of short range hops.

Super-regen designs are compact. With only one or two active components and 25 or so passives, they can fit onto a board the size of a postage stamp. They can also work at very low power - just a few microamps. There's a great patent for a super-regen by Thomas E. McEwan for a receiver that works down to 1 volt supply and 1uA of current

http://www.freepatentsonline.com/5630216.html

This makes it ideal for battery powered sensors which have to work for years from a single cell - or power harvest, from mechanical vibrations for example.

I had a go at copying the McEwan design - it worked to a point and had a range of about 3m when using a standard doorbell push as the transmitter. I suspect that it was working more as a crystal set than a super-regen though.

There is an renaissance in Super-regen designs as they can be implemented in silicon within the actual chip. This makes them ideal for say 2.4GHz bands where the antennas a much smaller. Such devices could be used as RF transponders for RF ID devices. They could possibly even harvest their operating power from the RF energy they receive - stored in a super-capacitor.

Super regenerative receivers are used in wireless doorbells, thermometers, remote control toys and other short range wireless gadgets. The photos show a commercial Laipac super-regenerative module and two doorbells - one in discrete through-hole components and the other in surface mount. These discrete designs will work at currents down to about 300uA.

The Laipac design is documented on the web and uses 2 transistors and an LM358 op-amp comparator device. The older doorbell achieves the receiver in just 3 transistors, and the newer doorbell does it with four. All low cost super-regens generally use a fixed capacitor and slugcore-tuneable inductor to get close to the required frequency. The inductor is clearly visible as the red plastic component on the back of the Laipac design.

Super regenerative receivers are also very easy to hack - in fact I first hacked one from a wireless doorbell I bought in Wilkinsons for under a fiver. You just have to find the serial output data - which is easy if you have a scope, or at a pinch a logic probe. Just connect this to your micro and start coding.

There's a well informed article about super regenerative receivers by Dr. Eddy Insam - well worth a read if you want to pursue them further

http://www.eix.co.uk/Articles/Radio/Welcome.htm

Eddie Insam talks about combining a super regen detector with a SAW stabilised transmitter device to make a cheap transponder or transceiver, but gives no detail away. He also mentions hacking a SAW stabilised transmitter module to make it into the heart of a super-regenerative receiver - tricky, but do-able, he says - I wish he would share some of that detail.

Over the next couple of posts I will share some of my recent findings.