This is a simple RF sensor network with small sensors sending measurements to a radio receiver board on top of a Raspberry Pi.

I’m using some cheap 433.92MHz sensors from Jaycar:

• ZW3100 transmitter

• ZW3102 receiver

They have digital I/O, low power and reasonable range. Toggling the data line on the transmitter should see a corresponding change in the output line of the receiver. Of course, there’s no provision for collisions etc.

The transmitters need to be battery powered and use low current so that they can be compact and last at least a year before the battery runs out.

The data rate I chose is 4800 baud, but we need to work out some way to synchronise the clocks between the transmitter and the receiver, otherwise we’ll never be able to work out where the data boundaries are. I chose to use Manchester coding, which used two bits for every data bit (i.e. halves the data rate), but this ensures a clock transition for every bit. This makes it much easier to synchronise the receiver’s clock with the data clock.

The protocol for sending a message comprises the following:

1. a preamble, comprising 32 x 0-bits, followed by 2 x 1-bits

2. a sync byte, marking the start of the message

3. a message length byte

4. one or more bytes of the message

5. a CRC calculated from the message length and content

The RF receiver has an AGC circuit which means that in the absence of any real signal, it will decode random noise. The purpose of the preamble to send a strong signal to the receiver, force the AGC to a reasonable level and allow the decoder to sync up with the 0→1 transitions that make a Manchester-encoded 0-bit.

The message format is designed to be compact, self-describing and to allow for future data to be added.

The following sensor types are currently supported:

• 1 - Temperature in C (x10)

• 2 - Pressure

• 3 - Message sequence number

• 4 - Light level

• 5 - Humidity

• 6 - Battery voltage in V (x10)

The sequence number is always included in a message.

The receiver has a sampling clock that samples the Data Out from the receiver at 16 times the expected baud rate. Incoming bits are rotated through a 16-bit window. When the window value looks like this:

x x x 0 0 0 0 0 1 1 1 1 1 x x x x

then a 0-bit is accepted. Similarly, which the window looks like this:

x x x 1 1 1 1 1 0 0 0 0 0 x x x x

then a 1-bit is accepted.

Once a bit transition has been matched, the receiver will initially look for the next bit after another 16 samples have been received. To cater for clock skew between the transmitter and the receiver, subsequent bits can be matched at small negative or positive offsets from this pattern.

Once a reliable self-clocked data stream is being received, the receiver FSM starts matching the various parts the make up the message protocol, until a validate message has been received. The receiver maintains a buffer of received message for a fixed number of stations.

Separately to this, the receiver acts as an I2C client for the Raspberry Pi. The Raspberry Pi uses the I2C bus to clock out the latest received messages in the buffer. It can then process the information as it chooses, for example making it available via its web server.

I wrote up a bit more about how this works here:

Building a simple RF-based sensor network