Environmental Sensor

is that change on the wind?

Environmental Sensor

OK, so it's just a thermometer and a light-dependent resistor, but environmental sensor sounds so much cooler. The sensor node puts changes on the network so that other nodes, such as the display and control node or the (theoretical at this point) thermostat node can act upon the information.

On the left is a picture of the environmental sensor node. It has a Picaxe 08M as the processor, and uses a DS1820 temperature sensor and a light dependent resistor from Radio Shack as the sensors.

Clockwise around the breadboard, starting from the lower left:

  • The blue block is a terminal block for connecting the sensor node to the two-wire network. You can see the diodes for the bridge rectifier above and below the terminal block.
  • On the top left of the breadboard are the two capacitors that make up the filter and storage circuit for the power supply. That's a 1000uF capacitor that stores power for the node during network communications cycles.
  • Just below and to the right of the power supply capacitors is the network interface circuit, consisting of 3 resistors and a single transistor.
  • Next to the interface circuit on the right and above the Picaxe is the serial download circuit. Usually when I'm working on the software for a node I use a couple of chips that I trade back and forth between the breadboard and a P.H. Anderson development board so I don't really use the serial download circuit, but it's three resistors and a diode, so it doesn't hurt to include it.
  • Next to the right is the LDR and the voltage divider resistor that make up the light sensor. The light sensor is connected to input 2 on the Picaxe.
  • Below the light sensor is the DS1820 in a TO-92 case. It's connected to the Picaxe on input 1.
  • Last is the indicator LED on Picaxe I/O 0 and the Picaxe chip itself.


  • 08M running network slave software.
  • DS18S20 temperature sensor.
  • LDR light sensor.
  • Network powered.

Node message tables

The environmental sensor node registers to send the following messages:

Message IDMessage textContent
1 MSGtemperatureChanged Contains the current and previous temperatures measured by the environmental sensor node. The sender info is the node ID, the high data byte is the current temperature and the low data byte is the previous temperature.
2 MSGlightLevelChanged Sent when the light level read by the LDR changes from the previous measurement. The sender info is the node ID, the high data byte is the new light level, and the low data byte is the previous light level measurement.
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