I’ve made some great progress with the sensor node project over the last two days. After finally getting around the problems I was having with the software bugs and the SPI restrictions I settled on the circuit you can see on the left.
This is a breadboarded Arduino made up of the ATMega328p chip with appropriate resistors and an oscillator. Connected via SPI is the RFM69CW radio Tx/Rx module and via I2C is an HTU21D humidity and temperature sensor.
I’ve also added an LED to Digital Pin 9 to give a brief flash after each transmission as feedback the unit is working,
Everything is powered by a 3.7v 850mAh lithium rechargeable battery. The battery passes through an MCP1702 voltage regulator to give a nice stable 3.3v for all of the devices. I’ve added some sleep functions to the code so that in-between transmissions the MCU powers down for 10 seconds before waking and repeating the data transmission. This means for the majority of the time the transmitter is using micro-amps and then only milli-amps for a few milli-seconds. Overall a very efficient state which should easily give a battery life over well over a year.
The data packet contains the node’s unique ID number, the battery voltage and the two sensor readings (Temperature and humidity).
I’ve got the circuit as small as possible on the breadboard (see pic). The next stage would be to transfer it onto a stripboard and solder it all together. Finally i’ll design and etch a PCB. I could use SMD parts to make the entire thing even smaller still and swap out the ATMega for an ATTiny which is a smaller chip.
I want to make 2 or 3 different sensor nodes with varying sensors on them (temperature, humidity, pressure, luminosity, etc.) so i’ve got packets coming in from several nodes. This will then allow me to start writing the code for the base station which will receive the data, time stamp it, store it on an SD Card and then sporadically upload it to a web server.
I’ve got 433MHz and some 868MHz RFM radio modules and also an SRF radio module from Ciseco which claims to have longer range. These will be all tested out in time to see which is best for penetration through walls, etc.