Experimental project around RFM radio modules using an ATmega328P MCU and librfm/librfm95.
To do something really extraordinary, the temperature reading of an MCP9808
sensor is periodically transmitted to the receiver.
To save battery power, the controller, radio module and temperature sensor
are put to power down/sleep mode in between transmissions. The idle current
is ~75µA, which is still quite a lot, but already better than 8mA 🙂
MCU, radio and temp sensor take about 5µA, so the TC1262 3.3V regulator seems
to account for ~70µA.
There is basic SD card support
to test sending larger amounts of data, but it might be useful for something
else like a data logger.
The receiver currently converts the raw temperature reading to °C and displays it with the RSSI value, CRC result and transmitter output power on a nice IPS TFT display. It responds to the transmitter as kind of ack with the RSSI, which is used for some very basic power management in the transmitter, reducing the supply current from 50 mA down to 22 mA. The transmitter waits for this response with a timeout so it won't be blocked and consumes a lot of power just because there is no response coming back.
Looking at the payload in the transmitted signal in URH (with an RTL-SDR Blog V4):
The four selected payload bytes are:
0b00000011Payload length (address byte + 2 byte temperature value)0b01000010Address (0x42)0b11000001Upper byte of raw temperature value from MCP98080b01010011Lower byte of raw temperature value from MCP9808
Calculating the temperature (assuming >= 0°C):
jshell> (0b11000001 & 0x1f) * 16 + 0b01010011 / 16f
$1 ==> 21.1875
So, 21.2 °C 🙂
The first 15 0b10101010 bytes are the preamble, then there are 4 sync word
bytes. After the 4 payload bytes, there are 2 CRC bytes as described in the
datasheet of the RFM69HCW:
