To increase the battery life of the little Funky wireless node I tried reducing the clock frequency of the ATTiny to 1MHz (internal 8MHz + CLKDIV fuse) but when set to 1MHz no communication was possible. With support from the JeeNode Forum I now know that for proper operation of the RFM12 a clock frequency of at least 4MHz is required.
As the current consumption in standby was measured to be about 50uA with a cheap digital volt meter and the DHT11 datasheet states an even higher standby current of min. 100uA for the sensor, “CapnBry” had a nice idea of reducing the power consumption by switching the DHT11 on and off via a port pin. As the DHT needs about 1 seconds after power on to deliver trustworthy measurement results one simply could sleep 1 second short, power on the sensor, sleep for another second, measure the result, turn of the sensor and transmit the data before going to sleep again.
I will test this procedure within the next days and keep you up to date…
Due to the minimum voltage requirement of 3V for DHT11, it is not possible to switch the sensor on and off using a port pin when running from 3,3V.
I bought some 1F/5V supercaps and charged one to 3.6V. The funky could run more than 16h from it reporting the temperature and humidity every 5 minutes. After that 16h the voltage dropped below 3V so the temperature and humidity could not be measured but the funky kept communication for about another 6h. So with a 1F super cap charged to 3.6V the funky can live for about 24h.