Power usage depends on the current of any sensors that you add, as well as how often you configure LoFi to check sensors and transmit. Transmitting consumes approximately 15 mA. Not transmitting, but constantly checking the sensors, consumes about 0.5 mA.

A more likely scenario for battery operation is where LoFi transmits sensor data periodically or is activated by a trigger switch, say every 10 minutes. In that case, LoFi sleeps most of the time. As you will see, this allows for extremely long battery life.

This weekend, I coded and tested the sleep mode. LoFi uses the ATtiny watchdog to wake every 1/4 of a second to check whether it is time for a periodic transmission. It also examines the receive pin to see if someone is trying to configure the device. The digital trigger input (which can be connected to vibration or door switches) does not require polling. Instead, it will wake the device immediately.

On the oscilloscope trace below, you can see LoFi wakes up briefly (narrow vertical spikes) before falling back to sleep. Each spike takes about 4.3 ms (oscillator start up + computations). Then, the transmit time occurs and LoFi takes about 200 ms to read the sensors and transmit the data.

If a transmission occurred every 10 minutes, here is the average current draw:

Only 18.5 microamps – thank you Atmel! Plug that into some common batteries:

So, theoretically, you could run LoFi from a coin cell for a year. If you add a diode, capacitor, and small solar panel, then the life of the coin cell could be extended, as it would act as a backup at night. Realistically, with a pair of AA cells, you could forget about it for many, many years.

Best of all, since LoFi includes the power supply voltage in the sensor data, you’ll know when it is time to change the batteries.